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Hydrogeology JournalOfficial Journal of theInternational Association ofHydrogeologists ISSN 1431-2174Volume 18Number 8 Hydrogeol J (2010)18:1747-1772DOI 10.1007/s10040-010-0672-3

Review: Recharge rates and chemistrybeneath playas of the High Plains aquifer,USA

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Review: Recharge rates and chemistry beneath playasof the High Plains aquifer, USA

Jason J. Gurdak & Cassia D. Roe

Abstract Playas are ephemeral, closed-basin wetlandsthat are hypothesized as an important source of rechargeto the High Plains aquifer in central USA. The ephemeralnature of playas, low regional recharge rates, and a strongreliance on groundwater from the High Plains aquifer hasprompted many questions regarding the contribution andquality of recharge from playas to the High Plains aquifer.As a result, there has been considerable scientific debateabout the potential for water to infiltrate the relativelyimpermeable playa floors, travel through the unsaturatedzone sediments that are tens of meters thick, andsubsequently recharge the High Plains aquifer. Thiscritical review examines previously published studies onthe processes that control recharge rates and chemistrybeneath playas. Reported recharge rates beneath playasrange from less than 1.0 to more than 500 mm/yr and aregenerally 1–2 orders of magnitude higher than rechargerates beneath interplaya settings. Most studies support theconceptual model that playas are important zones ofrecharge to the High Plains aquifer and are not strictlyevaporative pans. The major findings of this reviewprovide science-based implications for management ofplayas and groundwater resources of the High Plainsaquifer and directions for future research.

Keywords Review . High plains aquifer . Recharge .Playas . Nitrate . USA

Introduction

Playas are ephemeral, closed-basin wetlands that arecritical habitat for wildlife in the semiarid, shortgrassprairie, and agricultural landscape of the southern GreatPlains physiographic province, and have been hypothe-sized to be important zones of recharge to the High Plains(or Ogallala) aquifer (450,000 km2) in the United States(Fig. 1). Playas are similar to the topographical-depressionwetlands, or prairie potholes, of northern United Statesand Canada (van der Kamp and Hayashi 2009) and pans,which are closed basin, geomorphic depressions found inmany arid and semiarid regions of the world includingsouthern and central Africa, the Pampas and Pantanal ofSouth America, southern and western Australia, theeastern Gobi desert of Mongolia, the northeastern plainof China, and the western Siberian plain (Goudie 1991;Goudie and Wells 1995). In general, prairie potholes andpans are annual or seasonal zones of groundwaterdischarge and sometimes zones of groundwater recharge(van der Kamp and Hayashi 2009), whereas playas are notzones of groundwater discharge and are hypothesized aszones of groundwater recharge, as described in this paper.

There are approximately 66,000 playas throughout thesouthern Great Plains physiographic province (Fig. 1) andapproximately 61,000 playas are on the High Plainsaquifer (M. McLachlan, Playa Lake Joint Venture,personal communication, 2008). The highest density ofplayas is in the southern High Plains (or Llano Estacado)aquifer in Texas and in part of the central and northernHigh Plains aquifer in Kansas and Nebraska (Fig. 2a;Smith 2003; LaGrange 2005). Playas of the southernGreat Plains are essential habitat in one of the mostimportant inland migratory corridors in North America formany waterfowl, shorebirds, waterbirds, and for manyother migratory and resident birds. Playas are critical formaintaining biodiversity (Tsai et al. 2007) and arewetlands unique to the Great Plains because they arelikely zones of recharge and do not receive groundwaterdischarge as do prairie potholes and many other types ofwetlands. The floors of most playas are lined withrelatively impermeable clay soils and are commonlyseparated from the regional water table by tens of metersof unsaturated zone (vadose zone), which have generallyconfounded a detailed understanding of the role thatplayas have in recharging the High Plains aquifer. Since

Received: 10 January 2010 /Accepted: 13 October 2010Published online: 6 November 2010

© Springer-Verlag (Outside the USA) 2010

J. J. Gurdak ())San Francisco State University,1600 Holloway Ave, San Francisco, CA 94132, USAe-mail: [email protected].: +1-415-3386869Fax: +1-415-3387705

C. D. RoeCH2M Hill, Anchorage, AK 99508, USAe-mail: [email protected]

Hydrogeology Journal (2010) 18: 1747–1772 DOI 10.1007/s10040-010-0672-3

Author's personal copy

the early 1900s, many conceptual models about rechargebeneath playas have been proposed. Some early concep-tual models indicate that playas are evaporative pans thatdo not allow recharge beneath playas, whereas other morerecent models indicate that playas are effective rechargebasins. A variety of data supports various aspects of thesecompeting conceptual models.

The competing conceptual models have developedbecause of the sporadic nature of rainfall to the semiaridregion, the large number of playas in the region, a range ofphysical characteristics in playas, the relatively thickunsaturated zones (often greater than 30 m) separatingmost playas from the regional water table, and theinherently uncertain nature of most methods used toestimate recharge. An accurate understanding of rechargerates beneath playas is important from the perspective ofgroundwater management and the sustainability of ruralagricultural economies, particularly in light of the sub-stantial water-level declines in the High Plains aquifer

(McGuire et al. 2003). Other environmental concernsinclude erosion and transport of sediment and contaminantsfrom surrounding land and modification of playas forartificial recharge practices. Thus, accurate understandingof recharge is an important priority from the perspective ofwetland function and habitat health, protecting ground-water quality, and the substantial costs associated with landand water management, conservation, and regulation.

Although numerous studies have investigated the roleof playas in recharging the High Plains aquifer, relativelyfew have directly measured water and chemical move-ment beneath playas and interplaya settings. Most studiesrely on indirect methods to estimate water and chemicalmovement beneath playas. Although results from thesestudies indicate that playas enhance recharge at rateshigher than rates in interplaya settings (Scanlon andGoldsmith 1997), the water fluxes beneath playas arehighly variable in both space and time. No studies to datehave systematically characterized all the factors that

Fig. 1 Approximately 92% of the more than 66,000 playas of the southern Great Plains are located on the High Plains aquifer (modifiedfrom M. McLachlan, Playa Lake Joint Venture, personal communication, 2008)

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control spatial or temporal variability of water andchemical movement within and beneath playas. A moredetailed understanding of these controls is needed forbest management of the groundwater resources of theHigh Plains aquifer and of the ecosystems and wetlandhabitat within each playa. The objective of this paper isto provide a critical review of the pertinent literature onthe controls of recharge rates and chemistry beneathplayas of the High Plains aquifer. This paper alsosummarizes the published infiltration and recharge ratesbeneath playas and in some interplaya settings of theHigh Plains aquifer.

Playas of the High PlainsPlayas are small and shallow closed-basin wetlands thathave no external drainage and commonly contain ephem-eral lakes. About 80% of playas of the High Plains havesurface areas that are smaller than 0.2 km2 and aregenerally less than 1 m deep (Pool 1977; Haukos andSmith 1992; Fish et al. 1998). Within the southern GreatPlains, playas are most abundant in the southern andcentral High Plains of eastern New Mexico, westernTexas, the panhandle of Oklahoma, southeastern Colorado,and southwestern Kansas (Smith 2003). An estimated18,679 playas are on the southern High Plains at afrequency of 4–5 per km2 (Fish et al. 1998; Quillin et al.

2005), 15,033 playas are on the central High Plains,and 27,671 playas are on the northern High Plains(Fig. 1). Playas also are scattered throughout parts of thecentral and northern High Plains in Nebraska andWyoming(Smith 2003), and an estimated 16,000 playas are insouthwestern Nebraska. Playas in Nebraska are found ingreatest density in the areas of the Southwest Playas,Rainwater Basin, Todd Valley, and Central Table(LaGrange 2005; Fig. 1). Smith (2003) described in detailplayas in the Great Plains.

The surface area that drains into playas of the southernHigh Plains is estimated to total 77,700 km2 (Ward andHuddlestone 1979), an area that is ∼90% of the southernHigh Plains (Nativ 1992). Playas are important storagefeatures during floods and for irrigation and livestock,provide habitat for a variety of wildlife species, and arelikely an important source of recharge to the High Plainsaquifer (Steiert and Meinzer 1995; Luo et al. 1997). Thecapacity to hold water enables playas to support a diverseflora and fauna (Bolen et al. 1989; Haukos 1991; Haukosand Smith 1993, 1994; Hoagland and Collins 1997). Anumber of plant species are found exclusively in playas(Reed 1930), and many species of birds use playas duringthe winter, for breeding, and as migratory stopoverhabitats (Curtis and Beierman 1980; Davis and Smith1998; Smith 2003). In general, playas hold water and formlakes for many weeks to months, and thus the land within

Fig. 2 a Locations of the northern, central, and southern High Plains subregions; and b Hydrogeologic units of the High Plains aquifer(modified from McMahon et al. 2007)

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and generally immediately adjacent to playas is notsuitable for crops. Playas may be suitable for pasturesduring the dry periods, however, when the lakes are dry.

Many theories have been proposed to describe thephysical, chemical, and biological development andformation of playas (Zartman and Fish 1992): forexample, animal activity (Rettman 1981), wind erosion(Gilbert 1895; Reeves 1966; Kuzila 1994), and dissolutionof soil carbonate and piping of sediment into the subsur-face (Wood and Osterkamp 1984a). Finley and Gustavson(1981) noted a linear array of many playas and suggestthat playa location may be controlled in part by underlyinggeologic structures. Most recent interpretations concludethat playas formed as the result of complex pedogenic,geomorphic, hydrochemical, climatic, and biologic pro-cesses (Goudie 1991; Goudie and Wells 1995; Gustavsonet al. 1995; Holliday et al. 1996; Hovorka 1997).

Numerous studies have characterized the geomorphol-ogy of playas (Curtis and Beierman 1980; Osterkamp andWood 1987; Wood and Osterkamp 1987; Zartman andFish 1992; Gustavson et al. 1995). From a spatial-analysisperspective, the most valuable characterizations to datehave been the digitization of 20,577 playas across thesouthern High Plains aquifer by Fish et al. (1998), and thedigitization of 66,000 playas across the southern GreatPlains by McLachlan (M. McLachlan, Playa Lake JointVenture, personal communication, 2008). These geo-graphic-information-system data sets include attributes ofphysical and morphological features such as playa area,perimeter, soil type, elevation, depth to playa floor, andlength of shoreline. Some National Wetland Inventorydata are also available from the U.S. Fish and WildlifeService, and these data are available digitally for the entirestate of Nebraska (Fig. 1).

The spatial distribution of playas in the southern HighPlains may not be completely random (Zartman et al.2003). Playas tend to be more clustered north of theCanadian River, at the eastern edge of the Llano Estacadoescarpment, and in the southwestern High Plains region(Quillin et al. 2005). Lotspeich et al. (1971) noted largerbut fewer playas in the northern half of the southern HighPlains, which has finer soil cover than in the southernarea.

Playas have three distinct physical features (Fig. 3)—the playa floor, which is the flat floor of the playa that ischaracteristically lined by hydric soils; the annulus, whichis the sloped surface at the playa margin; and theinterplaya region, which is the area between the annuliof different playas and includes the uplands that drain intoplayas (Hovorka 1995). Most playas in the southern HighPlains are located within the Blackwater Draw Formation,which consists of silty clay loam sediments (Holliday etal. 1996). The playa floor is characterized by 1–2 m ofhydric soils and Vertisol clays (typically Randall clay inthe southern High Plains and Lodgepole, Fillmore, Scott,and Massie soil series in the northern High Plains), whichswell when wet and shrink when dry to form cracks asmuch as 1 m deep (Hovorka 1997). Clay-rich lacustrinesediments occur as much as 10 m below the floor (Parry

and Reeves 1968; Claborn et al. 1985) and are sometimesinterbedded with sand units that reflect the migration ofhistorical sand across the playa (Hovorka 1995). Addi-tionally, numerous paleosols are common beneath playasand were formed under past climate conditions that weremore stable and ideal for soil development (Hovorka1995; Bauchert 1996). The water table of the High Plainsaquifer is usually many tens of meters below thepaleosols. The annulus is characterized by interbeddedclay and loam that reflect past changes in the size of playalakes (Hovorka 1995). The interplaya settings contain siltyclay loam-soil horizons and caliche layers that are usuallymany tens of meters thick below land surface (Hovorka1995). Caliche is a cement-like layer of depositedcalcium-carbonate material that forms as the result ofevaporative concentrations of calcium carbonate in porewaters of soils and sediments. Several hundred test holeswere drilled in the floors of many playas in 1937 and 1938and indicate caliche layers at various depths below manyplayas (White et al. 1946). However, many of thesecaliche layers contained sand and were relatively perme-able (Nativ 1992). Solution channels that are common inthe caliche may provide pathways for water movementbelow the playa floor (Lotspeich et al. 1971; Nativ 1992).More recent test holes drilled in playas indicate a relativeabsence of carbonates beneath playa floors and areinterpreted as evidence of recharge beneath playas(Scanlon et al. 1994, 1995; Hovorka 1997). Additionally,Netthisinghe (2008) observed slickenside developmentwithin the playa floor sediments that indicate water flowthrough deeper playa sediments.

Geographic scope of reviewThe High Plains aquifer underlies ∼450,000 km2 in partsof eight States (Colorado, Kansas, Nebraska, NewMexico, Oklahoma, South Dakota, Texas, and Wyoming;Fig. 2a). The aquifer includes six primary hydrogeologicunits (Fig. 2b), of which, the Ogallala Formation is thelargest (McMahon et al. 2007). In 2000, the aquifer had anestimated 3.7 trillion (1012) m3 of water in storage

Fig. 3 Principal physical features of playas include the playa floorand annulus. The interplaya region is the land surface thatsurrounds playas and includes upland setting that drain into playas

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(McGuire et al. 2003), thus making it one of the largestaquifers in the world.

Use of groundwater from the aquifer as a source ofirrigation water has transformed the High Plains into oneof the largest and most productive agricultural regions inthe United States, earning it the nickname “breadbasket ofthe world” (Opie 2000). Groundwater withdrawals fromthe High Plains aquifer account for ∼20% of totalgroundwater withdrawn in the United States, of which97% is for irrigation (Maupin and Barber 2005). In 1989,the economic value of the aquifer was estimated to be 20billion (109) dollars (Moody 1990); this value was basedlargely on the agricultural production that relies on waterfrom the aquifer. Although public and domestic usesaccount for a relatively small percentage of the totalgroundwater use, these two uses provide drinking waterfor ∼82% of the 2.3 million people who live within theHigh Plains (Maupin and Barber 2005).

The sustainability of the High Plains aquifer is inquestion for a number of communities that rely on thisaquifer as their principal source of water for irrigatedagriculture and for public and domestic drinking supplies(Dennehy et al. 2002). The agricultural productivity of theregion has come at the cost of declining water tables andnonpoint-source contamination. Since the 1940s, aquiferdevelopment has lowered the water table more than 45 min parts of the region (McGuire et al. 2003). Water tableshave declined substantially since predevelopment times(prior to extensive groundwater pumping in the 1950s)because groundwater withdrawals, largely for irrigatedagriculture, have greatly exceeded recharge throughoutmuch of the aquifer. This imbalance is particularly true inthe central and southern High Plains. Such groundwaterdepletion has increased pumping costs and reduced waterdischarge to streams, among other things. Additionally,many natural and agricultural contaminants have beendetected in groundwater of this aquifer, including nitrate(Gurdak and Qi 2006; Qi and Gurdak 2006; Gurdak et al.2007b; Gurdak 2008) and arsenic (Fahlquist 2003; Reedyet al. 2007) at concentrations that exceed current max-imum contaminant levels (MCLs) for drinking water thatare set by the US Environmental Protection Agency(2008). Therefore, the question of sustainability of theHigh Plains aquifer is a function of changes in thequantity as well as the quality of groundwater, which islargely a function of recharge rates and chemical transportto the aquifer (McMahon et al. 2007; Gurdak et al. 2009).

Critical review

This review first discusses important recharge processes tothe High Plains aquifer and relevant hydrologic processesof playas, and then systematically syntheses the findingsfrom more than 175 publications about the importantprocesses controlling recharge quantity and qualitybeneath playas to the High Plains aquifer. The reviewends with a conceptual model of the role of playas inrecharging the High Plains aquifer. Because the vast

majority of the prior publications describe playas of thesouthern High Plains, this paper focuses on rechargebeneath playas of the southern High Plains. Approxi-mately 40 larger saline lakes, which are sites of ground-water discharge, exist in the High Plains (Wood andOsterkamp 1987) but are not included in this paper. Amore detailed review of recharge rates and chemistrybeneath playas of the High Plains aquifer is available inGurdak and Roe (2009).

Overview of recharge quantity and quality to the HighPlains aquiferRecharge refers to the amount or flux of water that entersgroundwater. Water that infiltrates the land surface andmoves downward through the soil and unsaturated zonebecomes recharge only after the water intercepts the watertable. As used in this paper, recharge is the vertical andvolumetric flux of water across the water table of theaquifer. Upward discharge of water from underlyingformations is another source of recharge to the HighPlains aquifer (Nativ 1992; McMahon et al. 2007), but isnot discussed in detail in this paper. Rates of rechargeaffect groundwater availability and sustainability, and arecommonly expressed in units of length per time. Accurateknowledge of recharge is important for making informeddecisions about groundwater management.

In many aquifers of the western United States,including the High Plains aquifer, recharge rates varyunder different land uses and with time owing to changesin climate that occur seasonally or during longer periodsthat are controlled by natural factors and (or) by humanactivities (Gurdak et al. 2007a). Accurate measurements ofrecharge are very challenging to obtain in most aquifersbecause there are no easy and direct methods forobserving and measuring moving water that interceptsthe water table. In the High Plains aquifer, for example,the water table is commonly many tens of meters belowthe land surface (Gutentag et al. 1984). Furthermore, themethods used to directly estimate recharge commonlyrepresent the rate of recharge at a particular space and timeand, therefore, may not adequately represent recharge atanother location in the aquifer or under different con-ditions that may affect recharge with time.

As a result of the inherent challenges in direct methodsof recharge estimation, indirect methods generally havebeen used in the High Plains aquifer to estimate rechargeas described in this paper. Indirect methods commonly useinformation from other components of the water cyclesuch as precipitation, evapotranspiration, streamflow, andinfiltration, to infer information about recharge. Becauseindirect methods do not directly measure moving waterthat intercepts the water table, recharge estimates fromindirect methods are subject to uncertainty. The degree ofuncertainty associated with any recharge estimate, whetherfrom direct or indirect methods, depends upon theassumptions used by the investigator and the accuracyand precision of any measurements or calculations.Uncertainty associated with recharge estimates are

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unavoidable given the spatial scale of the High Plainsaquifer and the historical and future time scales on whichgroundwater in this aquifer is managed. Those uncertaintyestimates that are reported with recharge rates are valuableinformation for groundwater managers. As demonstratedin this review, most studies do not address uncertainty ofthe recharge estimates.

Previous studies of recharge to the High Plains aquiferindicate that the direction and rate of water movement inthe unsaturated zone, and in turn recharge, are likelycontrolled by differences in land use and land cover(Scanlon et al. 2005, 2007), irrigation-return flow (Scanlonet al. 2003), spatial patterns in climate (McMahon et al.2006), temporal patterns in climate (Gurdak et al. 2007a),geomorphological features such as playas (Wood andSanford 1995a, b; Scanlon and Goldsmith 1997; Fryar etal. 2001) and other topographic depressions (Gurdak et al.2008), vegetation (Walvoord and Scanlon 2004), and soils(Keese et al. 2005). These and other controlling factors resultin slow and fast paths for recharge to the High Plains aquifer(McMahon et al. 2006). Slow paths are characterized bydiffuse recharge that may occur after rainfall, melting snow,or irrigation-return flows that infiltrate across a uniform areaof the aquifer, that percolates relatively uniformly throughthe unsaturated zone, and that eventually intercepts the watertable. Slow paths commonly occur in fine-grained sedimentsor under flat terrain (Scanlon and Goldsmith 1997).Conversely, focused recharge may result from fast pathsunder depressions in the land surface (Gurdak et al. 2008)such as playas (Wood and Sanford 1995a, b; Scanlon andGoldsmith 1997; Fryar et al. 2000, 2001) or other types ofpreferential-flow processes through the unsaturated zone(Hendrickx and Flury 2001). Focused recharge is charac-terized by water that follows rapid pathways to the watertable; those pathways bypass a large portion of the arealextent of the soil and unsaturated zone.

Water movement in the unsaturated zone is fundamen-tally controlled by differences in potential energy, which iscalled the total water potential and is the sum of gravity, soil-matric potentials, and osmotic forces. The water in theunsaturated zone moves from areas having a higher totalpotential to areas having a lower total potential. The totalpotential varies with depth in an unsaturated zone and isgenerally controlled by local precipitation and evapotranspi-ration rates and by the hydraulic properties of unsaturated-zone materials. Previous measurements of the total-potentialgradient in the northern High Plains of Nebraska indicatethe potential for downward water movement within theunsaturated zone, with little seasonal change below the rootzone (McMahon et al. 2006). In contrast, rangeland ofthe southern High Plains in Texas has total-potentialgradients that increase substantially with depth (McMahonet al. 2006). This finding indicates the potential for upwardwater movement from the water table to the zone of plantroots, which is consistent with interplaya observations byScanlon and Goldsmith (1997). These findings indicatethat substantial recharge is not likely to occur in interplayasettings under the current climate of the southern HighPlains.

The total-potential gradients in the unsaturated zoneultimately affect water movement and recharge rates.Estimated downward water fluxes (or recharge rates)across the High Plains ranged from 0.2 to 111 mm/yr(McMahon et al. 2006). Irrigated agricultural sites hadlarger fluxes (17–111 mm/yr) than rangeland sites (0.2–70 mm/yr). The largest water fluxes were observed at sitesin the northern High Plains (70–111 mm/yr) followed bycentral High Plains (5–54 mm/yr) and southern HighPlains (0.2–32 mm/yr). This order is due in part to climatedifferences from north to south and lower evapotranspira-tion rates in the northern High Plains than in the southernHigh Plains. McMahon et al. (2006) suggested that thedownward water flux (0.2 mm/yr) estimated using thechloride mass balance approach at the southern HighPlains rangeland site represents past hydrologic conditionsbecause upward total-potential gradients were observed.

The southern High Plains aquifer, where the majorityof the playas are located, was incised by the Canadian,Pecos, and Red Rivers (Fig. 2a) and cut off from the morehumid central High Plains and its recharge sources (Seni1980; Gustavson 1986; Nativ 1992). Nativ (1992) notedthat the exact portion of annual precipitation thatrecharges the southern High Plains aquifer has beendebated since at least the 1930s; estimates differ by morethan two orders of magnitude (0–41 mm/yr) under playasand diffuse-recharge settings and beneath sand dunes.Studies of diffuse-recharge settings in the southern HighPlains indicate that the recharge from direct precipitationis minimal (Nativ 1992). Fine-grained soil and caliche andclimate conditions likely limit recharge in diffuse-rechargesettings of the southern High Plains (Broadhurst 1942;Barnes et al. 1949; Ries 1981; Knowles et al. 1984).

The relatively thick unsaturated-zone sediments of theHigh Plains aquifer contain pore-water with chloride (Cl–)and nitrate (NO3

–) concentrations from natural evapocon-centration during thousands of years of precipitation(Walvoord et al. 2003) and from anthropogenic nitrogen(N) primarily from agricultural fertilizers (McMahon et al.2003, 2006, 2008). Groundwater quality in the HighPlains aquifer is potentially vulnerable to contaminationfrom these natural and anthropogenic Cl– and NO3

–

reservoirs (Gurdak and Qi 2006). Groundwater may becontaminated if processes mobilize and transport the Cl–

and NO3– reservoirs to the water table; such processes

might be conversion of rangeland to irrigated and rain-fedcropland (McMahon et al. 2006) or natural climatevariability (Gurdak et al. 2007a). McMahon et al. (2006)suggested that the downward displacement of NO3

– insome unsaturated zones was the result of mobilization byirrigation-return flow after rangeland was converted toirrigated cropland.

The chemical travel times from land surface to thewater table are substantially different beneath fast andslow recharge paths; this fact has important implicationsfor groundwater quality. McMahon et al. (2006) suggestedthat, for water moving from land surface to the water tablein the High Plains aquifer (Fig. 4), NO3

– fast-path traveltimes are faster within irrigated cropland (months to

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decades) but slower under rangeland (years to centuries).NO3

– slow-path travel times are faster under irrigatedcropland (decades to centuries) but slower under range-land (millennia). Some playas likely represent fast pathsfor recharge and chemical transport, whereas others mayrepresent slow paths, as discussed below.

The High Plains aquifer is limited in its ability tonaturally attenuate contaminants such as NO3

– throughdenitrification, and it has, in general, slow rechargerates—both of which suggest that, once the aquifer iscontaminated, it will remain so for decades and evenmillennia (McMahon et al. 2007). The slow denitrificationrates would require between 250 to 14,000 years to lowerNO3

– concentrations by 1 mg/L (as N) in groundwater ofthe High Plains aquifer (McMahon et al. 2007). Addition-ally, because travel times through the unsaturated zone aregenerally long—decades to millennia along slow flowpaths—the amount of chemical mass reaching the aquiferwill most likely increase with time. These results highlightthe importance of managing land use in the High Plainsto minimize NO3

– concentrations in recharge. Addition-ally, changes in water quality with time may affectthe groundwater resource in the High Plains aquifer(McMahon et al. 2007). The quality of groundwatergenerally has been overlooked because the primary focushas been on obtaining a sufficient water supply, and it hasbeen broadly assumed that the High Plains aquifercontains high-quality water. For the most part, findings

from McMahon et al. (2007) supported that assumption.At some local scales, however, particularly where pump-ing is intense or where topographic settings are conduciveto flow paths that are relatively fast, water quality may bea limiting factor for intended uses such as drinking wateror irrigation water.

Hydrology of playas: an overviewThe hydrology of playas is characterized by cycles ofinundation and drying. Once inundated, the hydroperiodof playas is variable but may last for many weeks to manymonths, although some may remain dry for years (Smith2003; Melcher and Skagen 2005a). The characteristic wetand dry phases of playa hydrology are a function ofclimate and the relatively thin permeable soils of the playafloor. The thick lacustrine sediments beneath playas thathave accumulated during many thousands of yearsindicate that playas have periodically flooded throughouttheir geologic history (Holliday et al. 1996; Hovorka1997).

The surrounding land use and runoff has an importantcontrol on playa hydrology. Ekanayake et al. (2009)present a method to delineate playa watershed andestimate runoff. The land surface of the southern HighPlains generally slopes from the northwest to southeast.As a result, the principal drainage area for most playas isnorth and west; a smaller region of drainage lies to thesoutheast (Claborn et al. 1985). The drainage area formost playas includes irrigated and non-irrigated croplandand rangeland that may be used for livestock grazing. Inthe city of Lubbock, Texas (Fig. 2a), and other urbanenvironments, playas are important for storm drainage andrecreation (Hertel and Smith 1994; West 1998). The runoffinto urban playas is commonly allowed to evapotranspireor infiltrate (West 1998). Playas were frequently used inagricultural-irrigation systems for tailwater storage andreuse (Fish et al. 1998). Guthery and Bryant (1982)reported that the number of modified playas on thesouthern High Plains increased from approximately 150in 1965 to 10,800 playas in 1980. Modification of playasfor irrigation systems has direct effects on the biomass,floral and faunal communities, soil erosion, and runoff,and it alters nutrient and pesticide input to the playas(Bolen et al. 1989).

Findings from many early investigations of playasindicate that much of the water that enters playas is lost toevapotranspiration (Theis 1937), and as little as 10%of water entering a playa infiltrates the subsurface(Schwiesow 1965). Therefore, many early investigatorsconcluded that recharge to the High Plains aquifer ispredominantly beneath interplaya settings. More recentinvestigations indicate that a substantial portion of thewater in playas may infiltrate into the subsurface and mayultimately recharge the High Plains aquifer (Wood andOsterkamp 1984b; Wood and Sanford 1994; and Wood etal. 1997; Zartman 1987; Scanlon and Goldsmith 1997).For example, Wood and Osterkamp (1984b) estimated thatapproximately 80% of the water collected in a playa is

Fig. 4 Chemical travel times from land surface to water tableunder fast and slow flow paths in rangeland and irrigated cropland(modified from McMahon et al. 2006)

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recharged through the playa annulus. Furthermore, Allenet al. (1972) found no minerals in the playa-floorsediments indicative of mineral precipitation from evapo-ration of precipitation.

The large number of playas and their ability to holdlarge volumes of water in an otherwise arid to semiaridclimate have attracted the attention of many who havestudied the hydrology of the High Plains aquifer (Nativ1992). However, the hydrology of playas has been thecenter of conflicting hypotheses for much of the last 60 ormore years. In general, the conflicting hypotheses differ inthe relative amount of inundation water that is lost toevapotranspiration and the amount of water that infiltratesand becomes recharge to the High Plains aquifer. Somestudies from the early 1900s indicate that most playawater is lost to evaporation and little remains for recharge,whereas other studies, many from the late 1900s, indicatethat substantial volumes of water infiltrate playas andrecharge the High Plains aquifer. The following details ofplaya hydrology outline the observations and estimatesthat have been used by those supporting the variousconflicting hypotheses.

Much of the rain in the High Plains falls from springthrough fall, which coincides with the period of highestannual evapotranspiration (Dvoracek 1981; Traweek1981; Haukos and Smith 1992, 1996). The southern HighPlains has a mean annual precipitation of 330 to 610 mmand a mean annual potential evapotranspiration of 1,651–1,753 mm (Dugan and Zelt 2000); thus, most precipitationis lost to evapotranspiration. However, surface runoff(“run-on” in the case of closed-basin playas) collects inplayas during moderate to intense rainstorms.

Surrounding soil texture directly influences the size ofplaya lakes. Grubb and Parks (1968) qualitatively notedthat playas in finer textured soils are larger, have a moreextensive drainage network, and have larger volumes perunit surface area than those playas in medium- to coarse-textured soils. In addition, for precipitation events of equalduration and intensity, surface runoff occurs more often,earlier, and for longer duration near playas in clayey andfiner-grained soils than near those playas in more loamysoils (Gustavson et al. 1995). Surface-water runoff toplaya lakes carries eroded sediment; clay particles aresuspended and tend to settle out further toward the middleof the playa floor (Reeves 1990; Gustavson et al. 1995).Thus, Vertisol soil is most common on playa floors andhas characteristic vertical soil structure built duringnumerous episodes of expansion and contraction (wetand dry cycles).

The volume of water that collects in playas of thesouthern High Plains is estimated to range from 2.2 to 7.0billion m3/year (Clyma and Lotspeich 1966; Hauser 1966;Hauser and Lotspeich 1968; Brown et al. 1978). Zartmanand Fish (1989) suggested that the annual volume of waterin playas, if recharged, is equivalent to approximately102–292 mm/yr of recharge throughout the irrigatedportion of the southern High Plains aquifer. However,the annual volume of water that collects in playas andultimately recharges the High Plains aquifer is difficult to

quantify and is not known. The annual volume of water inplayas is equal to a substantial percentage of the volumeof water that has been removed from storage by pumping.In Texas alone, the total loss of water in storage in theHigh Plains aquifer in the interval from predevelopment tothe year 2000 is estimated at 153 billion m3 (McGuire etal. 2003). Dividing the total loss of storage in Texas by thenumber of years since predevelopment (2000 – 1957=43 years) equals 3.6 billion m3 of water per year that waslost from storage, which is within the estimated range ofannual water that collects in playas (2.2–7.0 billion m3).

Brown et al. (1978) noted that the volume of water thatcollects in playas depends upon the frequency andintensity of precipitation and on the characteristics of thedrainage area. Runoff rates are slower and runoffgenerally contains less suspended sediment in playas withgreater surrounding coverage of vegetation (Brown et al.1978). Playas in natural settings are commonly flooded for1–3 months/year (Gustavson et al. 1994). Playas in urbanareas that are modified to hold stormwater may be floodedthroughout the year. James (1998) estimated that thevolume of water in five urban playas in Lubbock, Texas,ranged from 93,500 to 325,640 m3 of water at full stage.

Once water collects in playas, the rate of water loss toevaporation is substantial during the summer and fallmonths (Traweek 1981; Haukos and Smith 1992) and maybe as high as 13 mm/day (Brown et al. 1978). Otherestimates indicate as much as 55–60% of the availablewater in playas is lost to evaporation (Reddell 1965; Wardand Huddlestone 1979). However, Nativ (1992) andHarris et al. (1972) noted the lack of evaporite mineralswithin the playa-floor sediments and a lack of halophytic(salt loving) flora indicates that the playas do notaccumulate salts as a result of evaporation. Additionally,playa water generally has low salinity (Wells et al. 1970;Felty et al. 1972; Lehman 1972).

Water that is not lost to evapotranspiration may leave theplaya as infiltration into the subsurface. Infiltration in playashas been reported to follow three distinct stages (I–III;Table 1). In playas of natural setting that experience seasonalwet and dry periods, the infiltration rates during the stage Iare relatively high while the soil is dry. Claborn et al. (1985)noted that numerous researchers and farmers have observeda rapid decline in water levels of playa lakes immediatelyfollowing a large runoff event, followed by a much slowerdecline of water level as playa lakes become shallower. Therapid declines in water levels of playa lakes are hypothesizedto be due to rapid infiltration through cracks in the clay-linedfloors or to rapid infiltration through the playa annulus(Claborn et al. 1985). The amount of water in the soilcontrols the rate of infiltration during stage II infiltration. Asthe soil becomes wetter, infiltration rates slow during stageII. Stage III of infiltration occurs if the soil becomessaturated. In stage III, the infiltration rate is constant anddetermined by hydrologic properties of the soil andunsaturated zone. Playas in urban settings that are modifiedto hold storm-water drainage year round are likely to haveconstant infiltration rates that indicate stage III infiltrationprocesses.

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Table 1 Infiltration estimates beneath playas of the southern High Plains. (Infiltration rates compiled from studies that have directlymeasured infiltration or used water budgets from playas in the southern High Plains)

Study Infiltration rate Approach Setting Notes

Lehman and Clark (1975) — 1 playa studied1.00 mm/h (day 0) Constant head permeameter Playa floor Randall clay; feedyard runoff0.05 mm/h (day 1)0.02 mm/h (day 10)40.0 mm/h (day 0) Constant head permeameter Interplaya Permeable buried soil; feedyard runoff1.00 mm/h (day 8)0.05 mm/h (day 45)

Evans (1990) — 3 playas studied10.0 mm/min (minimum) Double-ring infiltrometer Playa floor Stage I infiltration46.0 mm/min (average)660 mm/min (maximum)22.0 mm/min (minimum) Double-ring infiltrometer Playa floor Stage I infiltration79.0 mm/min (average)1,041 mm/min (maximum)12.0 mm/min (minimum) Double-ring infiltrometer Playa floor Stage I infiltration163 mm/min (average)2,490 mm/min (maximum)0.0 mm/min (minimum) Double-ring infiltrometer Playa floor Stage III infiltration33.0 mm/min (average)38.0 mm/min (maximum)0.0 mm/min (minimum) Double-ring infiltrometer Playa floor Stage III infiltration12.0 mm/min (average)63.5 mm/min (maximum)0.0 mm/min (minimum) Double-ring infiltrometer Playa floor Stage III infiltration15.0 mm/min (average)38.0 mm/min (maximum)

Zartman et al. (1994a, 1996) — 1 playa studied2,946 mm/h (min. 1) 127–mm diameter cylinder infiltrometers Playa floor (center) (10 s fill time)610 mm/h (min. 5)1,524 mm/h (min. 1) 127–mm diameter cylinder infiltrometers Playa floor (outer basin) (10 s fill time)508 mm/h (min. 5)2,235 mm/h (min. 1) 127–mm diameter cylinder infiltrometers annulus (10 s fill time)559 mm/h (min. 5)269 mm/h (min. 1) 2,032–mm diameter basin infiltrometer Playa floor (center) (∼1 hr fill time)51.0 mm/h (min. 130)140 mm/h (min. 1) 2,032–mm diameter basin infiltrometer Playa floor (outerbasin) (∼1 hr fill time)25.0 mm/h (min. 130)81.0 mm/h (min. 1) 2,032–mm diameter basin infiltrometer Annulus (∼1 hr fill time)25.0 mm/h (min. 130)53.0 mm/h (min. 1) 8,890–mm diameter basin infiltrometer Playa floor (center) (∼1 hr fill time)9.40 mm/h (day 1)

Wood et al. (1997) — 2 playas studied1,140 mm/yr (minimum) Water budget Playa floor1,930 mm/yr (average)2,720 mm/yr (maximum)750 mm/yr (minimum) Water budget Playa floor1,185 mm/yr (average)1,620 mm/yr (maximum)

Parker et al. (2001) — 2 playas studied112 mm/h (min. 1) Flexible-wall permeameter Playa floor 1 (minimum) 15 samples0.10 mm/h (min. 5)0.10 mm/h (min. 60)276 mm/h (min. 1) Flexible-wall permeameter Playa floor 1 (average) 15 samples7.90 mm/h (min. 5)1.30 mm/h (min. 60)506 mm/h (min. 1) Flexible-wall permeameter Playa floor 1 (maximum) 15 samples25.0 mm/h (min. 5)3.30 mm/h (min. 60)193 mm/h (min. 1) Flexible-wall permeameter Playa floor 2 (minimum) 11 samples0.10 mm/h (min. 5)0.10 mm/h (min. 60)346 mm/h (min. 1) Flexible-wall permeameter Playa floor 2 (average) 11 samples30.5 mm/h (min. 5)2.30 mm/h (min. 60)502 mm/h (min. 1) Flexible-wall permeameter Playa floor 2 (maximum) 11 samples114 mm/h (min. 5)6.10 mm/h (min. 60)

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Caliche, which occurs widely throughout the southernHigh Plains, may act as a second barrier to water flow andchemical transport beneath the playa floor sediments(Knowles et al. 1984). However, Stone (1984) and Woodand Osterkamp (1984a) observed substantially less calicheand dissolved solids and salts in pore water of soilsamples beneath playas than beneath interplaya areas ofthe southern High Plains, which may indicate increasedflushing by percolating water (Nativ 1992) or thatdissolved solids and salts in pore water never accumulatedbeneath playas.

Recharge beneath playasFor the purpose of synthesizing the existing knowledge ofrecharge beneath playas, the following section has beensubdivided on the basis of four general types of studies toestimate rates of infiltration (Table 1) and recharge(Table 2) beneath playas of the southern High Plains.The four study types are water-budget studies, infiltrationstudies, unsaturated-zone studies, and groundwaterstudies. Results from each study type provide informationon a particular component of water movement through aplaya. Additionally, results from each study type mayrepresent different spatial and temporal scales of watermovement, provide a range of values, and have inherentuncertainty that is associated with each recharge-estimationmethod (Scanlon et al. 2003). Therefore, the most reliablerecharge estimates come from those studies that use manydifferent approaches in an effort to help reduce the inherentuncertainties in estimating recharge. Such considerationsare necessary when applying recharge estimates in scien-tific studies or management decisions. The followingsection ends with a discussion of the efforts to artificiallyincrease recharge and the effects of sedimentation and ofclimate change and variability on playa hydrology andrecharge.

Water-budget studiesWater budgets provide indirect (or residual) estimates ofinfiltration and recharge beneath playas (Reed 1994;James 1998; West 1998; Ganesan 2010). Studies that usewater budgets do not directly measure infiltration orrecharge beneath playas. The fundamental assumption ofa water-budget analysis is that the water entering a playaequals the water leaving a playa during the period ofinundation. In the case of playas, the runoff is assumed toleave the playa by means of either evapotranspiration orinfiltration (West 1998). Therefore, if the total volume ofwater runoff to a playa and the total volume of water thatleaves the playa because of evapotranspiration are known,then the residual value is assumed to equal the volume ofwater that infiltrates beneath the playa. Water-budgetstudies rarely, if ever, use more direct methods to estimateinfiltration or recharge for the purpose of evaluating theaccuracy and reliability of the water-budget estimates ofrecharge.

According to Reed (1994), water-budget analysesindicated that substantial volumes of water infiltratebeneath playas and that infiltration rates substantiallyexceed evaporation rates from playas. James (1998) usedwater budgets to estimate infiltration rates of playas inurban settings and reported that infiltration was substantialand controlled by several factors, including the year-roundsupply of water in urban playas near Lubbock, Texas.West (1998) estimated average-infiltration rates beneathurban playas that hold water year-round to range from 1.5to 14 mm/day. Similar infiltration rates were reported byJames (1998) for five urban playas; those rates rangedfrom 3 to 43 mm/day. The estimated volume of dailyinfiltration beneath the five urban playas ranged from 118to 869 m3/day. Interestingly, the water table beneathLubbock, Texas, rose substantially during the 1980s and1990s, while much of the southern High Plains aquiferexperienced substantial water-table declines (Rainwaterand Thompson 1994; McGuire et al. 2003). Kier et al.(1984) indicated that the rising water table beneathLubbock may have been caused by recharge from theapproximately 100 urban playas in Lubbock (West 1998)and the reduction in groundwater use within the city.

Awater-budget study of 22 playas in the southern HighPlains found that 30–50% of runoff into playas may beavailable to infiltrate through the playa annulus andmay ultimately become recharge (Claborn et al. 1985).However, Claborn et al. (1985) used indirect methods toestimate the volume of water above the clay-lined floorand did not collect data that could be used to verify eitherthe actual volume of water in the playa or whether wateractually infiltrated into the annulus. In a similar water-budget approach, Wood et al. (1997) incorporated arainfall-runoff process to estimate playa floor infiltrationrates of 750–2,720 mm/yr (Table 1) beneath playaslocated in the US Department of Energy’s Pantex Plantnear Amarillo, Texas. Interestingly, Wood et al. (1997)used indirect methods to estimate that of the totalinfiltration beneath the playa floor, macropore flux was25–100 times greater than interstitial (matrix) flux, but didnot collect data from within the unsaturated zone thatdirectly supported the estimated ratio of macropore tointerstitial water flux.

Although the water-budget method has many advan-tages, including ease and flexibility of use, a number ofsubstantial limitations reduce the accuracy and reliabilityof the recharge estimated by this method. The accuracy ofthe recharge estimate depends upon how accurately othercomponents in the water-budget equation are measured,particularly when the magnitude of the recharge rate issmall relative to that of the other variables (Scanlon et al.2003). To illustrate this point, Scanlon et al. (2003) notedthat errors of 5–10% in various terms of the water-budgetequation may result in errors in the recharge estimate ofmore than 100%.

Many types of unavoidable measurement errors areintroduced during water-budget calculations that may leadto uncertainty in the estimates of infiltration or rechargebeneath playas. For example, investigators commonly

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Table 2 Recharge estimates for the southern High Plains. (Recharge estimates compiled from water-budget, unsaturated-zone, and grou-ndwater studies in the southern High Plains. CMB chloride-mass balance; GW groundwater; UZ unsaturated zone)

Study type and publication Recharge (mm/yr)

Approach Setting

Water budgetJohnson (1901) 76–102 Observation RegionalGould (1906) 152 Observation RegionalTheis (1937) 3.2–17.0 Darcy′s law RegionalWhite et al. (1946) 1.5 Water budget RegionalBarnes et al. (1949) 2.5 Water budget Interplaya: nonspecificCronin (1961) 13 Darcy′s law RegionalHavens (1966) 20.6 Water budget RegionalRayner et al. (1973) 4.4 Water budget RegionalLansford et al. (1974) 0.40 GW modeling RegionalBrutsaert et al. (1975) 10 Water budget RegionalMorton (1980) 5.1–55.9 GW modeling RegionalTexas Department of Water Resources(1981)

12.7–25.4 GW modeling Regional

Bureau of Reclamation (1982) 23 Water budget RegionalBureau of Reclamation (1982) 25 Water budget Playa floorWood and Osterkamp (1984a) 2.5 Literature RegionalWood and Osterkamp (1984b) 40.0 Literature Playa annulusWood and Osterkamp (1984b) 60.0 From Luckey et al.

(1986)Playa annulus

Luckey et al. (1986) 2.5–25.4 GW modeling RegionalWood and Osterkamp (1987) 50.0 From Luckey et al.

(1986)Regional

Wood and Osterkamp (1987) 40.0 From Luckey et al.(1986)

Playa annulus

Nativ and Riggio (1989) 0.25–41 Water budget Playa floorDugan et al. (1994) 13.0–38.0 Water budget RegionalMullican et al. (1997) 10.2 GW modeling Interplaya: nonspecificLuckey and Becker (1999) 1.6–2.1 GW modeling Interplaya: nonspecificLuckey and Becker (1999) 16.0–24.0 GW modeling Interplaya: rangelandDugan and Zelt (2000) 2.5–38.1 Percentage of

irrigationInterplaya: irrigated cropland

Dutton et al. (2000) 3.6–42.7 GW modeling RegionalStovall et al. (2000) 15.2–139.7 GW modeling RegionalStovall et al. (2000) 25–50 GW modeling Interplaya: nonirrigated croplandUnsaturated zoneKlemt (1981) 4.8 Neutron probe RegionalKlemt (1981) 2.8–5.1 Neutron probe Interplaya: nonirrigated croplandKnowles et al. (1984) 1.5–14.5 Neutron probe Interplaya: nonspecificKnowles et al. (1984) 20.0 Neutron probe Interplaya—rangelandStone (1984) 0.2 CMB Interplaya: irrigated cropland.Stone (1984) 0.2–1.3 CMB Interplaya: rangelandStone (1984) 2.8 CMB Playa floorStone and McGurk (1985) 0.75 CMB Interplaya: rangelandStone and McGurk (1985) 12.2 CMB Playa floorWood and Sanford (1995a) 77 (±8) UZ tritium Playa annulusWood et al. (1997) 77 (±8) UZ tritium Playa floorScanlon and Goldsmith (1997) 0.1–4.0 CMB Interplaya: nonspecificScanlon and Goldsmith (1997) 6.0–10 CMB Playa floor (neglects runoff to playas)Scanlon and Goldsmith (1997) 60.0–100 CMB Playa floor (includes runoff to playas)Scanlon and Goldsmith (1997) 120 UZ tritium Playa floorWood et al. (1997) 27–31 UZ tritium Playa floorMcMahon et al. (2006) 17–32 UZ tritium Interplaya: irrigated croplandMcMahon et al. (2006) 0.2 CMB Interplaya: rangelandScanlon et al. (2007) 4.8–92 CMB Interplaya: nonirrigated croplandGroundwaterBrown and Signor (1973) 0.6–2.0 GW budget RegionalNativ and Smith (1987), Nativ (1988) 13.0–82.0 Tritium Playa floorMullican et al. (1994) 6 GW modeling Interplaya: nonspecificMullican et al. (1994) 99–219 GW modeling Playa floorWood and Sanford (1994) 9 CMB Playa annulusWood and Sanford (1995a) 11 (±2) CMB RegionalScanlon and Goldsmith (1997) 200–600 GW chemistry Playa floorWood et al. (1997) 22–44 CMB Playa floorWood et al. (1997) 145–257 CMB Playa floor (macropore pathway; macropore pathway and regional

pathways)Gurdak et al. (2007a) 198–200 Spectral analysis Regional: fast pathway, or macropore

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record the water-surface elevation in playas to estimate thevolume of water in a playa with time (West 1998). Thisapproach requires an accurate determination of the playa-floor and annulus geometry to accurately use water-surface elevation to determine the volume of water inthe playa. Surveying methods that are used to determinethe geometry usually introduce unavoidable errors thatmay lead to uncertainty in infiltration or recharge rates.Furthermore, studies that use a water-budget methodrarely collect data about subsurface water movement.These data could be used to determine if, in fact, waterthat infiltrates below playas actually intercepts the watertable as recharge. Evapotranspiration has the potential tocause subsurface water to move from depth toward theland surface, and such movement has been well docu-mented in interplaya regions of the southern High Plainsaquifer (Scanlon and Goldsmith 1997; McMahon et al.2006; Gurdak et al. 2007a). However, studies that useonly water-budget methods can not determine the poten-tial for such lateral or upward water movement. Forexample, a recent geophysical study by Netthisinghe(2008) indicated that lateral water flow (or through flow)through the playa sediments may be an importantmechanism for recharge to the High Plains aquifer, butdid not collect the subsurface verification data that areneeded to adequately support such a source of recharge.

Infiltration studiesA number of studies have directly measured infiltrationrates in playas using field-based infiltrometers (Evans1990; Koenig 1990; Zartman et al. 1994a, b, 1996; Huda1996; Scanlon and Goldsmith 1997). As Zartman et al.(1994a, b, 1996) noted, recharge beneath playas dependsupon infiltration within the playa. However, recharge ratesare not typically equivalent to infiltration rates for anumber of reasons, which are discussed below.

Reported infiltration rates range from 0 to 2,946 mm/hin playas and from 0.05 to 40 mm/h in interplaya settings(Table 1). Infiltration rates are generally reported to behigher near the playa center than in the perimeter of theplaya floor or in the annulus (Zartman et al. 1996;Table 1). High rates of infiltration in the playa center areattributed to preferential flow along desiccation cracks inthe clay floor (Zartman et al. 1996).

Zartman et al. (1994a, b) first observed that infiltrationbeneath a single playa was significantly and positivelyrelated to clay content of the floor. This apparent contra-diction to conventional wisdom (that is, high clay contentmeans low infiltration), appears to be caused by rapidwater movement down desiccation cracks in the clay floor.Immediately following ponding, large amounts of watercan apparently infiltrate though the desiccation cracks inthe playa floors. Measured infiltration rates are generallygreater during the initial flooding stage and tend tostabilize after the underlying sediments reach saturation(Evans 1990; Zartman et al. 1994a, b, 1996; James 1998).For example, Parker et al. (2001) reported the followingaverage infiltration rate for two playas at various times: 1-

min infiltration rates of 276 and 346 mm/h; 5-mininfiltration rates of 7.9 and 30.5 mm/h; and 60-mininfiltration rates of 1.3 and 2.3 mm/h (Table 1). Rapidinitial infiltration rates decrease as ponding water causesclays to swell and, thus, seal desiccation cracks and closepreferential flow paths (Zartman et al. 1994a, b).

Although stage I infiltration rates are high in playa soilsbecause of flow along desiccation cracks, infiltration ratestypically slow as the desiccation cracks seal and reachrelatively low stage III infiltration rates that are based onthe saturated-hydraulic conductivity of the soil (Evans1990). Scanlon and Goldsmith (1997) suggested that evenwhen playa floor sediments are fully saturated, they arenot completely impermeable, and cite a saturated-hydraulic conductivity of 7.2×10–4 mm/h from a playafloor as evidence. Such a saturated-hydraulic conductivityis approximately equivalent to a ∼6 mm/yr water fluxthrough the playa floor (Scanlon and Goldsmith 1997).Parker et al. (2001) reported a similar range of saturated-hydraulic conductivities for Randall clay soils from floorsof two playas in natural settings. These saturated-hydraulic conductivities (4.3×10–4–1.9×10–1 mm/h) areequivalent to water fluxes through the playa floor of ∼4and 1,682 mm/yr, respectively. Because playas areinundated for only a fraction of the year, these waterfluxes likely overestimate the actual annual water fluxesand need to be divided by the period of inundation for amore accurate annual water-flux estimate.

Interestingly, Scanlon and Goldsmith (1997) concludedthat recharge is strongly related to the volume of pondingin a playa and depth of infiltration. The volume of pondedsurface water is directly related to the physical character-istics of the individual playa, drainage pattern of theinterplaya setting, and climate patterns. As evidence oftheir findings, Scanlon and Goldsmith (1997) reported thatpreferential flow was greater beneath ponding locations inplaya floors where surface sediments were initially drierand cracks were more evident than in other locations suchas the playa center, that were more frequently flooded.Preferential-flow paths in playa floors may includedesiccation cracks, interpedal pores, root tubules, andother types of macropores (Scanlon and Goldsmith 1997).

Unsaturated-zone studiesThe relatively thick unsaturated zones of the High Plainsaquifer are ideal for application of unsaturated-zonetechniques for estimating recharge, which are commonlyused in semiarid and arid regions (Scanlon et al. 2003).These studies typically use physical and chemical-tracertechniques and sometimes numerical models to estimaterecharge. Recharge estimates from these techniques gen-erally represent a small spatial scale. Physical techniquesusually include the use of water or matric potentialsensors installed in the unsaturated zone to estimate thetotal potential gradient (Scanlon and Goldsmith 1997).Chemical-tracer techniques typically include the use ofapplied tracers such as bromide and nontoxic and visibledyes (Scanlon and Goldsmith 1997), and historical or

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environmental tracers (such as tritium, 3H) that result fromhuman activities or natural evapoconcentration of saltsfrom precipitation (chloride, Cl–, and nitrate, NO3

–;Scanlon and Goldsmith 1997; McMahon et al. 2006).Although numerical models have been used extensively inother semiarid and arid regions to estimate recharge, theuse of numerical models may not be used as commonlybecause of complications posed by shrink-and-swellprocesses that are typical in the clay-lined floors of playas.Most techniques used during unsaturated-zone studiesprovide estimates of water fluxes through the unsaturatedzone and do not directly measure recharge. Therefore,researchers commonly assume that water fluxes in theunsaturated zone (estimated below the depth influenced byevapotranspiration) represent actual recharge rates.

Unsaturated-zone studies report recharge rates thatrange from 2.8 to 120 mm/yr in playa floors and rechargerates that range from 0.1 to 92 mm/yr in interplayasettings (Table 2). The major findings of these studies arein general agreement that recharge rates are higher beneathplayas than beneath interplaya settings of the southernHigh Plains. Scanlon and Goldsmith (1997), who con-ducted one of the most comprehensive unsaturated-zonestudies of playas to date, concluded that playas increaserecharge because of the observed results that watercontents, water potentials, and tritium concentrations weremuch higher and chloride concentrations were much lowerbeneath playas than beneath interplaya settings.

The findings reported by unsaturated-zone studies arein less agreement about which processes are mostimportant in controlling recharge beneath playas. Woodand Osterkamp (1984a, b; 1987) suggested that the playaannulus acts as the primary recharge zone during periodsof ponding. Furthermore, they suggest that organicmaterial in the playa is oxidized to CO2, which dissolvesin water and forms carbonic acid. The carbonic acid maypromote dissolution of the underlying caliche, formationof solution channels, and increased subsurface porosity.However, the comprehensive data sets of Scanlon andGoldsmith (1997) generally support the conceptual modelthat infiltration occurs through playa floors and is notnecessarily restricted to the annular regions around playas.

Many studies in the southern High Plains (Wood andOsterkamp 1984a, b, 1987; Claborn et al. 1985; Osterkampand Wood 1987) concluded that recharge is relatively higherthrough the annulus than through the playa floor; thesestudies cited coarser sediments in the annulus as evidence.Scanlon and Goldsmith (1997), however, reported onlyslightly coarser sediments in the near-surface sediments ofthe annulus as compared with sediments in the correspond-ing zones beneath the playa floor. Scanlon and Goldsmith(1997) used total-potential profiles to suggest that waterdrains more consistently under playa floors than beneathplaya annuli. The total-potential profiles beneath some playaannuli indicate higher water fluxes than beneath correspond-ing playa floors, whereas other annular potential profilesindicate lower water fluxes than beneath playa floors(Scanlon and Goldsmith 1997). Lower reported chlorideand carbonate concentrations in sediments beneath playas

than in sediments beneath interplaya settings may beevidence of high water fluxes; they may indicate thatchloride and carbonate never accumulated or that they wereflushed or dissolved out of the playa profiles (Scanlon andGoldsmith 1997).

The role of interstitial (matrix) versus macropore(preferential) flux beneath the playa floor has been debated(Wood 1999; Scanlon 1999). Scanlon and Goldsmith(1997) reported qualitative evidence of preferential (fastpath) flow beneath some playa floors; however, thepreferential flow is apparently terminated at underlyinglayers of coarser sand in the relatively small number ofplayas that were studied. Therefore, the interbedded layersof sediment of different origins and hydrologic propertiesthat are common in the unsaturated zone beneath someplayas (Gustavson 1996; Hovorka 1997) may impederapid or preferential flow toward the water table. Assum-ing that most recharge occurs because of preferential flowthrough these macropores or desiccation cracks, Wood etal. (1997) estimated that recharge beneath playas could beas high as 145–257 mm/yr (Table 2). These rates areconsistent with the recharge rates of 198 to 200 mm/yr(Table 2) reported by Gurdak et al. (2007a) that areinterpreted as temporal response to climate variability andindicative of fast path or macropore flow processes suchas beneath playas.

Findings from most geologic studies of sediments beneathplayas generally support the conclusion that recharge ratesbeneath playas are greater than rates beneath interplayasettings (Holliday et al. 1996; Hovorka 1997). Hovorka(1997) reported no evidence of increased salinity or perma-nent ponding beneath selected playas and offered theinterpretation that typical playas have been recharging theunderlying aquifer throughout their geologic history. Hovorka(1997) concluded that recharge has always drained playasbefore evaporation concentrated solutes, and neither carbo-nate nor more soluble salts have accumulated in typical playasediments. Thus, playa water remains relatively freshcompared with water in the approximately 40 saline lakes inthe High Plains (Sanford and Wood 1995), because infiltra-tion and possibly recharge to the aquifer exceeds evaporation.

Other evidence of frequent ponding and rapid waterflux are the low chloride concentrations and lack ofcalcium carbonate (caliche) profiles in the unsaturatedzone beneath playas. Maximum chloride concentrations ininterplaya-soil water exceed those in soil water beneathplayas by as much as three orders of magnitude (Woodand Sanford 1995a; Scanlon and Goldsmith 1997). Lowchloride concentrations in sediments suggest that chloridenever accumulated or that it was flushed out by rapidwater movement. In contrast, several thousand years ofchloride accumulation are required to create the chlorideconcentrations found in sediment of interplaya profiles(Scanlon and Goldsmith 1997). Gustavson et al. (1995)observed that all major interplaya-soil series that havedeveloped on the southern High Plains are calcic soils,which contain substantial secondary accumulations ofcalcium carbonate, primarily from evaporation and evap-otranspiration. Low concentrations of calcium carbonate

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in playa sediments are caused by surface-water ponding;dissolution of calcium carbonate is facilitated by acidicprecipitation, physical flushing by rapid water flux, and bylimited plant growth in playas that minimizes depositionof calcium carbonate normally facilitated by evapotrans-piration of soil water.

The associated uncertainty from recharge rates that areestimated from unsaturated-zone studies may be substan-tial. Scanlon et al. (2003) suggested that estimates ofwater flux beneath playas that are based on chloride dataare highly uncertain because of relatively large uncertain-ties in the chloride input to the system (Scanlon andGoldsmith 1997). Uncertainties in the deposition valuesare reported as a factor of –0.5 to 2, which would result inuncertainties in water fluxes of ∼3–20 mm/yr (Scanlonand Goldsmith 1997). Wood and Sanford (1995a, b)provide a recharge estimate (77 mm/yr) with an errorestimate (8 mm/yr; Table 2). Wood et al. (1997) also notethe large uncertainty and spatial heterogeneity that existsin estimating the average chloride concentration in theunsaturated zone, which is used in the chloride-massbalance approach to estimate recharge. Climate change orvariability that occurs on relatively shorter time scalesthan the residence time of water in the unsaturated zonemay also lead to errors in unsaturated-zone-based rechargeestimates (Wood et al. 1997). Wood et al. (1997)addressed uncertainty of recharge estimates by includingthe standard deviation of input parameters to the rechargeequations. They estimated that of the total regionalaverage annual recharge to the southern High Plains,macropore recharge flux beneath playas may account for60–80%, interstitial (non-macropore) recharge fluxbeneath playas may account for 15–35%, and interstitialrecharge flux beneath interplaya settings may account for5% (Wood et al. 1997).

Groundwater studiesThe groundwater studies of playa recharge have generallyused tracer-based techniques that include dating ofgroundwater age (tritium, 3H) and environmental tracers(chloride, Cl–). Recharge estimates from groundwaterstudies represent recharge across much larger spatialscales than recharge estimates from unsaturated-zonestudies (Scanlon et al. 2002). Therefore, recharge esti-mates based on groundwater studies in areas of playasmay be more appropriate for groundwater-resource inves-tigations, because groundwater studies provide a morespatially averaged recharge rate than the point estimatesobtained from unsaturated-zone studies (Scanlon et al.2003). However, the spatially averaged recharge ratesfrom groundwater studies may not provide the spatialresolution to determine effects from any single playa orgroup of playas.

Early groundwater based recharge estimates, includingthat of Brown and Signor (1973), reported that less than2 mm/yr (Table 2) are added to storage to the High Plainsaquifer as recharge by infiltration from natural rainfall,whereas more than an average of 305 mm/yr of water is

being removed from storage because of pumping. Using3H in groundwater as a tracer, Nativ and Smith (1987)estimated recharge beneath playas to range from 13 to82 mm/yr. On the basis of a comparison of estimatedrecharge rates in diffuse settings (0.2–14.5 mm/yr; Barneset al. 1949; Klemt 1981; Knowles et al. 1984; Stone andMcGurk 1985; Stone 1990), Nativ and Smith (1987)suggested that the High Plains aquifer is predominantlyrecharged by focused percolation from playa lakes. Woodand Sanford (1995b), using a chloride-mass balanceapproach from groundwater (Table 2), provided a regionalestimate of 11 mm/yr recharge to the northern part of thesouthern High Plains. Similarly, Fryar et al. (2001)reported solute and isotopic data from shallow monitoringwells near playas receiving wastewater discharge; thesedata indicate a sequence of episodic precipitation, evap-orative concentration of solutes, runoff, and infiltrationbeneath playas. The water in these wells also indicatedreturn flow from wastewater and irrigation.

Using results from a groundwater flow model, Mullicanet al. (1994) estimated that focused recharge beneath playascould be as high as 219 mm/yr (Table 2); however, thesemodel-based estimates were determined by assuming that allwater from a regional recharge rate of 6.0 mm/yr is focusedthrough playas. No data sets were collected to validate actualrecharge rates beneath the playas.

Artificial rechargeThe large storage volume of playas has prompted manyquestions regarding the ability of playas to supplementwater resources of the region (Aronovici et al. 1970;Aronovici and Schneider 1972; Palacios 1981), includingartificial recharge to the High Plains aquifer (Valiant1964). Artificial recharge refers to any manmade mod-ification of playas intended to increase flow of watertoward the water table of the aquifer. Artificial rechargehas been explored as an approach to stabilize or replenishgroundwater supplies from the High Plains aquifer(Schwiesow 1965).

In order to increase infiltration and recharge byreducing evaporation losses from playas, playa floorshave been modified to confine water in smaller, deeperimpoundments with less surface area (Dvoracek 1981). Toincrease infiltration, surface drainage wells have beeninstalled that use gravity to allow playa water to flow tothe High Plains aquifer (Valiant 1964). These wells havebeen unsuccessful because the high silt content of theplaya water quickly clogs the wells and the sediments inthe aquifer (Claborn et al. 1985). Claborn et al. (1985)reported reasonable artificial-recharge rates using waterfrom playa lakes in a pressure injection system atpressures of 344,737–551,580 N/m2 (50–80 psi). Energycosts and logistical considerations, however, restrict use ofthis approach (Claborn et al. 1985).

An alternative to artificial recharge is the direct use ofplaya water for irrigation (Dvoracek 1981), whicheliminates many of the problems associated with artificialrecharge. Jones and Schneider (1972) suggested that the

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demand on the High Plains aquifer could be reduced by asmuch as 30% by direct pumping from playa lakes forirrigation supplies in combination with recycling irrigationtailwater and artificially recharging the aquifer using playawater. However, the direct use of playas for irrigation hasmany limitations. Soil moisture is generally sufficient foragricultural requirements in those seasons when playas fillwith water. Therefore, playa water needs to be stored untillater in the season when irrigation water is required.Dvoracek (1981) proposed various playa modificationschemes that may have various degrees of success inreducing water loss to evapotranspiration and increasingthe efficiency of playa-water storage. These modificationschemes have economic costs associated with installation,maintenance considerations because of sedimentation, andeffects on playa ecosystems. Additionally, unmodifiedplayas likely provide the best resource and habitat forwaterfowl and other species (Pence 1981).

One of the first systematic evaluations of the use ofplayas to support artificial recharge to the High Plainsaquifer was conducted by Brown et al. (1978). Thisevaluation used results from at least six field experimentsand numerous prior publications regarding the use ofplayas for artificial recharge (Hauser and Lotspeich 1968;Schneider et al. 1971; Aronovici et al. 1972; Brown andSignor 1973; Reeder 1975; Wood and Bassett 1975).Brown et al. (1978) suggested that under specificconditions, using water from playas in water-spreadingbasins or injection wells may be suitable for artificialrecharge of the High Plains aquifer. Artificial rechargefrom playa lakes is more likely to be successful if thewater is free of suspended sediment and recharged inzones of the aquifer that have high infiltration rates and noclay or low-permeability zones in the unsaturated zone.

Schneider and Jones (1984), who investigated infiltra-tion in playas that had been modified by excavation of thetop layer of soil, reported infiltration rates that weresubstantially greater than in unmodified playas. Infiltrationrates in these modified plays were initially high(1,000 mm/day), followed by slower rates (433 mm/day;Schneider and Jones 1984). Dvoracek and Peterson (1970)observed similar infiltration rates (469 mm/day) inmodified playas. However, suspended sediments in thewater column of the modified playas were identified as thecause of surface sealing and a reduction of infiltrationrates (Schneider and Jones 1984). Therefore, Schneiderand Jones (1984) concluded that in order to maintain highinfiltration rates, modified playas require periodic andcostly maintenance to remove or reduce surface seals.Urban and Claborn (1984) reported that geotextilematerials buried beneath playas have had some successat filtering sediments.

Although previous studies report greater infiltrationrates in modified playas than in unmodified playas, thefindings from such artificial recharge studies need to beconsidered in light of infiltration studies (see sectionInfiltration studies) that report substantial infiltration ratesin unmodified (or natural) playas. Therefore, any cost-benefit analysis of artificial recharge needs to evaluate the

potential added benefit that playa modification might haveon increasing infiltration and recharge above the naturalrates and the potential effects of playa modification on theecology of the playa-wetland system.

The quality of playa water used for artificial recharge isof concern because of its possible effects on groundwaterquality (Felty et al. 1972). For example, Mollhagen et al.(1993) observed detectable levels of triazine herbicidesand aldicarb insecticides in playa water. Water-qualityconcerns stem from legal constraints prohibiting water-quality degradation in existing aquifers in Texas. Thefuture use of playas for artificial recharge remainsuncertain. Manmade attempts at modifying playas for thepurposes of increasing recharge are uncertain because ofthe logistical and economic challenges, legal consider-ations that differ by State, potential Federal wetlandclassifications for playas, water-quality concerns for theHigh Plains aquifer, and the importance of playas ashabitat for various flora and fauna. There is a lack ofevidence in the literature that describes the possible effectsof playa modification for artificial recharge on chemicalmobilization and water quality of the High Plains aquifer.

SedimentationSmith (2003) noted that “…sedimentation is likely thesingle largest immediate threat to the continued existenceof properly functioning wetlands in the Great Plainstoday.” The accumulation of sediments from uplanderosion has shortened the hydroperiod, decreased watervolume, and increased water loss of playas due toevaporation (Tsai et al. 2007). These changes to thenatural hydrology of playas may reduce the diversity offlora and fauna habitat and increase flooding and propertyloss, and they may have effects on recharge to the HighPlains aquifer (Luo et al. 1997; Smith and Haukos 2002;Tsai et al. 2007).

Playas are typically surrounded by cultivated croplandand rangeland that may be used for grazing livestock.Although cultivated cropland and livestock grazing in aplaya drainage area may contribute to a reduction in thecover of perennial vegetation and increase the potential forsoil erosion and sediment transport to the playas, studiesindicate that the sediment load from cultivation-dominateddrainage areas is substantially larger than the load fromrangeland-dominated drainage areas (Luo et al. 1997; Tsaiet al. 2007). Playas in selected cropland settings arereported to contain 8.5 times as much sediment as thoseplayas in rangeland settings (Luo et al. 1997). Averagesedimentation rates (∼5–10 mm/yr) of playas in croplandsettings are substantially larger than average sedimenta-tion rates (∼0.7–0.8 mm/yr) reported for rangelandsettings (Luo et al. 1997). Luo et al. (1997) concludedthat if sedimentation rates remain approximately constant,sediment could fill nearly all cropland playas in less than100 years.

As a result, Federal, State, and privately fundedprograms focus on buffering playas to protect them fromsedimentation and contamination while simultaneously

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enhancing wildlife habitat (Melcher and Skagen 2005a, b).Efforts such as the Conservation Reserve Program, whichhas established more than 6,880 km2 of perennial grass inthe southern High Plains, have likely slowed sedimenta-tion rates (Luo et al. 1997). Conservation practices thatsupport native vegetation surrounding playas are likely totrap sediment and reduce sedimentation rates to playas(Luo et al. 1997). Given the large number of playas, theremoval of sediment can be restrictively expensive.Melcher and Skagen (2005a, b) summarized wetlandprotection strategies and best-management practices,including mitigation buffers that are most appropriate forreducing sedimentation and nonpoint-source contamina-tion in playas of the High Plains region.

Sedimentation of playas has adverse effects on wetlandstructure and function (Luo et al. 1997; Smith 2003).However, the effects of sedimentation on infiltration andrecharge are not clear because of a lack of supportingevidence from published scientific studies. Smith (2003)proposed several factors about sedimentation that likelyaffect infiltration and recharge beneath playas. Smith(2003) hypothesized that the coarser-grained sedimentthat typically erodes from interplaya settings may mixwith the clay soils of playas and fill desiccation cracks inthe playa floor. Hovorka (1997) identified and attributedsubsurface silt mixed with ancient lacustrine clay layers asevidence of silt deposition during dry periods within thegeologic history of the playa. However, no studies wereidentified during this literature search that provideevidence that coarser-grained materials are currently fill-ing desiccation cracks in clay-lined playa floors during dryperiods. Furthermore, Parker et al. (2001) found limitedevidence of more permeable material deposited in desic-cation cracks of playa-floor soils that would provideconduits for water flow even after the cracks had sealed asthe result of wetting. As Smith (2003) noted, it isunknown whether this process is occurring and increasinginfiltration, thus enhancing recharge.

Sedimentation results in shallower playas that have lesstotal volume and possibly larger surface areas, and that aremore likely to overflow and flood areas outside the playafloor and annulus (Smith 2003). Larger surface areas overshallower playas are subject to more rapid evaporationthan playas that have deeper annuli and smaller surfacearea (Smith 2003). Less water may be available forinfiltration if evaporation rates increase as playas fill withsediment.

Finally, sedimentation may result in a clay-lined playafloor completely covered with sediment from interplayasettings. It is unknown how such a surface layer will affectthe playa floor’s shrink-swell properties, which result indesiccation cracks that have been identified as importantcontrols on rapid infiltration and recharge. The sedimentalso may allow the underlying clay to maintain a highermoisture content, thus preventing the formation ofdesiccation cracks. Future studies are needed to determineif the desiccation cracks form near the surface under layersof sediment and how infiltration characteristics maychange in playa floors under sedimentation.

Climate change and variabilityAnthropogenic climate change and natural climate varia-bility are likely to have substantial effects on global waterresources, including those across the Great Plains (Inter-governmental Panel on Climate Change 2007). Matthews(2008) outlined possible effects of climate change andvariability on playa habitat and biodiversity. Climatechange and variability may have important effects oninfiltration and recharge beneath playas. Gurdak et al.(2007a) demonstrated that natural climate variability oninterannual to multidecadal timescales affects rechargerates (198–200 mm/yr, Table 2) and likely promotesrecharge along fast pathways or macropores such as inplayas. However, no other studies to date (2010), and tothe knowledge of the authors, have specifically exploredhow climate change and variability may alter rechargebeneath playas. Therefore, the following section brieflyoutlines a few climate projections noted by Matthews(2008) and possible responses of recharge beneath playas;these responses are based on the playa hydrology andrecharge processes outlined in this paper.

During the next 30–100 years, the Great Plains mayreceive less snowfall in winter, the snow will begin fallinglater and melt earlier, and more winter precipitation willbe rain rather than snow (Intergovernmental Panel onClimate Change 2007). Under such a climate scenario ofmore winter rainfall, winter recharge beneath playas mightincrease because of the relative lack of water loss due toevapotranspiration during winter as compared with sum-mer evapotranspiration loss. Research is needed to testthis hypothesis.

The Intergovernmental Panel on Climate Change(2007) reported that annual precipitation across parts ofthe Great Plains is likely to decrease; the largest decreasesare predicted in the southern High Plains region, espe-cially New Mexico and Texas. Under such climatescenarios of less annual precipitation, the potential forrecharge beneath playas may decrease because there isless water to run off and collect, infiltrate the playasediments, and ultimately recharge the aquifer. Research isneeded to test this hypothesis.

In contrast to the southern High Plains, the northernHigh Plains, especially part of Nebraska, may havesubstantial increases in precipitation during the summerduring the next 30–100 years (Intergovernmental Panel onClimate Change 2007). Precipitation across the GreatPlains region, however, is likely to continue to be highlyrandom with great local variation in amounts and intensity(Nippert et al. 2006), which could result in local droughtsand regional flooding (Covich et al. 1997). Underincreased summer precipitation in the northern HighPlains, recharge beneath playas may be increased. How-ever, higher rates and intensity of precipitation mayincrease erosion and sedimentation rates of playas(Matthews 2008). Sediment may bury the playa floorslined with shrink-swell clay, which have been previouslyidentified as important conduits for infiltration andrecharge. Therefore, recharge beneath playas, as well asthe wetland habitat, may be reduced under such climate

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projections. Research is needed to test these and otherhypotheses regarding the effects of climate change andvariability on the function of playa wetlands and rechargeto the High Plains aquifer. Gurdak et al. (2009) outlinesadditional research questions about the effects of climatechange and variability on groundwater resources of theHigh Plains aquifer and other regional aquifers of theUnited States.

Recharge chemistry beneath playasThe chemistry of recharge water beneath playas is a morerecent topic of study that has been motivated by concernsabout groundwater quality of the High Plains aquifer andthe detection of elevated concentrations of nitrate (NO3

–

as N), dissolved solids, pesticides, and other chemicals ingroundwater that may be harmful to humans and animals.The biodiversity of playas is also at risk from nonpoint-source contamination from soil erosion, agricultural run-off, and direct dumping of wastes into playas. Forexample, an estimated 1–2 billion m3/yr of irrigationtailwater, which is ∼20% of the irrigation water pumpedfrom the High Plains aquifer, flowed into playas duringthe 1960s and 1970s alone (Bolen et al. 1989). Possiblenonpoint-source contaminants in playa water may includenutrients, chemical fertilizers and pesticides, feedlot run-off, manure fertilizer, urban wastewater, organic chem-icals, and trace metals (Irwin et al. 1996). The increasedrecharge rates beneath playas could be of concern if therecharge chemistry is of poor quality. The followingsection synthesizes the movement and reactions ofchemicals from water in playa lakes and subsurfaceprocesses underlying the playa affecting recharge chem-istry to the High Plains aquifer.

Water quality of playa lakesMore than 25 studies have collected various types ofwater-quality data from playas of the southern High Plainsaquifer (Casula 1995). The objectives of most studies aresynoptic in nature and include data collection of playa-water quality at a particular time and place (Sublette andSublette 1967; Rekers et al. 1970; Bureau of Reclamation1982; Nelson et al. 1983; Buck 1989; Huang 1992). Theobjectives of other water-quality studies vary and includethe evaluation of playas for mosquito habitat (Ward 1964);the suitability of playas as a source of water for irrigation(Lotspeich et al. 1969); the effects of agricultural-waste-water runoff and land-use effects on playa-water quality(Felty et al. 1972; Mollhagen et al. 1993; Pezzolesi 1994;Irwin et al. 1996; Thurman et al. 2000; Purdy et al. 2001a,b; Hudak 2002); the suitability of playas as storagereservoirs (Reeves 1970); the potential effects on ground-water quality from artificial or natural recharge beneathplayas (Wells et al. 1970; Wood and Osterkamp 1984a, b;Ramsey et al. 1988, 1994); the effects on wetland habitat(Horne 1974; Parks 1975; Becerra-Muñoz 2007); and thepresence of waterborne-bacterial pathogens (Westerfield1996; Warren 1998; Hamilton 2002).

The water quality of playa lakes has been reported todiffer greatly in space and time because of physicalcharacteristics of the playa floor and annulus, soil andland-use characteristics of the interplaya settings, andvariability in the annual and interannual cycles ofprecipitation, evaporation, and infiltration that affecterosion and runoff chemistry (Curtis and Beierman 1980;Casula 1995; Hall et al. 1995; Willig et al. 1995; Fish etal. 1998). Runoff and material transported into playas isproportional to drainage area. Casula (1995) reported thatmany water-quality constituents show moderate positivecorrelations between playa drainage area; those constitu-ents include total dissolved solids (TDS), chloride, sulfate,alkalinity, pesticides, and pH. Lake area is reported toinversely correlate with TDS, specific conductivity,chloride, sulfate, pH, and many pesticides and maydirectly correlate with dilution of chemical constituents(Casula 1995). Casula (1995) did not obtain strongstatistical relations between playa characteristics and waterquality and attributes those findings to a lack of land-usevariables in the statistical models. Fish et al. (1998)suggested that temporal variability that is likely caused bychanges in climate may present a substantial challenge tounderstanding the effects of land use on spatial variabilityof playa-water quality.

Playa lakes commonly contain water with less than200 mg/L dissolved solids and 400–500 mg/L suspendedsolids (Wood and Osterkamp 1987; Zartman et al. 2001),which is characteristic of freshwater lakes and differentfrom the approximately 40 saline lakes present in theregion (Wood and Osterkamp 1987). The lack of salineplaya water, lack of salt accumulation in the playasediments, and presence of freshwater flora in playasindicates that evaporation that produces salts is not adominant process affecting water quality. Many research-ers suggest that if playa water is lost solely fromevaporation, salts and minerals would be concentrated inthe water and sediment and more halophytic flora wouldbe present (Smith 2003).

One of the more recent and spatially extensive surveys ofwater-quality conditions in 99 playa lakes throughout thesouthern High Plains reported elevated concentrations ofnitrate (1.64–4.23 mg/L as N) and arsenic (5.10–67.0 μg/L),and numerous pesticide compounds (Mollhagen et al. 1993).Although the range of nitrate concentrations exceeds thebackground concentration of 4 mg/L (as N) in groundwaterof the High Plains aquifer (Gurdak and Qi 2006), theseconcentrations do not exceed the MCL for drinking water(10 mg/L as N; US Environmental Protection Agency 2008).However, 59 playas from this survey contained arsenic atconcentrations that exceed the MCL for drinking water(10 μg/L; Mollhagen et al. 1993; US EnvironmentalProtection Agency 2008). Arsenic concentrations inplaya lakes sampled by Mollhagen et al. (1993) rangefrom 5.10 to 67.0 μg/L and have an average concentrationof 13.1 μg/L.

Playa water is not used for direct human consumption;even so, elevated arsenic in recharge water could pose ahealth concern. Fahlquist (2003) detected elevated arsenic

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concentrations in groundwater of domestic-supply wells atconcentrations ranging from 1.7 to 107 μg/L, and 14 (of48) samples located in the southern half of the southernHigh Plains exceeded the MCL. Arsenic concentrations ingroundwater of the High Plains aquifer have beensuggested to originate from organic-rich shale, volcanicash, discharge from saline lakes, or oilfield brines(Fahlquist 2003; Scanlon et al. 2009). Others suggest thathistorical use of arsenic-based pesticides and defoliantselevated the background concentrations of arsenic in thesoil surrounding the playa (Mollhagen et al. 1993).Furthermore, Thurman et al. (2000) reported the detectionof a number of the major cotton and corn herbicides andmany of their metabolites in playa water. However,Thurman et al. (2000) did not collect groundwater qualitydata to determine if the herbicides or metabolites havereached the groundwater beneath playas.

The southern High Plains is well known for its largeconfined-beef-cattle feeding operations (Parker et al.2001). An estimated 7 million cattle are fed in theseoperations (Southwest Public Service 1999). Approxi-mately one-half of the confined-animal feeding operationsin the High Plains use playas as collection basins forfeedyard runoff and as storage basins until solid manure canbe dredged for use as agricultural fertilizer (McReynolds1994). Playas in such operations are often modified toinclude primary storage ponds of sedimentation basinsbetween the feedyard and playa (Purdy et al. 2001a). Thesemodifications help to catch storm-water runoff that maycontain manure, sediment, and other chemicals.

The effects of confined-animal feeding operations onwater quality of the playas have not been extensivelystudied (Purdy et al. 2001a). Those studies of waterquality of playas in such operations reported elevatedconcentrations of nutrients, salts, and pathogens, andelevated biochemical oxygen demand (Sweeten 1994);those playas generally have lower quality water thannatural playas (Parker et al. 2001).

Purdy et al. (2001b) studied the effects of feedyards onendotoxin concentrations, fecal coliform count, and otherwater-quality conditions during winter and summer inplayas that are located in confined-animal feeding oper-ations. Although Purdy et al. (2001b) found that suchactivities reduce playa water quality (including endotoxinconcentrations and general water quality), and increasefecal coliform counts, they suggested that such deterio-rated playa-water quality likely does not pose a threat tohuman or animal health or the environment if the waterremains in the playas. The authors based their conclusions“on the premise that feedyard playas play a minor role inrecharging groundwater” (Purdy et al. 2001b). However,Purdy et al. (2001b) also stated that there is an urgent needto examine groundwater recharge from playas because it isunknown what role playas used in these feeding oper-ations play in recharging the perched aquifers and thedeeper High Plains aquifer. Purdy et al. (2001a, b)concluded that livestock should not be allowed to accessplayas that receive runoff from confined-animal feedingoperations and that water removed from playas in these

feeding operations may have serious effects on the healthof cattle and humans.

Although recharge is not well characterized beneathmost playas in such feeding operations, conditions infeedyard playas may help to minimize chemical mobi-lization. It has been hypothesized that animal wastes and acertain bacterium create elastic slime that may help to sealthe playa floors (Lehman and Clark 1975; Stewart et al.1994; Purdy et al. 2001a, b). Additionally, reports ofgroundwater quality near selected beef cattle feedyardsindicated no substantial effects on groundwater qualityfrom the feeding operations (Sweeten et al. 1995).

Subsurface processes affecting recharge chemistryPrevious studies provide evidence of direct relationsbetween the water quality of playa lakes, subsurfaceprocesses, and resulting chemistry of recharge to ground-water of the High Plains aquifer. The biogeochemicalprocesses in saturated or inundated playa sediments canhave substantial effects on the chemistry of recharge(Pezzolesi et al. 2000). The inundation of and biologicalactivity in playas affects dissolved oxygen in playa waters,which influences the movement of nutrients, trace metals,and organic chemicals and, in turn, the decomposition oforganic matter (Pezzolesi et al. 1995, 2000).

Because of the dominance of cotton productionthroughout the southern High Plains and the historicaluse of arsenic-based and organochlorine pesticides on thiscrop, greater concentrations of arsenic and other tracemetals have been hypothesized to occur in the soils ofplayas surrounded by cotton crops (Irwin et al. 1996;Venne et al. 2006). Although some studies have shownarsenic concentrations in playa sediments that are gen-erally 6–7 times as great as worldwide soil-backgroundlevels, results generally indicate no substantial differencesin trace-metal concentrations found in soils from playas incropland and rangeland settings (Irwin et al. 1996; Venneet al. 2006, 2008). Moreover, Venne et al. (2006) foundthat trace-metal concentrations in sediments were at least 5times as high as concentrations in amphibian tissue, whichindicates that bioaccumulation of metals did not occur.This study concluded that no apparent relation existsbetween land use (cropland and natural grassland), trace-metal concentrations in playa sediments, and trace-metalconcentrations in amphibians. Trace-metal concentrationsmay be ubiquitously distributed in playa sediments of thesouthern High Plains (Venne et al. 2006).

Evidence supports substantial differences in soil chem-istry between playas receiving wastewater from confinedanimal feeding operations and those playas in naturalsettings. Stewart et al. (1994) reported total soil N rangingfrom 3,000 to 4,000 mg/kg and soil phosphorus (P) to be2,000 mg/kg from playas receiving wastewater from beefand dairy lots. By comparison, soils of playas notreceiving feedlot wastewater had approximately 168 mg/kgtotal N and 28 mg/kg total P (Haukos and Smith 1996).However, other studies have shown that playa wetlands

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are effective at filtering nutrients (N and P) through biomassuptake (Pezzolesi et al. 1998).

Numerous studies have shown that NO3– is attenuated

in soils of playas receiving runoff from confined-animalfeeding operations and in treated sewage and industrialwaste (Fryar et al. 2000, 2001). At playas in the USDepartment of Energy’s Pantex Plant near Amarillo,Texas, Fryar et al. (2000) observed that ponded surfacewater impeded oxygen diffusion and caused anaerobicconditions in the near surface of playa-floor sediments,thus promoting denitrification of nitrate within playas.Additionally, chloride concentrations in sediments beneathplayas receiving feedyard wastewater have been observedto increase with time and depth (Clark et al. 1975).However, nitrate concentrations did not increase withdepth or time below playas receiving feedyard wastewater,which likely indicates that denitrification is removingnitrate (Clark et al. 1975). As a result, smaller concen-trations of nitrate are likely present in recharge beneathsome playas.

However, the previously mentioned studies focused onplayas that were continuously flooded. Fryar et al. (2000)noted that the removal of nitrate by denitrification is likelyto be more temporally and spatially variable in playas thatare not continuously flooded such as those found innatural settings. Fryar et al. (2000) reported elevatednitrate in groundwater in the vicinity of one playa thatreceived wastewater. Playas that have short and frequentepisodes of flooding and drying are more likely to havedesiccation cracks that promote aerobic conditions in thesoil as well as the potential for rapid macropore flow.Denitrification in the playa subsurface limits but does notpreclude groundwater contamination resulting from waste-water discharge to playas or from other playas that focusrecharge (Fryar et al. 2000). In a statistically based study,Enwright and Hudak (2009) report a weak positivestatistical relation between nitrate concentration ingroundwater of the southern High Plains aquifer anddistance of the wells to the nearest playa, which theysuggest may be an indication of denitrification beneathplayas resulting in smaller nitrate concentrations in wellsnearest the playas. Furthermore, geochemical conditionsthat promote denitrification may promote mobilization anddelivery of trace metals and some organic compounds inrecharge. For example, Thurman et al. (2000) speculatedthat metabolites from cotton herbicides may have theability to leach from subsurface sediment beneath playasinto the groundwater. However, very few studies haveused unsaturated-zone methods beneath playas or installedmonitoring wells immediately downgradient from playasto evaluate recharge chemistry from playas. Consequently,and as first noted by Fryar et al. (2000), additionalmonitoring of groundwater quality near playas—especiallythose that receive feedlot wastewater—is warranted.

Additionally, no research examined the effects ofartificial recharge on the fate and mobilization ofcontaminants in the modified playas. The practice ofremoving clay-lined floors and dredging playa sedimentsmay reduce the natural attenuation capacity of the playa-

wetland system and possibly increase the mobilization ofsome contaminants moving toward the water table.

Conceptual model of recharge beneath playasThree prominent conceptual models of recharge beneathplaya have emerged from the literature:

1. Playas are evaporation pans (for example, Lehman1972; Claborn et al. 1985).

2. Playas are not exclusively evaporation pans, andrecharge is restricted to the annulus of playa (forexample, Osterkamp and Wood 1987; Wood andOsterkamp 1987).

3. Playas are not exclusively evaporation pans, andrecharge is focused through clay soils of the playafloor (for example, Broadhurst 1942; White et al. 1946;Wood and Sanford 1995a, b; Scanlon and Goldsmith1997; Wood et al. 1997).

This paper demonstrates that playa-focused recharge ispossible at substantially (1–2 orders of magnitude) higherrates than in interplaya areas of the southern High Plainsaquifer (Fig. 5; Table 2); it thus provides evidence againstinterpreting playas as strictly evaporative pans (conceptualmodel 1). Higher recharge rates beneath playas aresupported by high water flux, contents, and downwardpotentials; by low chloride and high tritium concentrationsin the pore water; and by low caliche content in thesediments. Additionally, infiltration rates are significantlyand positively related to clay content of the floor. Thisapparent contradiction of conventional wisdom is causedby rapid infiltration down desiccation cracks. The rapidinfiltration rate decreases as ponded water causes expan-sion of the soil matrix and sealing of desiccation cracks.Studies report strong correlations between ponding anddepth of infiltration and recharge, evidence of watermovement beneath clay-lined playas, and limited move-

Fig. 5 The recharge estimates listed in Table 2 are summarized foreach recharge setting; they generally indicate larger recharge ratesbeneath playas than beneath interplaya settings (note: x-axis islogarithmic). The recharge settings labeled as Nonspecific were notdescribed in the literature and may include a range of rechargesettings

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ment through the playa annulus; these studies do notsupport the interpretation that recharge is restricted to theplaya annulus (conceptual model 2).

Many questions remain regarding factors that controlrecharge beneath playas; however, conceptual model 3 isbest supported by most published findings for recharge tothe High Plains aquifer (Fig. 5). Reported recharge ratesbeneath playa floors range from ∼0.25 to more than500 mm/yr, whereas most interplaya settings in croplandsand rangelands have recharge rates reported to range from∼0.25 to 25 mm/yr (Fig. 5). Although reported rechargeestimates through the playa annulus range from ∼10 to100 mm/yr and are generally higher than most estimatesreported for interplaya settings, a number of rechargeestimates through the playa floor are higher than thereported rates through the annulus (Fig. 5). The reportedrecharge rates for nonspecific regional recharge settings(Fig. 5) range from less than 1.0 to almost 200 mm/yr,which is similar to the range reported for recharge beneathplaya floors. However, the nature of regional-rechargemethods makes it very difficult to distinguish focused-recharge processes from diffuse-recharge processes. Thenonspecific regional recharge estimates may reflect anaverage or integration of recharge beneath playas andrecharge beneath interplaya settings (Fig. 5). Therefore,without recharge contributions from playas, regionalrecharge to the southern High Plains aquifer couldpossibly be 1–2 orders of magnitude smaller. Properlyfunctioning playa wetlands, which have shrink-swell soilsthat produce desiccation cracks and rapid infiltration rates,are thus important for the overall recharge contribution tothe southern High Plains aquifer.

The literature synthesis described in this paper did notevaluate recharge beneath playas of the northern HighPlains because no published studies were identified.Therefore, until future research on playas of the northernHigh Plains is published, it is unknown if rechargeprocesses beneath playas in that region are similar tothose in the southern High Plains.

Summary of major findingsUnderstanding how playas affect the quantity and qualityof recharge to the High Plains aquifer has importantimplications for the sustainability of the High Plainsaquifer, human and ecosystem health, the sustainability ofrural agricultural economies, and the substantial costsassociated with land and water management, conservation,and regulation. The major findings of the literaturesynthesis are outlined in this section and yield science-based implications for assessing and managing playas andgroundwater resources of the High Plains.

Movement of recharge and chemicals to the water tablefollows fast and slow pathways. Different pathways areavailable for recharge and chemical transport to reach thewater table, and some paths are relatively faster thanothers. In locations that represent diffuse recharge (slowpaths), estimated time of chemical transport from landsurface to the water table exceeds the period of agricul-

tural activity (more than 100 years in some locations) andimply that agricultural chemicals should not be present atthe water table yet. In fact, agricultural chemicals arecommonly detected in groundwater. This apparent dis-crepancy is explained by local fast paths that may enablewater and chemicals from the land surface to reach thewater table in months to decades. By comparison, slowpaths may enable water and chemicals from the landsurface to reach the water table in centuries to millennia.

Playas help recharge the High Plains aquifer. Mostplayas represent fast pathways for recharge and provide animportant component of recharge to the High Plainsaquifer. Although the exact amount of recharge to theHigh Plains aquifer from any individual playa or group ofplayas is unknown without detailed investigation, sub-stantial evidence in the literature shows that some portionof water that is stored seasonally in playas is able toinfiltrate and eventually intercept the High Plains aquiferas recharge.

Recharge from interplaya settings is relatively lowcompared with playa settings. Interplaya settings generallyrepresent slow paths for recharge and chemical transportbecause of high evapotranspiration and low precipitationrates in the southern High Plains. Reported interplayarecharge rates average 1–2 orders of magnitude smallerthan most estimated recharge rates beneath playas.

Playa recharge varies in space and time. Largevariations in estimated recharge rates beneath playasindicate that recharge is controlled, in part, by the spatialand temporal patterns in the physical characteristics of theplayas, in climate, and in surrounding land-use practices.The physical characteristics of playas that have apparentinfluence on recharge rates are the drainage area, playavolume, depth of the playa floor, vertical extent of shrink-and-swell clay that lines playa floors, depth of sedimentoverlying clay-lined floors, unsaturated-zone sedimentsunderlying the playa, and depth to the water table. Climatefactors that affect the shrink-and-swell characteristics ofthe playa floors are likely to have important controls onchanges in recharge with time. Some land-use practicessuch as cultivation, increase sedimentation to playas andthus affect the physical characteristics that influenceinfiltration and recharge beneath playas. However, exist-ing studies do not provide data to support the developmentof a reliable predictive model (or models) of rechargebeneath any individual playa or group of playas. Futurestudies are needed to develop models that predict therecharge rates beneath playas.

Methods used to estimate recharge have inherent andunavoidable uncertainty. The same is true for the methodsused by studies to estimate recharge beneath playas.However, these studies rarely report errors or uncertaintiesassociated with recharge estimates. Furthermore, manystudies use only a single method to estimate recharge.Recent research has shown that the use of many differentmethods can help constrain recharge estimates and reduceuncertainty. Thus, future studies that use as many differentapproaches as logistically and financially possible toestimate recharge will likely help answer important

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remaining questions about recharge rates and chemistrybeneath playas.

Future researchA number of gaps remain in understanding and predictingrecharge rates and chemistry beneath playas of the HighPlains aquifer. The conditions in and around playas thatcontrol recharge rates and chemistry have a direct effecton the diversity of flora and fauna in the playa, land-usecharacteristics for farmers and ranchers, and the futuresustainability of groundwater in the High Plains aquifer.Therefore, a number of important research needs remain.These needs are posed as the following questions to helpaddress existing gaps in the current state of knowledgeabout recharge and chemical transport beneath playas ofthe region. The existing literature does not bring data tobear on important questions that include the following:

1. What are the effects of current and future rates ofsedimentation on infiltration and recharge beneathplayas?

2. How much of the water that infiltrates beneath playas islost to lateral subsurface flow and subsequent evapo-transpiration before reaching the water table, and howdo such processes affect the results of studies thatassume that all water infiltrated beneath playasbecomes recharge?

3. Are innovative and wetland-friendly approaches forartificial recharge beneath playas available?

4. How much contamination reaches the groundwaterbeneath playas, and does playa modification thatincreases artificial recharge also increase transport ofcontaminants to the water table?

5. How important are playas for recharge to the northernHigh Plains aquifer, for which comparatively littleresearch has been reported?

6. How will climate change and climate variability affectrecharge beneath playas?

These and other questions may be answered usinginterdisciplinary studies of water and movement ofchemicals through the playa-wetland system to the HighPlains aquifer as recharge.

Additional data are needed to support an understandingof the subsurface rate of water movement and fate ofchemicals after infiltration in the annulus or through theplaya floor. For example, the potential for lateral move-ment of water from the annulus to interplaya sedimentsand subsequent loss to evapotranspiration is unknown.More important, a relatively small number of rechargeestimates have used unsaturated-zone or groundwaterstudies, which provide much more meaningful anddetailed estimates of recharge than water-budget orinfiltration studies. Additionally, current research doesnot clearly describe how playa modification for artificialrecharge affects the fate of contaminants in playas andmobilization toward the water table. A number of specificknowledge gaps remain and include the effects of

sedimentation on infiltration rates in playa floors and theshrink-swell characteristics of Vertisol soils, transport oforganic chemicals and trace metals under anaerobicsubsurface conditions in playa sediments and under playamodifications for artificial recharge, and the effects ofclimate variability and climate change on the hydrologyand recharge potential of playas. Future studies thatdevelop predictive models of recharge rates and chemistrybeneath playas will likely provide valuable tools for playaand groundwater management and conservation.

As Melcher and Skagen (2005a) suggested, interdisci-plinary and collaborative scientific studies are needed.Collaborative studies between geologists, hydrologists,ecologists, biologists, agronomists, and land-conservationscientists will likely result in knowledge that best fills theremaining gaps in information about playas. Futurestudies concerned with the role of playas in rechargingthe High Plains aquifer will likely refine conceptual model3 of recharge by using a systematic approach on variousspatial and temporal scales and by using a wide range ofhydrologic, biogeochemical, and isotopic methods. Asdescribed by Nativ (1992), a systematic approach wouldinclude data collection of the amount of precipitation andrunoff to a playa, the volume of water stored andvariations with time, the evapotranspiration of water fromthe playa, and changes with water contents and total-potential gradients beneath playas and correspondinginterplaya areas with time. Additional information maybe gained by using biogeochemical and isotopic indicatorsthat trace water and chemical directions and rates ofmovement within the subsurface.

Acknowledgements The authors thank the Playa Lakes JointVenture (PLJV) for its support of this study. Constructive commentson earlier versions of this paper were provided by B. Johnson(Texas Parks and Wildlife Department), T. LaGrange (NebraskaGame and Parks Commission), S. Paschke (US Geological Survey),M. Carter (PLJV), J. Ver Steeg (PLJV), C. Rustay (PLJV), M.McLachlan (PLJV), D. Slobe (PLJV), and three anonymousreviewers.

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