e 2006 american society

Transactions of the ASABE

Vol. 49(2): � 2006 American Society of Agricultural and Biological Engineers ISSN 0001−2351 1

WEIGHING LYSIMETERS FOR EVAPOTRANSPIRATION

RESEARCH IN A HUMID ENVIRONMENT

X. Jia, M. D. Dukes, J. M. Jacobs, S. Irmak

ABSTRACT. Three weighing lysimeters were developed for evapotranspiration research at the University of Florida, Instituteof Food and Agricultural Sciences, Plant Science Research and Education Unit near Citra, Florida. The lysimeter designfollowed accepted procedures as well as aspects unique to the study site, including a foundation designed for a perched watertable outside the lysimeters, fetch distance, deep drainage, and lightning protection. Each lysimeter has a planted surfacearea of 2.32 m2 and a soil depth of 1.37 m. The soil in each lysimeter is reconstructed sandy soil originally from theexperimental site. The lysimeter facility includes monitoring wells, an automatic pumping system, and additional lightningprotection system for load cells and soil moisture sensors. The construction materials and installation cost (excluding labor)were $63,443 for the three lysimeters. Lysimeter on-site maintenance, operation, and performance are discussed. Four loadcells with an accuracy of 0.02% (0.12 mm) are used to weigh the average 5.8 Mg lysimeter mass, including the steel lysimetertank and soil. Initial data show that the three lysimeters provided a consistent hourly evapotranspiration (ETc) measurementover a five-day period in the summer season, although many field activities and precipitation events occurred. An additional30 days of daily bahiagrass ETc resulted in a 0.82 ratio between the ETc and Penman-Monteith reference evapotranspirationin November 2003.

Keywords. Evapotranspiration, Humid environment, Weighing lysimeters.

rop evapotranspiration (ETc) determination is im-portant to guide irrigation scheduling and to man-age water resources. Lysimeters are the mostreliable research tool for direct measurement of

ETc (Burman et al., 1983; Howell et al., 1985; Burman andPochop, 1994).

For ETc research, a lysimeter is a tank containing a soilprofile and plants of interest. More specifically, lysimetersare “tanks filled with soil in which crops are grown undernatural conditions to measure the amount of water lost byevaporation and transpiration” (Jensen et al., 1990). Bymonitoring the change in water storage in the lysimeters,along with other components in the water balance (e.g., pre-cipitation, irrigation, and drainage), the actual evapotran-spiration rate can be obtained over the measurement interval.Resultant measurements can provide daily evapotranspira-tion values for grass to within 0.05 mm or 1% of accuracy(Allen and Fisher, 1990), and to 0.43 mm per day over three

Submitted for review in August 2005 as manuscript number SW 6022;approved for publication by the Soil & Water Division of ASABE inFebruary 2006.

The authors are Xinhua Jia, ASABE Member Engineer, PostdoctoralResearch Associate, and Michael D. Dukes, ASABE Member Engineer,Assistant Professor, Department of Agricultural and BiologicalEngineering, University of Florida, Gainesville, Florida; Jennifer M.Jacobs, Associate Professor, Department of Civil Engineering, Universityof New Hampshire, Durham, New Hampshire; and Suat Irmak, ASABEMember Engineer, Assistant Professor, Department of Biological SystemsEngineering, University of Nebraska, Lincoln, Nebraska. Correspondingauthor: Xinhua Jia, Department of Agricultural and BiologicalEngineering, 259 Frazier Rogers Hall, University of Florida, P.O. Box110570, Gainesville, FL 32611-0570; phone: 352-392-1864 ext. 259; fax:352-392-4092; e-mail: [email protected].

growing seasons for shallow-rooted crops (Martin et al.,2001).

The design, construction, and operation of lysimeter areoften complicated. The summary of lysimeter developmentand design by Howell et al. (1991) indicates that evapotran-spiration accuracy is influenced by the measurement dura-tion, lysimeter shape, weighing mechanisms, andconstruction materials as well as site maintenance. Allen andFisher (1990) further stated that environmental consider-ations related to lysimeter design and data corrections couldcause the evapotranspiration results to be impractical to use,or lead to inaccurate conclusions. These considerationsinclude accurate estimation of evaporative, vegetative, andlysimeter rim areas and differences between soil moisturecontent inside and outside the lysimeters.

In humid environments, such as in Florida, the design andconstruction of lysimeters is particularly challenging due toshallow water tables, loose soil structure, and frequentprecipitation events. The purpose of this article is to describethe design and construction of three relatively large weighinglysimeters in central Florida. These lysimeters were devel-oped to measure plant water use for grass and other shallowrooted crops such as vegetables. The design and constructiontakes into account the standard considerations, as well assite-specific requirements. Environmental factors in lysime-ter maintenance and calibration of the lysimeters arepresented. Preliminary performance results are provided.

EXPERIMENTAL SITEThe experimental site is located at the University of

Florida, Plant Science Research and Education Unit(PSREU), near Citra in Marion County, Florida. The PSREU

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is between Gainesville and Ocala and has 445 ha of landavailable for research. The geographic location is at latitude29° 24′ N and longitude 82° 10′ W with an elevation ofapproximately 20 m above sea level. The climate in thiscentral region of Florida is humid subtropical, with anaverage annual rainfall of 1,350 mm (Purdum, 2002). Mostof the rainfall (60%) occurs in June, July, August, andSeptember. Afternoon thunderstorms are frequent during thisperiod, and it rains almost every day. Annual averageevapotranspiration is about 945 mm, which is 70% of the totalrainfall (Purdum, 2002).

The PSREU Phase 1 field, where the lysimeters arelocated, is divided into six large blocks for experiments, eachone with an area of 20 to 30 ha. As shown in figure 1, a FAWN(Florida Automated Weather Network) weather station islocated near the entrance to the PSREU, Eddy1 is a weatherstation 80 m away from the lysimeters, and Ref-ET is thesecond weather station about 500 m away from the lysime-ters. All three stations are in grass fields. The lysimeters arelocated in Block 6, a 23 ha grass plot. The location has at least230 m of fetch distance in all directions. These three weatherstations are used to estimate reference evapotranspiration(ETo).

The soil type at this site is located in the Arredondo-Gainesville association (Thomas, et al., 1979), which isnearly level to sloping, well drained soils, with some sand toa depth of more than 100 cm, loamy below, and others sandythroughout. Tischler (2003) stated that the soils at the fieldare Sparr, Millhopper, or Adamsville, where Sparr is a poorlydrained loamy, siliceous, subactive, hyperthermic AquicArenic Paleudult; Millhopper is loamy, siliceous, semiactive,hyperthermic Grossarenic Paleudults; and Adamsville ishyperthermic, uncoated Aquic Quartzipsammernt. The watertable is deeper than 2 m year round, and bedrock is greaterthan 1.5 m (Thomas et al., 1979). The measured bulk densityof the soil for the lysimeter site at 15, 30, 45, 60, 75, 90, and

105 cm depths are 1.36, 1.39, 1.41, 1.42, 1.43, 1.42, and1.43 g/cm3, respectively. The Sparr, Millhopper, and Adams-ville fine sands are composed of 94% to 97% sand and 2% to5% silt in the upper 100 cm (Carlisle et al., 1989). The fieldcapacity at this site is about 10% of soil volumetric content,and soil water holding capacity is 8%. This is a very loosesandy soil, and it presented challenges during constructiondue to soil instability and a perched water table.

Bahiagrass, a typical turf grass used in humid regions, waspreviously established as a pasture area in Block 6. This typeof grass requires limited maintenance and is categorized asa warm-season grass, which has a higher degree of stomatalcontrol with lower potential evapotranspiration rates than acool-season grass (Jensen et al., 1990). Linear-move sprin-kler systems are used for irrigation in the research areas.Typically, the fields are irrigated twice a week (~19 mm eachirrigation event) when rainfall does not occur. The grassfields are mowed as needed to ensure a 12 cm grass height,as defined in the reference evapotranspiration concept inASCE-EWRI guidelines (ASCE-EWRI, 2005). The field isvisited at least weekly to download data and ensure thatinstruments are functioning properly.

LYSIMETER DESIGN AND CONSTRUCTIONThe Citra, Florida, lysimeters described in this article

consist of a large concrete base, three small concrete bases,one for each lysimeter, double tanks reinforced with squaresteel tubing, an in-situ weighing system, automatic pumpingand drainage systems, a data acquisition system, a soilmoisture monitoring system, water table monitoring, threenearby meteorological weather stations, and a lightningprotection system. The lysimeter construction is grouped intothree stages: foundation construction, lysimeter tank fabri−cation, and tank installation and instrumentation, as de-scribed in the following sections.

Figure 1. PSREU field layout with each weather station marked on the plot (modified from http://plantscienceunit.ifas.ufl.edu/images/location/p1.jpg).

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FOUNDATION CONSTRUCTIONThe on-site construction of the lysimeter foundation

began with soil excavation from the experimental site inMarch 2003. The soil was removed by an excavator from thesurface to a depth of about 3 m, and temporarily stored bylayer according to soil horizon near the construction site. Atotal of 600 to 700 Mg of soil was extracted from this site,leaving a 16.8 m long, 6.6 m wide, and 3 m deep rectangularhole (fig. 2). The initial plan was for each lysimeter to haveits own concrete base. The wet spring season prior toconstruction resulted in a perched water table at the time ofconstruction. The foundation was redesigned by a geotechni-cal engineer to prevent foundation instability under saturatedconditions. The final concrete foundation was one large basewith three small concrete structures on the top and is

underlain by a gravel layer to provide a well drained base forthe foundation (fig. 3).

According to Terzaghi’s bearing capacity theory, the soildirectly beneath the foundation is counteracted by frictionand adhesion between the soil and the foundation. Becausethis resistance is against lateral spreading, the soil immedi-ately beneath the foundation remains permanently in a stateof elastic equilibrium, and behaves as if it was a part of thefoundation and may sink with the foundation even with aheavy load. However, for the soil zone that is adjacent to andbeneath the foundation, the soil tends to slide or risedepending on the soil properties beneath the foundation(Murthy, 2002). Accordingly, the three small foundationswould likely settle independently, making the tanks unstable

Figure 2. Lysimeter foundation: construction of metal frame above gravel layer (from east to west direction).

Figure 3. Cross-section view of lysimeter foundations (from south to north direction).


because one foundation was located at the rising zone ofanother foundation. The large foundation essentially spreadsthe force over a larger area, reducing the chance that the tankswould settle.

Foundation construction began by leveling the groundsurface inside the excavated site. A 5.1 cm thick concretelayer was then poured and cured to provide a stable base andprevent gravel from sinking. A 45 cm thick layer of 1.9 cmdiameter gravel was placed above the thin concrete layer andtamped to level it. The gravel was lined with thick plastic filmover the leveled gravel layer to prevent clogging of the gravelpore spaces. The dense gravel beneath the foundation, whichhas a large angle of friction coefficient (35° to 45°), canprovide a higher bearing capacity and less sliding area(Murthy, 2002). During the rainy season, the water table mayrise to the foundation level, as occurred during the fall of2004. The occurrence of water table reduces the total bearingcapacity of the soil if the water table lies at any intermediatedepth between the soil surface and 8.20 m below the surface(5.41 m foundation width plus the 2.8 m foundation depth)(Murthy, 2002). A nearby groundwater level monitoringstation shows that the averaged water table for 30 years is13.0 m above average sea level and the elevation of thestation is 19.7 m, similar to that at the PSREU. In October2004, the water table was 5.9 m below the soil surface. Thelowest water table (8.0 m below soil surface) occurred on2 July 2001, and the range of the water table for 30 years is2.3 m (USGS, 2006). Therefore, the shallow groundwatermay influence the soil’s bearing capacity, but it may notlikely rise to the soil surface and overflow in the lysimeter.

Above the gravel, rebar (#6, 60 grade) was used to forma double matrix frame as a support around and above the base(fig. 2). For each lysimeter, additional rebar was used acrossthe lysimeter base to strengthen the structure. Small ironrebar (#3, 60 grade) was used to wrap the #6 double rebar. Thedistance between the two #6 rebar spans is 15 cm vertically

and 30 cm horizontally. The distance between the #3 stirrupsused to bind each two of the #6 rebar connections is 30 cm.At the corner of the metal structure, L-shaped #6 rebar wasused to strengthen the frame, extending 80 cm into thestructure. The framework bottom was 7.5 cm above thegravel to allow for 7.5 cm of concrete between the metalframe bottom and the gravel. All the gaps within the metalframe (15 cm height) were filled with concrete. In addition,the concrete layer extended 7.5 cm above the metal frame.The total height of the concrete layer above the gravel was30 cm, consisting of a 15 cm metal and concrete layer, 7.5 cmbelow and above this layer (fig. 3). The finished largefoundation is 5.41 m wide, 15.61 m long, and 0.80 m tall.

To equalize any differential pressure that might buildunder the foundation due to high water table conditions, six15 cm diameter schedule 40 PVC pipes were installed 20 cminto the gravel layer, 3.5 m apart in pairs. Each pair is located1 m from one side of a lysimeter. These pipes extend abovethe ground surface and are open to the atmosphere. Each ventpipe is fitted with two 90° elbows so that rainfall cannot enterthe vent. Nylon screens are used to cover the end of each ventpipe to prevent entry of insects.

After the large concrete base was constructed, three smallconcrete bases were built 3.35 m apart centered on the largeconcrete foundation. Each base supports a lysimeter. Thebases are 1.75 m square, 30 cm high, and reinforced with #6rebar metal frames. Two I-beams were installed in aneast-west direction inside each frame. Looking at the sitefrom the ground surface, the finished foundation is amonolithic concrete base with three square bases on top andsix pipes protruding upward (fig. 4). The foundationconstruction took two months.

LYSIMETER TANK FABRICATIONEach of the three lysimeters consists of two tanks. The

outer tank is a large square, hollow tube (1.78 × 1.78 ×

Figure 4. Isometric view of foundation, lysimeter outer tanks, and pressure release pipes (from southwest to northeast direction).

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Figure 5. Top view of one lysimeter showing outer tank, inner tank, and drainage system.

2.02 m), extending from the top of the soil surface andenclosing each lysimeter base. The inner tank is a smallsquare tank (1.52 × 1.52 × 1.37 m) (fig. 5). The top surfaceof each small concrete base serves as the bottom of an outertank. The inner tank holds the soil and crop. Load cells were

installed between the concrete base and the inner tank so thatthe inner tank can be weighed. Both the outer and inner tankswere constructed at the University of Florida, Department ofAgricultural and Biological Engineering machine shopfacilities and transferred to the field prior to the installation.

Figure 6. Cross-section view of lysimeters with monitoring well, soil moisture sensors, and load cells (from west to east direction).


The material used for the construction of lysimeters was4.8 mm thick steel plate. To increase the wall strength of thelysimeters, square metal tubes, 7.6 cm on a side and 4.8 mmwall thickness, were attached across the four outer sides ofboth lysimeter walls. For the outer tank, five horizontal tubeswere used and the distance between tubes was 30 cm (figs. 4and 6). For the inner tank, five horizontal square tubes,spaced 25 cm, were installed across the tank. Six tubes, thesame size as the wall tubes, were welded to the bottom of eachinner tank to increase the bottom strength (fig. 6). Afterconstruction, the lysimeters were coated with green epoxypaint to protect them from corrosion.

TANK INSTALLATION AND INSTRUMENTATION

After the concrete bases were completely cured, thelysimeters were installed on 22 April 2003. The outer tankwalls were placed as one unit and fastened with L-shaped10.2 × 10.2 × 0.6 cm angle steel from the inner wall to thetop of the small concrete base, and from the outer wall of theouter lysimeter to the top of the large concrete base. In thisarrangement, the outer tank was stabilized between the twoL-shaped angle steel brackets around the inner perimeter(fig. 6).

Once the outer tank was secured, the soil excavated fromthe construction site was backfilled around the threelysimeters. After repacking, the site was watered andrepacked several times until the soil was completely levelwith the surrounding area.

When the soils outside the lysimeters had been recon-structed, the three outer tanks remained open at the site. Priorto inserting the inner tanks, two steel channels, 15 × 10 cmand 1.3 cm wall thickness, were welded across the I-beamsthat were embedded in each concrete base inside the outertank. Four single-ended shear beam SBS-10K load cells(Measurement Specialists, Inc., Huntsville, Ala.) werebolted to the top of the steel tube at the four corners parallelto the steel channels. A bolt was attached at the end of the loadcell arm. A 1.6 cm thick steel plate was welded at each cornerin the bottom of the inner lysimeter. The bolt of the load celland the steel plate at the bottom of the inner lysimeters formthe contact points. All the lysimeter mass, including the steellysimeter, soil, plants, and soil moisture probes, wassupported by the four contact points in each tank (fig. 7). Theinner lysimeter was precisely positioned on the four loadcells.

The load cells are temperature-compensated, stainlesssteel cantilever-type load cells that give a linear change in

Figure 7. Load cell side view (from west to east direction).

resistance in response to an applied weight. Each load cell iscapable of measuring 4,536 kg (10,000 lb). The ratedexcitation signal is 5 V DC/AC and the full-scale output is3.0 mV/V. The temperature-compensated range is from−10°C to 50°C, and the safe load limit is 150% of ratedcapacity. The next size load cell was an SBS-4K model witha capacity of 7.3 Mg that did not provide enough capacitycompared to the worst case of a saturated soil in the lysimeter(8.6 Mg).

To extract the water accumulated in the bottom of eachlysimeter, two large permeable ceramic plates (Filtros, Ltd.,East Rochester, N.Y.), 15 cm diameter and 2.5 cm thick, werefastened on the bottom center of the inner lysimeter. Thethreaded holes were fitted with a nipple for attachment of thedrainage tubing. The copper drainage tubes were attached tothe nipple on the lysimeter inner tank and routed under theinner tank and between the inner and outer tanks to the soilsurface. The drainage tubes are connected, by plastic tubing,to a vacuum system to remove the drainage (fig. 6).

After the inner tank was installed, there was a 4.7 cm gapbetween the inner and outer tanks. The two drainage tubes aswell as load cell output wires were routed through the gap. Arubber sheet, 0.5 cm thick and 18 cm wide, was painted greenand used to cover the gap. The rubber was bolted on the outertank rim only. The free side of the rubber is slightly higherthan the bolted side. This ensures free vertical movement ofthe inner lysimeter, and precipitation falling on the rubbercovers flows away from the lysimeter, instead of flowing intothe lysimeter.

For a 1.42 g/cm3 bulk density soil, the total void porespace of the soil is 46.4%; this is equivalent to a water depthof 636 mm. At field capacity, the soil moisture storage isapproximately 64 mm, and the remaining soil void space canhold 572 mm of water depth. Surface runoff is unlikelybecause the water table is maintained well below the surfaceand the sandy soils have high infiltration rates. Routinepumping occurs at intervals well within the requiredaccumulation time period. Additionally, Gregory et al.(2006) reported that the infiltration rate is 225 mm/h for thepasture site where the lysimeters are located. For soil with ahigh infiltration rate, surface runoff into the lysimeters isminimal because the lateral flow movement is negligiblecompared to vertical flow (Sumner and Bradner, 1996).Furthermore, the lysimeters are located at a relatively higherland surface within the field. Surface ponds were observed atsome lower points during the hurricane season in 2004, butnot near the lysimeters.

A PVC pipe, 1.8 m long, was inserted through the centerof the gap between the inner and outer tanks to the top of thefoundation on 22 April 2005. The bottom end of the pipe wascut in a cross shape so that water could be pumped out, andthe top end of the pipe was covered by a cap so that no insectscan get into the gap. The joint between the pipe and the rubbercover was glued. This access pipe is used to pump outoverflow in the tanks from extreme rainfall events, such ashurricanes. During these events, lysimeter monitoring isdisabled. Thus, losing drainage to the lysimeter tanksinterstitial space does not affect the crop water use calcula-tions. A final view of the lysimeter area is shown in figure 8.

When all three lysimeters were installed, the soil profilewas reconstructed inside the lysimeters. A 15 cm layer ofcoarse sand was placed at the bottom of the lysimeter toprovide a storage reservoir for gravity drainage. The soil was

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Figure 8. Plan view of the lysimeter site (not to scale), solid lines show the soil and water system and dashed lines show the connections between sensors,dataloggers, pump, timer, and the power supply.

repacked in 5 cm increments and tamped close to its originalbulk density for each layer. To monitor soil moisture contentand temperature, five Vitel probes (Stevens Water Monitor-ing Systems, Inc., Beaverton, Ore.) were installed near thecenter of each lysimeter, at depths of 5, 10, 20, 50 and 100 cmfrom the soil surface (fig. 6). Five Vitel probes were alsoinstalled at the same depths outside the lysimeters in anundisturbed area.

After the soil was repacked, a slotted PVC pipe wasinstalled at the northwest corner of each lysimeter andextended 10 cm above the soil surface (fig. 6). This pipe isused to monitor the water table inside the lysimeter and toensure that the lysimeter bottom is not flooded. As describedby Allen et al. (1991), grass growing in soil with a shallowwater table can depress root growth. Thus, as previouslyexplained, the drainage pumping was conducted to maintainfree drainage. However, if the pumping were scheduled morefrequently, the gravel layer at the bottom of the lysimeterwould be drier than the surrounding area at the same depth.Small moisture differences at the bottom of the lysimetershould have little effect on grass growth because there is littleupward water movement in a sandy soil. Since the foundation

was more than 2.5 m deep, it would likely not affect cropwater use studies because the majority of the roots are within1 m depth at this site, based on observations during lysimetersite excavation.

As of 9 June 2004, the pumping system was changed frompumping one lysimeter at a time to pumping all lysimeterssimultaneously by connecting the tanks in parallel. Anautomatic pumping system was installed on 8 October 2004.After the automatic pumping system was installed, pumpingwas typically scheduled at night or in early morning, whenETc is minimal.

As shown in figure 8, a vacuum pumping system is locatedon a mobile trailer approximately 18 m from the lysimeters.The three large tanks (303 L) collect the drainage from eachlysimeter. A small tank (114 L) serves as a backup vacuumcontainer. The maximal pumping rate for free water is 70 L/h.When the pumping time is limited to within 4 h, the storagecapacity of the tanks is sufficient. A vacuum pump and anautomatic timer are enclosed in a waterproof cabinet besidethe storage tanks. The entire apparatus is mounted on themobile trailer so that it can be moved for field operations(fig. 9).


Figure 9. Photograph showing the lysimeters in the foreground and the automatic vacuum pumping system in the distance (from southwest to northeastdirection).

On the north side of the lysimeters, a CR23X datalogger(Campbell Scientific, Inc., Logan, Utah), programmed for afour-wire full bridge, is used to record the voltage output fromthe lysimeter load cells every minute and averages every 10 minto a data file. Three enclosures, containing a lightning protectionsystem for each lysimeter, are located next to the datalogger. Onthe south side of the lysimeters, a CR10X datalogger isconnected to the Vitel probes to record soil moisture measure-ments. This data logger and the probes also have a lightningprotection system. Soil heat flux measurements, taken from themiddle lysimeter, are also recorded by the CR10X. There is1256 m2 of area (20 m radius) surrounding the lysimeters withsigns placed to prevent any vehicles or foot traffic that couldaffect the readings of the load cells.

CALIBRATIONA lysimeter calibration was conducted to develop a

relationship between the voltage of the load cells and theequivalent weight of the tank. Prior to backfilling the innertanks, a known volume of water was added to the inner tankand the corresponding change in the load cell voltage (fouron each tank) was recorded after the readings stabilized.Stabilization typically occurred within 1 to 2 min. A linearregression analysis was performed to obtain a calibrationfactor for the average load cell signal in each tank. Theresulting regression coefficients for each lysimeter are shownin figure 10. Here, the linear regression equation is repre-sented as y = ax + b, where y is the measured output (mV), x isthe applied load (kg of water), a is the calibration slope(mV/kg), and b is the intercept (mV).

For each weight increase, the output readings were recorded.The average millivolt output was plotted against the weightincrease in each lysimeter. With the manufacturer’s stated loadcell accuracy of 0.02%, this calibration shows that the expectedaccuracy is about 0.12 mm equivalent depth of water over thelysimeter area (5.8 Mg average lysimeter weight).

OPERATIONThe Citra, Florida, weighing lysimeters have been in

operation since 28 May 2003. However, the site was not

y = 0.000165 x + 0.142742

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Figure 10. Lysimeter calibration curves determined by adding loads on 10June 2003.

completely established with bahiagrass sod and collectingdata until July 2003. On 4 August 2003, a severe thunder-storm resulted in a power surge that damaged at least one loadcell in each lysimeter tank. The load cells were replaced, and

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each load cell was connected to a surge arrestor (modelE280-6v, Citel, Inc., Miami, Fla.) to prevent lightningdamage in the future. The repaired lysimeters have been inoperation since 13 October 2003.

Rainfall and irrigation at the lysimeter site are monitoredby a tipping bucket and recorded using a Hobo event logger(Onset Computer, Bourne, Mass.) and a manual rain gauge.The manual rain gauge readings are recorded two to threetimes per week during the summer. Both instruments areinstalled at a 60 cm height and are subject to overheadirrigation. A second tipping bucket rain gauge is located 80 maway from the lysimeter site at 3 m height. Total rainfalldepth from all rain gauges is compared to the increase inlysimeter weight during storm events. However, only the tworain gauges near the lysimeters were used for precipitationdepth measurement.

Site operation procedures were established to maintainconditions in the lysimeters comparable to those of thesurrounding area. In summary, the conditions addressed andcorresponding procedures are as follows:

� Soil moisture conditions: Uniform irrigation is appliedover the lysimeters and the rest of the field using a lin-ear-move sprinkler irrigation system. Storm event tim-ing and amounts are recorded by the tipping bucket raingauge and verified by the manual rain gauge. Soilmoisture sensors inside and outside the lysimetersmonitor soil water conditions. The water table insideeach lysimeter is measured regularly during field visits.If there is any difference in the water table heightamong the three lysimeters, individual pumping is per-formed.

� Grass height: Grass height inside the lysimeter is mea-sured during field visits and maintained at the sameheight as the surrounding grass. The grass inside thelysimeters is clipped manually from outside the outertank to avoid machine or human disturbance. In the sur-rounding fetch area outside the lysimeters, the grass ismowed with a small lawn mower.

� Grass variety: The same bahiagrass was planted at theinitial stage. Persistent weeds are treated with appropri-ate herbicides to maintain the uniformity of the grassoutside the lysimeters. Because it is difficult to ensureproper herbicide concentration within such a smallarea, the weeds inside the lysimeters are manually re-moved.

� Soil conditions: Soil pH values are measured once ayear inside each lysimeter and in the field outside eachlysimeter. Corrective actions are performed as re-quired.

� Soil structure: The soil was repacked layer by layer insmall increments to maintain similar bulk densities in-side and outside the lysimeters. The native sandy soildoes not have any other structure. However, in Florida,fire ants are tenacious pests. Green et al. (1999) statedthat ant mounds in the soil could increase the soil hy-draulic conductivity, as well as create a preferentialflow in the soil. These changes in soil properties wouldinfluence the water movement in the lysimeter. Antsinside the lysimeters are eliminated immediately afterthey are observed.

� Drainage pumping: An automatic pumping systemwith a time control program is used for drainage pump-ing. The lysimeters are connected to the pump in paral-

lel to ensure that the pumping is under a similarpressure in all tanks. The drainage volume is measuredmanually with a 4 L graduated cylinder and recordedat the end of the pumping process. During the summerrainy season, pumping is conducted on a daily basis.Normally, pumping is scheduled in the early morning,on the day after rainfall or irrigation. The pumping timeis scheduled for alternate hours, which means pumpingfor an hour, rest for an hour, and then pumping foranother hour, etc. Due to the coarse nature of the soil,a constant vacuum cannot be maintained; rather, pump-ing is performed to maintain soil moisture conditionssimilar to those of the surrounding area. The drainagesystem was used to remove free drainage that accumu-lated at the bottom of the tanks. The duration of pump-ing depends on the precipitation amount. The pumpingduration is determined by rainfall occurrence and theamount of rainfall recorded by a manual rain gaugenear the lysimeter site. For example, a 13 mm rainfallevent requires 2 to 4 h of pumping, while a 25 mm eventrequires 4 to 8 h of pumping. Pumping is not normallyconducted when it is raining, but scheduled after rainevents.

� Agronomic practices: Field activities, such as irriga-tion, pest control, and fertilization, are conducted bythe PSREU staff upon request. The grass at the site ismowed once per week in the summer and as needed inthe winter to maintain an 8 to 15 cm height over a totalarea of 0.17 ha near the lysimeters and on the 23 ha fieldsurrounding the lysimeters.

DATA COLLECTION AND PROCESSINGData collection at the lysimeter site includes all compo-

nents of the mass balance required to determine evapotran-spiration. The lysimeters are considered to be the controlvolume of interest. The voltage changes from the load cell at10 min intervals are converted to change in mass using thecalibration equations. Inputs to the lysimeters are precipita-tion and irrigation. These inputs are recorded as weightincrease in the lysimeter and verified by the readings from theHobo event logger and the manual rain gauge. Outputs fromlysimeters are evapotranspiration and drainage. The volumeof drainage is measured manually following pumping eventsand converted to units of weight. The weight loss from thesoil and crop surface due to evapotranspiration is measuredand recorded as the precipitation minus drainage and changein the lysimeter mass.

Field data are subjected to routine quality assurance andquality control procedures. After the data is downloaded inthe field, a preliminary screening is performed to diagnoseerrors. Data are analyzed on a monthly basis. During the10 min measurement interval, there are frequently smalloscillations in load cell voltage, even during the nighttime.This might be due to wind effects, or vibrations recorded bythe load cells. A small amount of signal noise is smoothed byusing a curved equation (Allen et al., 1994).

EXPENSESThe total initial cost for the lysimeters, including materi-

als, foundation, construction, and installation, was $63,443


Table 1. Total design, construction, and installationcosts for the Citra, Florida, lysimeters.

Category Subcategory Cost ($)

Materials Gravel 9,492Steel 11,788Sod 7,444

Foundation Design, construction, and concrete 7,149

Scale Load cells (12) 3,240

Data Dataloggers (3) 4,828collection Multiplexers (2) 1,046system Solar panel 1,297

Surge protectors (27) 1,832Tipping bucket 430Vitel probes (20) 6,700

Miscellaneous Ceramic plates (6) 148Painting 1,382Welding 1,790Pumping system 1,207Small supplies 3,670

Total 63,443

(table 1). The costs do not include the labor cost for thelysimeter design, construction, installation, and maintenanceas well as associated weather stations and software.

PERFORMANCEEvapotranspiration rates for a five-day period (8 to 12 July

2003) during the initial stage are provided to demonstratelysimeter performance. This period is representative of wetconditions, as rainfall occurred almost every day. Althoughsome load cells were damaged on 4 August 2003, the initial

design and calibration were validated for a short time.Frequent rainfall events between 13 July and 3 August 2003prevented ETc calculations for a longer continuous period.While future studies of plant water use will require extendeddata over extended time periods, for demonstration purposesthe five days of hourly data provide a qualitative assessmentof the lysimeters and is comparable to previous studies byMarket et al. (1988) (0 days of ETc), Barani and Khanjani(2002) (12 days of daily ETc), and Allen and Fisher (1990)(6 days of 30 min ETc and 6 months of daily ETc).

Figure 11 shows the hourly ETc calculated for the five daysin the three lysimeters (L1, L2, and L3 for lysimeters 1, 2, and3; and the average of the three lysimeters). During this initialperiod of operation, pumping was scheduled in the middle ofthe day on day 189 (8 July) and precipitation occurred at5:00 p.m. on day 190, 8:00 p.m. on day 191, 12:00 p.m. onday 192, and 4:00 p.m. on day 193.

For the wet period, the water mass balance included bothprecipitation and drainage. Processing the hourly data waschallenging, as pumping and precipitation differed bylysimeter. The mass change recorded by the lysimeters wasused in the calculation, and the precipitation and drainagemeasurements were used as verification. The hourly ETcmeasurements in the three lysimeters were not statisticallydifferent according to an analysis of variance (p = 0.96) andhad a similar diurnal pattern. All three ETc values were highduring the day and low at night, interrupted by maintenanceor precipitation events. The standard deviation among thethree lysimeters varied from 0 to 0.41 mm/h during the fivedays, with the highest values during the day when ETc wasalso high. This variability may be due to the numericalresolution of the CR23X datalogger. A single-bit change inthe sensed excitation loss within the 15-bit datalogger could

−0.2

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189 189.5 190 190.5 191 191.5 192 192.5 193 193.5 194

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L1 L2 L3 Average

Figure 11. Hourly ETc measured in weighing lysimeters on days 189 to 193 (8 to 12 July) of 2003.

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189 189.5 190 190.5 191 191.5 192 192.5 193 193.5 194

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Figure 12. Average hourly ETc in the three lysimeters and the PM ETo at Citra, Florida, on days 189 to 193 (8 to 12 July) of 2003.

11Vol. 49(2):

change the load cell measurement by approximately 0.18 kg(average lysimeter mass 5.8 Mg × 2−15) or 0.29 mmequivalent water depth, similar to results discussed by Allenand Fisher (1990).

The measured average hourly ETc in the three lysimeterswas plotted against the estimated hourly ETo (fig. 12), whichused the weather data from the nearby (80 m) weather station.The ETo was estimated by the standardized ASCE-EWRIPenman-Monteith equation (PM ETo) (ASCE-EWRI, 2005).

The lysimeter ETc followed a similar pattern as the PMETo during most of the daytime hours. The lag time betweenthe change in weather conditions and activities on thelysimeters, such as drainage pumping, caused minor varia-tions between ETo and ETc measurements. The PM EToprovided very stable values during the nighttime, and theestimated values were highly responsive of total availableenergy in humid climate (Yoder et al., 2005; Jia et al., 2005);however, the measured ETc values in the lysimeters werehighly sensitive to any mass changes.

Table 2 shows the daily total ETc for the three lysimetersand the PM ETo from the weather stations for the five days.The ratio of ETc to ETo (Kc) ranged from 0.97 on 10 July to1.20 on 12 July and averaged 1.07 over the five-day period.The highest variability among lysimeters coincided with themidday rainfall event on day 192. Use of these data requiresdetailed examination of field activities and precipitationevents. Table 3 shows the weather conditions near theweighing lysimeters during the five days.

After the load cells were replaced on 23 September 2003,daily bahiagrass ETc values were calculated using the same

procedure as for the five-day hourly data in figures 11 and 12.Thirty days of daily average ETc in November 2003 areplotted in figure 13. The evaporative area used in the ETccalculations was 6.8% higher than the inner lysimeter area(2.5 m2), which accounted for 2.5 cm extended grass lengthestimated from the grass height.

The average standard deviation among the three lysime-ters was 0.54 mm/d for the 30 days, similar to the valuereported by Martin et al. (2001). Higher deviations occurredwhen either precipitation or pumping occurred. Figure 13shows the data period with minimal management activities,but six pumping events and five precipitations events wererecorded during the period. The average ETc was 2.02 mm/d,and the average ETo was 2.45 mm/d. The average ratio of ETcto ETo was 0.82 while the grass was growing vigorouslyduring that time, corresponding with the field records.

Table 2. Comparison of daily evapotranspiration (ETc) valuesmeasured in the lysimeters and estimated PM ETo: L1, L2, and L3 arelysimeters 1, 2 and 3, respectively, Avg. and SD are the daily average

and standard deviation, respectively, of the three lysimeters.

Date(2003)

Day ofYear

ETc (mm/d) PM ETo(mm/d)L1 L2 L3 Avg. SD

08 July 189 5.47 5.34 5.92 5.58 0.30 5.6309 July 190 3.96 4.65 4.18 4.26 0.35 3.8810 July 191 5.12 4.53 3.83 4.50 0.65 4.6211 July 192 3.14 4.65 4.18 3.99 0.77 3.6512 July 190 3.38 3.95 4.07 3.80 0.37 3.16

Table 3. Environmental conditions near the weighing lysimeters on 8 to 12 July 2003 at Citra, Florida.

Date(2003)

Precipitation(mm)

Air Temperature(°C)

Relative Humidity(%)

Wind Speed(m/s)

Incoming SolarRadiation (W/m2)

Avg. Max. Min. Avg. Max. Min. Avg. Max. Min. Avg. Max.

8 July 0.0 27.8 35.1 22.1 77.4 97.3 44.7 1.14 3.16 0.32 293 10459 July 3.8 27.6 34.5 22.9 80.7 96.3 50.4 1.03 2.90 0.36 211 94910 July 19.3 27.7 34.1 23.0 79.7 95.8 53.7 1.05 3.09 0.31 239 99011 July 12.4 26.2 32.9 21.9 84.4 96.6 57.2 1.17 3.98 0.20 201 95112 July 1.5 25.2 31.7 21.8 84.3 96.8 60.8 1.23 3.47 0.22 162 832

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29 Oct. 1 Nov. 4 Nov. 7 Nov. 10 Nov. 13 Nov. 16 Nov. 19 Nov. 22 Nov. 25 Nov.

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Figure 13. Daily average ETc with the error bars among the three lysimeters and the PM ETo at Citra, Florida, from 29 Oct. to 27 Nov. 2003.


CONCLUSIONThree weighing lysimeters with 2.32 m2 surface area and

1.37 m deep were developed for evapotranspiration researchat the University of Florida and located near Citra, Florida.The system includes double tanks and four load cells in eachlysimeter. The cost of construction materials and installationof the three lysimeters was $63,443. The design, construc-tion, installation, operation, calibration, and performance ofthe lysimeters were discussed.

Initial results from the three lysimeters provided aconsistent hourly ETc measurement over a five-day period inthe wet summer season with an accuracy of 0.29 mm and amaximal ETc variability of 0.41 mm/h among the threelysimeters, although many field activities and precipitationevents occurred. The hourly average ETc values in thelysimeters were statistically similar to PM ETo using theweather data from the nearby the weather station.

There are numerous challenges for the design andoperation of lysimeters in the southeastern U.S. Daily rainfallduring the rainy season requires routine pumping. Extremeevents, including hurricanes, may result in the water tablereaching the soil surface. A unique foundation system to limitthe impact of near-surface water table was designed andconstructed. Analysis of longer periods will be required toassess the impact of annual water cycles, including dry andwet, on the lysimeter performance.

ACKNOWLEDGEMENTS

The authors wish to acknowledge George W. Triebel,Danny Burch, Larry Miller, and Steve Feagle for support andassistance with project. This research was supported by theFlorida Agricultural Experiment Station and a grant from theSt. Johns River Water Management District and approved asJournal Series No. R-10978.

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ASCE-EWRI. 2005. The ASCE standardized referenceevapotranspiration equation. ASCE-EWRI Standardization ofReference Evapotranspiration Task Committee Report. NewYork, N.Y.: ASCE.

Barani, G. A., and M. J. Khanjani. 2002. A large electronicweighing lysimeter system: Design and installation. J. AmericanWater Resources Assoc. 38(4): 1053-1060.

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Carlisle, V. W., F. Sodek, M. E. Collins, L. C. Hammond, and W. G.Harris. 1989. Characterization data for selected Florida soils.Soil Science Research Report 89-1. Gainesville, Fla.: Universityof Florida, Institute of Food and Agricultural Sciences.

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