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IntroductionIrrigation is required for profitable commercial potato production in Idaho.

Maximum economic return requires, among other things, that soil watercontent be maintained within rather narrow limits throughout the growingseason. Potatoes are often considered to be a high water use crop, when infact many other crops grown in Idaho have equal or greater seasonal wateruse requirements. This misconception arises from the fact that potatoes aresensitive to water stress compared to most other crops, have a relativelyshallow root-zone depth and are often grown on soils with low to mediumwater holding capacities. These conditions necessitate that reliable irrigationsystems capable of light, frequent, uniform water applications be used tooptimally control soil water availability throughout the growing season. Theseconditions also dictate that an effective potato irrigation management pro-gram include (i) regular quantitative monitoring of soil water, (ii) schedulingirrigations according to crop water use and soil water holding capacity, and(iii) a water supply and irrigation system that is capable of providing theneeded irrigation on schedule.

The sensitivity of potato yield to irrigation management is depicted infigure 1. The results were obtained from a 1995 research study of watermanagement practices on 45 commercial potato fields in southeast Idaho(Stark, 1996). Potato yield is reduced by both over- and under-irrigation. Amere 10 percent deviation from optimum water application for the growingseason may begin to decrease yield. This sensitivity to water management isattributable to the sensitivity of potato plants to water stress, coupled withvery little buffering of the soil-plant system to water management errorsresulting from limited soil water storage. Yield reductions due to over-irrigation can be attributed to poor soil aeration, increased disease problems,and leaching of nitrogen from the shallow crop-root zone. Efficient irrigationmanagement can increase marketable yield while reducing production costsby conserving water, energy, and nitrogen fertilizer, as well as reducingpotential ground water contamination. Efficient irrigation management is aprerequisite for consistent maximum economic return from commercialpotato production in Idaho.

Potato DevelopmentGrowth Stages The physiological development of the potato can be dividedinto five growth stages. These growth stages are shown in figure 2 in relationto typical curves representing leaf area index (LAI), root-zone depth, and

byBradley A. KingandJeffrey C. Stark

PotatoIrrigation

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M A N A G E M E N T

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tuber yield. Leaf area index is thedimensionless ratio of leaf surfacearea to ground surface area. Growthstage I spans the period of plantingto emergence and ranges from 20 to35 days depending upon varietaldifferences, cultural practices, andenvironmental conditions. Growthstage II encompasses early vegeta-tive development from emergence totuber initiation and ranges from 15 to25 days depending upon the site-specific conditions. Stolons begin todevelop during growth stage II, buttubers are not yet present. Tubersform at the tips of the stolons over a10-to-14-day period, which is called“tuberization” or tuber initiation andrepresents growth stage III. Duringthis growth stage, the LAI is gener-ally in the range of 1 and 2, whichcorresponds to 50-80 percent rowclosure depending upon site-specificconditions and variety.

Tuber enlargement or “bulking”occurs largely throughout growthstage IV. The increase in tuber sizeis approximately linear with time overa 30-to-60-day period under optimalenvironmental conditions. Near theend of growth stage IV, LAI reachesa maximum range of 3.5 to 6.0,depending upon variety and environ-mental conditions (Wright and Stark,1990). Water use or “transpiration”by the potato plant also reaches amaximum at this time. Near the endof growth stage IV, the growth rate ofthe canopy begins to decline.

During growth stage V, plantsbegin to die and lose leaves. Tubergrowth rates decline as the result ofreduced leaf area and photosyn-thetic activity, and tuber skins beginto mature. The remaining tubergrowth results primarily from translo-cation of plant materials from stem,leaf, and roots to the tubers.

Root System Potato plant rootsystem development is relatively

Figure 1. Total tuber yield as influenced by the difference between irrigation andevapotranspiration (ET) on 45 commercial potato fields in southeasternIdaho.

Figure 2. Generalized seasonal progression of rooting depth, leaf area index (LAI),and tuber yield of potato.

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thetic products (starch and sugars)by the plant and their translocationfrom the leaves to the tubers. Potatoyield and quality depend uponmaximizing the steady accumulationof photosynthetic products in thetubers. When production of theseproducts exceeds that needed forrespiration and continued plantgrowth, they are stored in the tubers.

One of the first physiologicalresponses affected by plant waterdeficits is the expansion of leaves,stems, and tubers. Water deficitsreduce plant growth by reducing theinternal water pressure in plant cells(turgor pressure), which is necessaryfor expansion. Reduced vine andleaf growth limits total photosyntheticcapacity, while the reduced rootdevelopment limits the plant’s abilityto take up water and nutrients. Waterdeficits also disrupt normal tubergrowth patterns by reducing orstopping tuber expansion. Tubergrowth resumes following relief ofplant water deficits, but the disrup-tion of the normal tuber expansionrate may result in tuber malforma-tions such as pointed ends, dumb-bells, bottlenecks, and knobs. Widelyfluctuating soil water contents createthe greatest opportunity for develop-ing these tuber defects. Growthcracks are also associated with widefluctuations in soil water availabilityand corresponding changes in tuberturgidity and volume of internaltissues.

Potatoes are particularly sensitiveto water stress during tuber initiationand early tuber development. Waterdeficits at this time can substantiallyreduce U.S. No. 1 yields by increas-ing the proportion of rough, mis-shapen tubers. Early-season waterstress can also reduce specificgravity and increase the amount oftranslucent end.

Water stress during tuber bulkingusually affects total tuber yield more

than quality. A large photosyntheti-cally-active leaf surface area isnecessary to maintain high tuberbulking rates for extended periods.Maintenance of this large active leafsurface area requires continueddevelopment of new leaves toreplace older, less efficient ones.Water stress hastens leaf senes-cence and interrupts new leafformation, resulting in an unrecover-able loss of tuber bulking.

Soil water content at harvest hasa significant influence on mechanicaldamage sustained by tubers duringthe harvesting process. Tubers thatare dehydrated as a result of low soilwater content at harvest are moresusceptible to blackspot bruise.Tubers that are turgid as a result ofhigh soil water content at harvest aremore susceptible to shatter bruiseand thumbnail cracking.

Potato yield and quality aresusceptible to excess soil water aswell. Excess soil water from frequentor intensive irrigation or rainfallduring any growth stage leachesnitrate nitrogen below the plant rootzone, potentially resulting in nitro-gen-deficient plants, reduced fertil-izer use efficiency, and an increasedhazard to ground water. Saturationof the soil profile for more than 8-12hours can cause root damage due toa lack of oxygen required for normalrespiration. Excess soil water atplanting promotes seed piece decayand delays emergence due todecreased soil temperature. Pota-toes that are over-irrigated duringvegetative growth and tuber initiationhave a greater potential for develop-ing brown center and hollow heart,and are generally more susceptibleto early die problems. Excess soilwater can also lead to tuber qualityand storage problems.

Irrigation ManagementThe coarse-textured soils and

shallow, 18-24 inches, with themajority of roots in the surface 12inches. The shallow rooting depth isattributed to the inability of itsrelatively weak root system topenetrate tillage pans or otherrestrictive layers. Soil compaction byfield vehicle traffic can greatly restrictpotato root penetration. Soil watercontent at the time of tillage opera-tions has a major influence on thedegree of compaction resulting fromfield traffic.

Many soils in Idaho have aweakly cemented calcium carbonatelayer within 24 inches of the soilsurface, which restricts potato rootpenetration but not water movement.Field determination of actual potatoplant rooting depth is of primaryimportance to proper irrigationmanagement.

Potato Growth and Soil WaterAvailability The sensitivity ofpotatoes to plant water stress islikely due to their rather shallow rootsystem and complex physiologicalresponses to moderate plant waterdeficits (Curwen, 1993). The firstphysiological response is closure ofthe leaf stomata: the small pores inthe leaf that control gas exchangebetween internal leaf cells and theenvironment. Evaporation of waterfrom within the leaves serves to coolthe leaves, resulting in a plantcanopy temperature below airtemperature under well-wateredconditions. The stomata in the leafclose under plant water deficits as adefense against further water loss.The physical indication is an in-crease in canopy temperature as aresult of reduced evaporative coolingof the leaves.

While stomatal closure reduceswater loss through the leaves, it alsoreduces carbon dioxide diffusion intothe leaf. This slows photosynthesis,reducing the production of photosyn-

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hot, dry summers that are character-istic of southern Idaho make irriga-tion essential for producing reliable,economically sustainable potatoyields. The purpose of irrigationmanagement is to maximize potatoyield and quality by maintaining soilwater content within specified limitsthroughout the growing seasonthrough timely and controlled waterapplication to the crop.

Optimum Soil Moisture ContentMany field research studies havefocused on determining optimum soilwater content for irrigated potatoproduction. Most studies on thewater stress sensitive RussetBurbank variety indicate that avail-able soil water (ASW) in the rootzone (0-18 inches) should bemaintained above 65 percent toavoid yield and quality losses.Results from research studies usingirrigation frequencies of one to threedays on silt loam soils have shownthat intermittent ASW levels below65 percent may not reduce tuberyield and quality. In general, how-ever, the average ASW of the rootzone should be maintained between65 and 85 percent during the activegrowth period for optimum results. Inpractice, ASW in the root zone willfluctuate above and below this rangefor short periods of time immediatelybefore and after irrigation. This isparticularly true with set-movesprinkler systems and furrow irriga-tion systems. Drip irrigation systemsand solid-set, center-pivot, andlinear-move sprinkler systems allowfor light, frequent irrigations and canbe managed to minimize soil waterfluctuations.

The optimal range for watercontent at planting is about 70 to 80percent ASW. This soil water levelwill provide ideal conditions forplanting and early sprout develop-ment. Excessively wet soil conditions

may slow soil warming and delaysprout development and emergence.Cool, wet soil conditions can in-crease seed piece decay andincrease physiological aging of seed,resulting in higher stem and tubernumbers. Excessively dry soilsshould be irrigated prior to plantingto avoid potential seed piece decayproblems that sometimes result fromirrigating between planting andemergence.

During the latter part of thegrowing season (growth stage V)plants begin to senesce and cropwater use rates markedly decrease.Consequently, care should be takento adjust irrigation amounts to avoiddeveloping excessively wet soilconditions. High soil water contentsduring this period can produceenlarged lenticels that provideopenings for soft rot bacteria to enterthe tubers. Pink rot and PythiumLeak infections are also increasedby excessive late-season soil water.

Available soil water should beallowed to decrease to about 60 to65 percent at vine kill to provideoptimal conditions for promotingtuber skin set and russeting. Driersoil conditions at vine kill increasethe chances of developing stem-enddiscoloration.

Pre-harvest irrigation should betimed to optimize soil conditions andtuber hydration levels at harvest.Tubers that have matured underrelatively dry soil conditions (lessthan 60 percent ASW) will likely bedehydrated, which will increase theirsusceptibility to blackspot bruise.Under these conditions, fields shouldbe irrigated at least one week priorto harvest to completely rehydratetubers. If ASW has been kept above60 percent during tuber maturation,fields can be irrigated two to threedays prior to harvest. Care shouldalso be taken to avoid getting fieldstoo wet at harvest because of

increased potential for shatterbruise, greater difficulty separatingsoil from tubers, and storage rotproblems.

Soil Water Holding Capacity Soilserves as the reservoir for plantnutrient and water needs. Soil has afinite capacity to hold water againstgravity, which is called water holdingcapacity. A graphical representationof how water is held in soil is shownin figure 3. A given volume of soilconsists of solids composed ofminerals and organic matter andpores filled with air and water. Whenthe soil pores are completely filledwith water, the soil is said to besaturated (figure 3a). Under condi-tions of free drainage, the force ofgravity will drain water from thelargest pores. This free-drainingwater is called gravitational water.After 12 to 48 hours, drainage willdecrease to a rather negligible rate.The soil water content at this point iscommonly called field capacity orupper drained limit (figure 3b). In thepresence of an active root systemand actively transpiring plants, someof the gravitational water will beutilized by the plant, reducing theactual volume of drainage below thecrop-root zone, effectively providingshort term soil water storage.Irrigation should not be managed toproduce gravitational water, butrather should only be applied inamounts necessary to bring root-zone soil water content back to fieldcapacity.

Water is held in the soil as a filmaround soil particles by molecularattraction and by water surfacetension forces producing what iscommonly called capillary action.Hence, water held in soil pores iscalled capillary water (figure 3c) andis available for plant use. As plantsremove water from the soil, water isextracted from progressively smaller

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a b

dc

GravitationalWater Field Capacity

Capillary Water PermanentWilting Point

Figure 3. Graphical representation ofsoil water states.

example, if the volumetric soil watercontent of a silt loam soil with a bulkdensity of 1.37 g/cm3 is 32 percent,soil water content on a weight basisis then 23.4 percent (32÷1.37=23.4).Soil water content measured on avolumetric basis is preferred forirrigation management computationsbecause bulk density of the soil isnot required. Soil water content usedin this publication is expressed on avolumetric basis. General soil watercontents at critical points, along withwater holding capacities for agricul-tural soils, are given in table 1.Inspection of available water listed intable 1 reveals that soils having asignificant portion of silt have thegreatest water holding capacities,offering the greatest flexibility inpotato irrigation management.

Evapotranspiration and YieldEvapotranspiration (ET) representsthe sum of water used by plants fortranspiration and water loss due toevaporation from plant and soilsurfaces. Evapotranspiration variesaccording to meteorological condi-tions, plant and soil surface wetness,crop type, soil water content, andamount of crop cover (LAI). Themeteorological parameters thataffect ET are solar radiation, relativehumidity, ambient air temperature,and wind speed. Since these canvary considerably from day-to-day,so will ET. Furthermore, seasonal ETwill vary from year-to-year in re-sponse to annual meteorologicaltrends.

Evapotranspiration for potatoesthroughout the 1993 and 1994growing seasons for three locationsin southern Idaho (Parma, TwinFalls, and Rexburg) are shown infigure 5. The dependence of ET onmeteorological conditions is evidentby the variation in daily ET through-out the growing season. The 1993growing season had a cool, wet

agement. Therefore, the energy perunit volume required to extract waterfrom soil at the permanent wiltingpoint is usually accepted to be 1500kPa (15 bars). Since energy input isrequired to extract water from thesoil, the energy potential of water inthe soil at permanent wilting point is-1500 kPa (-15 bars). Energyrequired to extract water from soil atfield capacity is not as well definedbecause coarse-textured soils reachfield capacity (negligible drainage) ata higher energy status than soilsconsisting of finer particles. Thus,coarse-textured soils with largervoids have a higher soil waterenergy potential at field capacitythan silt-loam-textured soils withsmaller voids. The energy potentialof soil water at field capacity isusually -20 kPa (centibars) for siltloam and clay soils, and -10 kPa(centibars) for sandy soils with agradual transition between them.

Each soil has a unique relation-ship between soil water content andsoil water energy potential called thesoil water release curve. Thisrelationship, which is highly depen-dent on soil texture, is showngraphically in figure 4 for four soiltextures. The rather flat curve of atypical loamy sand soil indicates anarrow range in water contentbetween field capacity and perma-nent wilting point, indicating lowwater holding capacity. In contrast,the sloping curve of the silt loam soilhas a much wider range in soil watercontent between permanent wiltingpoint and field capacity, indicatinggreater water holding capacity.

Soil water content is often ex-pressed as a percentage on either aweight or volumetric basis. Caremust be taken to make sure whichwater content basis is used. Conver-sion between the two requiresknowledge of soil bulk density, whichis dry soil mass per unit volume. For

pores until the remaining waterexists as a film around soil particles.The attraction of soil particles to thisthin film of water is so strong that agreat amount of energy is required toremove the remaining water from thesoil–to the degree that plants cannotobtain water and consequently wiltand die. The soil water content atthis point is called the permanentwilting point and is graphicallyillustrated in figure 3d. The volume ofwater held in the soil between fieldcapacity and permanent wilting pointis called available water. It is com-monly expressed as inches of waterper inch or foot of soil depth andreferred to as the water holdingcapacity of the soil.

The actual field soil water contentfor field capacity and permanentwilting point depend upon manyfactors (e.g. soil structure, soiltexture, crop, etc.) and need to bedetermined by field and laboratorytesting procedures. However, thesomewhat vague definitions of fieldcapacity and permanent wiltingpoint, coupled with field spatialvariability, make precise determina-tion impractical for irrigation man-

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spring and a cool summer, whichresulted in potato quality problemsacross the state that were attributedto excessive soil water and cool soiltemperatures in growth stages II andIII. The 1994 growing season had awarm spring and an unusually hotsummer, which resulted in manyirrigation systems having difficulty inmeeting crop water use. The 1993and 1994 growing seasons likelyrepresent the near extremes inseasonal ET for potatoes in southernIdaho.

Differences in the start, peak, andend of daily ET values shown infigure 5 for the three locations aredue to differences in planting andharvest dates, and seasonal meteo-rological conditions. Daily ETthroughout the season is noticeablyreduced for the Rexburg areacompared to the other locations dueto the cooler average daily tempera-tures throughout the growing seasonattributable to geographical location.Published daily ET values, as shownin figure 5, provide a basis to de-velop an irrigation managementprogram. In-field soil water measure-ment is also required to account forsite-specific differences in ET basedon type of irrigation system used,soil type, and local meteorologicalconditions such as wind andprecipitation.

The typical response of potatoyield, total and U.S. No. 1, to totalseasonal water application (includingprecipitation) is shown in figure 6 forthree potato cultivars at Kimberly,Idaho (Wright and Stark, 1990). Yieldis linearly related to seasonal waterapplication. Yield decreases whentotal seasonal water applicationexceeds seasonal evapotranspira-tion. The differences in total waterapplication that maximize yieldreflects the differences in growingseason length between the cultivarsand not necessarily differences in

Figure 4. Soil water release curves for four soil textures common in agriculture.

Table 1. Soil water contents for agricultural soils.

Soil Water Content on Volumetric Basis (%)

Permanent Water HoldingField Capacity Wilting Point Available Water Capacity (in/ft)

Texture Class Average Range Average Range Average Range Average Range

Sand 12 7-17 4 2-7 8 5-11 0.96 0.60-1.32

Loamy Sand 14 11-19 6 3-10 8 6-12 0.96 0.72-1.44

Sandy Loam 23 18-28 10 6-16 13 11-15 1.56 1.32-1.80

Loam 26 20-30 12 7-16 15 11-18 1.80 1.32-2.16

Silt Loam 30 22-36 15 9-21 15 11-19 1.80 1.32-2.28

Silt 32 29-35 15 12-18 17 12-20 2.04 1.44-2.40

Silty Clay Loam 34 30-37 19 17-24 15 12-18 1.80 1.44-2.16

Silty Clay 36 29-42 21 14-29 15 11-19 1.80 1.32-2.28

Clay 36 32-39 21 19-24 15 10-20 1.80 1.20-2.40

(Source: Jensen et al., 1990.)

daily ET rates. The seasonal waterapplication that maximizes yield willvary year-to-year in response tometeorological trends and differ-ences in growing season length. Thetrend in yield response to waterapplication observed under con-

trolled research conditions (figure 6)is similar to that observed undercommercial field conditions (figure 1).

Irrigation MethodPotatoes can be grown with all

types of irrigation, however, some

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are better suited than others forconsistently obtaining high qualitytubers. The water sensitive nature ofpotatoes, combined with its shallowroot zone, favors irrigation systemsthat are capable of light, frequent,and uniform water applications.Using these criteria as a basis forranking the suitability of commonirrigation methods, the order ofpreference from highest to lowestwould be: drip, solid-set (portable),linear-move, center-pivot, side-roll,hand-move, and furrow. In practice,economics are the overriding factorin irrigation system selection. Com-patibility with soil type, crop rotation,and cultural practices are alsoimportant considerations. Buried dripis expensive, incompatible withconventional potato productionpractices, and is not suitable forestablishing stands of some cropscommonly grown in rotation withpotatoes, especially in coarse-textured soils. Solid-set portable isexpensive, as is linear-move.Center-pivots are highly susceptibleto excessive runoff under the outertowers unless conservation tillagepractices, such as basin or reservoirtillage, are utilized. Side-roll andhand-move sprinkler systems areprone to wind skips under the windyconditions common to southernIdaho. Furrow irrigation is suscep-tible to poor water applicationuniformity, excessive deep percola-tion, and leaching. Sprinkler irriga-tion is the most common methodused for potatoes in Idaho, withcenter-pivot, side-roll, and hand-move being widely used.

Irrigation SchedulingPotato irrigation scheduling for

maximum profit requires that thetiming and amount of water applica-tion be determined and applied tominimize soil water fluctuationsthroughout the growing season.

Figure 5. Daily evapotranspiration for potatoes at three locations in southern Idahothroughout 1993 and 1994 growing seasons.

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Successful irrigation managementrequires regular quantitative monitor-ing of soil water and knowledge offield crop water use, soil waterholding capacity, and crop-rootingdepth. Excess irrigation usuallyresults from applying too much waterat a given irrigation rather than fromirrigating too frequently. This isparticularly true for side-roll andhand-move sprinkler systems wheresoil water holding capacity and crop-rooting depth are frequently overesti-mated; and furrow irrigation, whereapplication depth is difficult tocontrol. These situations lead toplant water stress when soil waterfalls below acceptable limits two tothree days before irrigation, andsubsequent irrigation applicationsare in excess of soil water storagecapacity. This characteristic problemcan generally be attributed toinadequately designed systems,irrigation system equipment limita-tions, or improper irrigation manage-ment.

Determining the appropriatetiming of irrigations usually involvesthe use of daily ET estimates basedon local meteorological data tomaintain a daily soil water balancethroughout the irrigation season.This technique, combined withperiodic quantitative measurementsof soil water to adjust the computedsoil water balance to actual fieldconditions, provides a cost effectivemeans for determining the timing ofirrigations. This approach has theadded benefit of implicitly determin-ing the irrigation application amountas well. The computational mechan-ics of the soil water balance ap-proach are provided in the publica-tion Irrigation Scheduling UsingWater-Use Tables, CIS 1039,University of Idaho, College ofAgriculture. The basic steps involvedare:

Figure 6. Total and U.S. No. 1 tuber fresh yield as influenced by total seasonal waterapplication for Russet Burbank (RB), Kennebec (K) and Lemhi Russet (LR).(Source: Wright and Stark, 1990.)

1 Estimate field capacity andpermanent wilting point basedon the predominate soil texturein the field.

2 Estimate current crop-rootingdepth.

3 Maintain a daily soil waterbalance based on publishedvalues of ET.

4 Irrigate when daily soil waterbalance approaches 65 to 70percent ASW, applying the netamount required to increase thesoil water content to fieldcapacity or less in the case oflight, frequent irrigation.

5 Periodically monitor soil watercontent or soil water potentialand adjust the daily soil waterbalance to match actual fieldconditions.

Several methods are available toquantitatively measure soil watercontent. Only some are suitable forpotatoes, however, because of thecritical threshold level of availablesoil water and the limited root-zonedepth. Many of the methods are

labor intensive and require training,experience, and expensive equip-ment. This has led to the develop-ment of crop consulting firms spe-cializing in irrigation management,which often provide crop nutrient andpest management services as well. Adetailed discussion of soil watermeasurement methods is provided inthe publication Soil Water Monitoringand Measurement, PNW 475,University of Idaho, College ofAgriculture.

Tensiometers have been used tosuccessfully monitor soil wateravailability in potato fields. Goodcontact between the soil and tensi-ometer tip is essential for properoperation. Tensiometers are ofteninstalled in the potato hill at twodepths, such as 8 and 16 inchesbelow soil level. Typically, the uppertensiometer is used to track soilwater potential within the bulk of theroot zone, while the lower one isused to determine whether soil waterpotential at the bottom of the rootzone is increasing or decreasingover time.

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Figure 7. Generalized soil water release curve for sand andloamy sand soils.

Figure 8. Generalized soil water release curve for sandy loamand loam soil.

Figure 9. Generalized soil water release curve for silt loamand silt soil.

Figure 10. Generalized soil water release curve for silty clayloam and silty clay soil.

The neutron probe is likely themost precise and reliable tool for soilwater measurement since it deter-mines volumetric soil water content.However, licensing, training, andassociated operational costs limittheir use to consulting firms andlarge farms. Time domainreflectometery (TDR) offers manyfeatures that make it well suited to

soil water measurement in potatoes.However, the initial equipment costis quite high. Current researchefforts to develop less expensiveTDR units may make it the methodof the future. Other methods are alsoavailable and may be suitable.

A soil water release curve isneeded to relate soil water potentialto volumetric soil water content. The

generalized soil water releasecurves shown in figures 7 through 10can be used to relate soil waterpotential to volumetric soil water,ASW and water deficit. These curvesrepresent the primary soil waterrelationships needed for the devel-opment of an effective irrigationmanagement program. They allowsoil water content or water potential

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Table 2. Soil water potential and volumetric water content ranges correspondingto 65 percent ASW.

Soil Water Soil WaterPotential Content

Soil Texture (kPa) (% by volume)

Sand, Loamy Sand -25 to -35 9-12

Sandy Loam, Loam -35 to -50 19-22

Silt Loam, Silt -50 to -65 24-26

Silty Clay Loam, Silty Clay -65 to -75 29-31

Table 3. Center-pivot irrigation scheduling example.

Computed MeasuredSoil Soil

Net Water WaterET Rainfall Irrigation Deficit Deficit

Date (inches) (inches) (inches) (inches) (inches) Comments

7/9 .29 — — 0.91

7/10 .31 — 0.5 0.62 0.81 Adjust deficit to 0.8, irrigate

7/11 .27 — — 0.89

7/12 .25 — 0.5 0.64

7/13 .22 — — 0.86

7/14 .18 .05 0.5 0.54 Neglect rainfall <0.1”

7/15 .25 — — 0.79 0.9 Adjust deficit to 0.9, irrigate

7/16 .31 — 0.5 0.71

7/17 .28 — 0.5 0.49

Table 4. Set-move sprinkler irrigation scheduling example.

Computed MeasuredSoil Soil

Net Water WaterET Rainfall Irrigation Deficit Deficit

Date (inches) (inches) (inches) (inches) (inches) Comments

7/9 .29 — — 1.25

7/10 .31 — 1.41 0 1.1 Adjust deficit to 1.1 in, irrigate 9.4 hrs

7/11 .27 — — 0.27

7/12 .25 — — 0.52

7/13 .22 — — 0.71

7/14 .18 .05 — 0.92 Neglect rainfall <0.1”

7/15 .25 — — 1.17

7/16 .31 — 1.56 0 1.25 Adjust deficit to 1.25 in, irrigate 10.4 hrs

7/17 .28 — — 0.28

measurements to be used to calcu-late the net irrigation applicationamount needed to fill the soil waterreservoir to field capacity. Forexample, if tensiometers show anaverage soil water potential of -40kPa (centibars) on a sandy loamsoil (figure 7), then ASW is 62percent, which indicates it’s time toirrigate with a net application of 0.36in/ft of crop root-zone depth. Soilwater monitoring alone can be usedfor irrigation scheduling if performedon a real-time basis and used todirectly control an irrigation systemcapable of immediate response. Inpractice though, most field scaleirrigation systems are not capable ofimmediate response. Thus, a soilwater balance is computed dailyusing both estimated and forecasteddaily ET to anticipate when the nextirrigation should occur and amountto apply. This computed soil waterbalance is reconciled to actual fieldconditions through use of the soilwater release curve and quantitativesoil water measurement.

The range of soil water potentialand volumetric soil water content inthe potato root zone at which timeirrigation should occur to maintainASW above 65 percent is shown intable 2. These values are obtainedfrom the generalized soil waterrelease curves shown in figure 7through 10. These values are notabsolute, but serve as a generalguide for effective irrigationmanagement.

An example of irrigation schedul-ing for a center-pivot system over anine day period in July is outlined intable 3. On the morning of 7/10, theaverage reading for several tensiom-eter locations aligned radiallyoutward from the center of a center-pivot irrigated potato field is -40 kPa.The predominate soil texture issandy loam, which at -40 kPa has anASW of 67 percent and water

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depletion of 0.54 inches per foot,based on the generalized soil waterrelease curve (figure 8). The totalsoil water deficit is 0.81 inches for an18-inch effective crop-root zone. Thecomputed soil water deficit, basedon estimated daily ET ending on 7/9,is 0.91 inches. Since it is greaterthan the measured value, an adjust-ment is necessary. The actual soilwater deficit is greater than the netapplication of 0.5 inches from asingle 36-hour center-pivot rotation,so irrigation should be continued.The soil water balance is maintainedthrough 7/14 using estimated dailyET. Precipitation amounts less than0.1 inches are neglected whencomputing the daily soil water deficit,but are effectively accounted for byperiodic adjustment to measured soilwater deficit. On the evening of 7/15,the average tensiometer reading is -45 kPa, which translates to an ASWof 64 percent and total soil waterdeficit of 0.9 inches. This is greaterthan the computed soil water deficitof 0.79 inches and greater than the0.5 inch net application from a singleirrigation. Therefore, the soil waterbalance should be adjusted to actualfield conditions and irrigation shouldbe continued. This process contin-ues throughout the growing seasonand when the actual or computedsoil water deficit is less than the netapplication by a difference of ap-proximately the daily ET, thenirrigation should be discontinueduntil the computed or measured soilwater deficit is greater than the netapplication depth.

An example of irrigation schedul-ing for a side-roll or hand-movesprinkler system over a nine dayperiod in July is shown in table 4.The irrigation system is designed toprovide a net irrigation applicationrate of 0.15 inches per hour with aminimum six days between irriga-tions based on two sets per day. On

the morning of 7/10, the averagereading for several tensiometersplaced so that they all receiveirrigation nearly the same day is -52kPa. The predominate soil texture issilt loam. Based on the generalizedwater release curve (figure 9), it hasan ASW of 67 percent and waterdepletion of 0.66 inches per foot fora total soil water deficit of 1.1 inchesfor a 20-inch effective crop-rootzone. The computed soil waterdeficit based on estimated daily ETending on 7/9 is 1.25 inches, whichis greater than the measured value,so an adjustment in the computeddeficit is necessary. The net irriga-tion application required for the dayis 1.1 inches plus one day’s ET ofapproximately 0.31 inches for a totalof 1.41 inches. The irrigation set timeis then 9.4 hours (1.41 inches/0.15in/hr). This set time is used through-out the remaining five-day period,unless significant rainfall occurs. Theirrigation set time should be reducedone hour for every quarter inch ofrainfall. The soil water balance ismaintained through 7/15.

On the morning of 7/16, theaverage tensiometer reading is -65kPa, which translates to an ASW of62 percent and total soil water deficitof 1.25 inches. This is slightlygreater than the computed soil waterdeficit of 1.17 inches, so the soilwater budget needs to be adjustedto actual field conditions. The netapplication required for the day is1.25 plus one day’s ET of approxi-mately 0.31 inches for a total of 1.56inches. The irrigation set time is then10.4 hours (1.56 inches/0.15 in/hr).This process is continued throughoutthe growing season and the irrigationset times are adjusted to match thewater use over the six-day irrigationinterval.

Irrigation System OperationalParameters The primary irrigation

system information needed forirrigation scheduling is net irrigationapplication amount or rate. Forcenter-pivot and linear-move irriga-tion systems, the net applicationamount is dependent upon systemcapacity, wet run time betweenirrigations, and system applicationefficiency. For side-roll, hand-move,and solid-set sprinkler systems, thenet application rate depends uponsprinkler spacing, flow rate, andapplication efficiency. Systemapplication efficiency is a measure ofhow much water exiting the irrigationsystem actually goes to fulfilling cropwater requirements. Water is lostdue to wind drift and evaporationunder sprinkler irrigation, and todeep percolation resulting from non-uniform water application with allirrigation systems. Typical irrigationsystem application efficiencies forIdaho are given in table 5.

The first step in calculating netirrigation application is to determinegross water application. Gross waterapplication depth per rotation for acenter-pivot irrigation system, as afunction of system capacity androtation time, is presented in figure11. System capacity in terms of gpm/acre is needed to use the curves infigure 11 and can be obtained fromthe system sprinkler packagespecifications or by dividing totalsystem flow rate by the acreageirrigated. Net application depth for 80percent application efficiency can beobtained directly from the right-sideaxis of figure 11. Net applicationdepth for any application efficiencycan be calculated as:

Example:

0.8 in 85% =Net

Depth x

100

0.68 in =

ApplicationEfficiency (%) =

NetDepth

GrossDepth x

100

12

Gross water application rates forset-move and solid-set sprinklersystems, as a function of sprinklerflow rate and spacing, are presentedin figure 12. Sprinkler flow rate canbe obtained from figure 13 forstraight-bore nozzles, as a functionof nozzle size and pressure. Netapplication rates for 70 percentapplication efficiency are given infigure 12 (right-side axis). Netapplication rate for any applicationefficiency can be calculated as:

Example:

Management Under LimitedWater Supply Many researchstudies have focused on investigat-ing the effects of water stress timingon tuber yield and quality. Whenwater resources are limited, the bestpractice is to schedule irrigations tocover the period from tuber initiationthrough mid-bulking, and selectcultivars that use less water and/orare less sensitive to water stress.Results from a few studies haveindicated that water stress can bestbe tolerated during the early vegeta-tive growth and late tuber bulking.Actual water stress effects on yieldand quality depend on ET rate, soilwater holding capacity, irrigationfrequency, crop growth stage, andcultivar.

Irrigation SystemManagementCenter-Pivot Sprinklers Center-pivot systems are generally notdesigned with sufficient capacity tomeet peak period daily water use.Instead, soil water-banking (buildingup soil water reserves in the root

zone) is used to supply a smallfraction of daily ET over the durationof the peak period. This allows forreduced system capacity, resulting insmaller pump size, lower electricaldemand charges, and reduced waterapplication rates. Water-banking isallowed because center-pivotsystems are capable of providinglight, frequent irrigations. It applies tolinear-move systems as well, but to areduced extent to account for dry runtime during repositioning. Water-banking can potentially be applied toany irrigation system capable oflight, frequent irrigations such as dripand solid-set sprinklers. The degreeto which water-banking can beutilized is directly proportional to soilwater holding capacity and crop-rooting depth. Potatoes grown oncoarse-textured soils having waterholding capacities less than 1 inchper foot do not allow for water-banking and must have a net systemcapacity equal to peak daily ET. For

example, if peak ET is 0.34 in/daythen the net system capacity mustbe 6.4 gpm/acre [0.34 in/day x 18.86(gpm/acre)/(in/day)] or a grosssystem capacity of 7.5 gpm/acre, ifapplication efficiency is 80 percent.

Center-pivot systems that utilizewater-banking must be managed toensure that the soil water reservoir isfull at the beginning of the peakwater use period. This requiresplanning and field soil water monitor-ing to the full depth of the crop-rootzone. Failure to do so will likelyresult in crop water stress near theend of the peak-use period; theextent depends on soil and climaticconditions. The timing of the peak-use period varies season-to-season.For example, a center-pivot systemoperating in 1994 near Twin Falls,having a net system capacity of 0.28inches per day, would need to havebeen managed so that the soil waterreservoir was full by 6/20 when ETbecame greater than 0.28 in/day

Figure 11. Center-pivot application depth as a function of system capacity androtation time (x-axis) for system capacities ranging from 5 gpm/acreto 8 gpm/acre.

0.154 in/hr0.22 in/hr 70% =Net

ApplicationRate

x

100

=

ApplicationEfficiency (%)

NetApplication

Rate =

GrossApplication

Rate x

100

13

However, in the second case, lowASW values fall below recom-mended limits, resulting in periodicplant water stress. When this occursthere is no corrective course ofaction as system capacity is fixed.The ultimate tuber yield and qualitydepend upon the seasons climaticconditions as they drive daily ET.

The natural tendency is to speedthe center-pivot system up whencrop water deficits (stress) develops.This action only serves to reduceapplication efficiency because ET isincreased by evaporation from wetsoil and vegetation. Increasing thespeed of a center-pivot produceslighter applications and more fre-quent wetting of the soil and plantcanopy, thereby increasing the totalamount of water lost to evaporationand decreasing the amount stored inthe soil. Thus, system speed shouldremain the same or be reducedwhen crop water deficits (stress)develop. This will increase irrigationefficiency by storing a greaterpercentage of water applied in thesoil.

Minor changes in applicationefficiency can result in a significantdifference in center-pivot systemperformance. A 3 to 8 percentdifference in application efficiencywill occur between nighttime anddaytime irrigation, resulting indifferences in soil water storage. Asa result, center-pivot speed shouldbe adjusted so that rotation time isnot a multiple of 24 hours. Other-wise, areas of the field consistentlywatered during the daytime will have3 to 8 percent less water stored inthe soil for crop use. This smalldifference accumulated over timecan result in water stressed areaswithin the field.

Conservation tillage practices,such as basin or reservoir tillage, arerequired to achieve optimum infiltra-tion uniformity with potatoes under

Figure 13. Sprinkler discharge as a function of straight bore nozzle diameter (x-axis)and pressure in psi (lines).

Figure 12. Set-move and solid set sprinkler application rate as a function of sprinklerdischarge (x-axis) and sprinkler spacing in feet (lines).

(figure 5). However, a full soil waterreservoir was not necessary in 1993,since peak ET seldom exceeded0.28 in/day (figure 5). Figure 14depicts available soil water through-out the irrigation season for a center-pivot system that is managed so that

soil water is replenished to fieldcapacity (100 percent ASW) early inthe season (figure 14a), compared toonly 85 percent ASW (figure 14b).Under both scenarios, the character-istic gradual drawdown of ASWoccurs during the peak-use period.

14

center-pivot irrigation. The hilling ofpotato plants causes water toconcentrate in the furrow under highapplication rates and when com-bined with even slight slopes willcause runoff. Water collects in lowareas causing excessive infiltration,while up-slope areas have reducedinfiltration and become waterstressed. The field-scale cumulativeeffect results in reduced yield andquality, lower water and nutrientefficiency, and localized leaching ofchemicals from the root zone.

Set-Move Sprinklers Side-roll andhand-move sprinkler systems arenormally designed to deplete soilwater storage between irrigationsduring the peak-use period. Thus,soils with greater soil water storageallow for longer irrigation intervals,reducing equipment and capitalcosts.

This characteristic operatingprincipal is contrary to the need tominimize soil water fluctuations foroptimum tuber yield and quality.Typical potato irrigation managementproblems that occur with set-movesprinkler systems include schedulingirrigations too far apart and applyingmore water than the root zone canhold. This may be a result of overestimating soil water holding capac-ity and crop-rooting depth, or aninsufficient number of sprinklerlaterals requiring too many days totraverse the field. The maximumirrigation interval can be calculatedas:

Maximum irrigation intervals basedon a peak ET of 0.33 in/day for

Table 5. Typical irrigation system application efficiencies.

Application Efficiency

System Type (%)

Surface Systems

Furrow 35-65

Surge 50-55

Cablegation 50-55

Sprinkler Systems*

Set-move 60-75

Solid-set 60-85

High pressure center-pivot 65-80

Low pressure center-pivot 75-85

Linear-move 80-87

Microirrigation

Drip 90-95

* Use lower efficiencies with larger spacing and windy conditions.(Source: Sterling and Neibling, 1994.)

Root-ZoneDepth

(ft)

Soil WaterHoldingCapacity

(in/ft) x =MaximumIrrigation

Interval Peak Daily ET (in/day)

+ 1 day irrigation time

x

AllowableDepletion

(0.35)

Table 6. Maximum irrigation interval for set-move sprinkler systems based on0.33 in/day peak ET plus one day irrigation time.

Texture Class Root-Zone Depth (inches)

14 16 18 20 22

Sand, Loamy Sand 2.2 2.3 2.5 2.7 2.9

Sandy Loam, Loam 3.1 3.4 3.7 4.0 4.3

Silt Loam, Silt 3.4 3.7 4.0 4.4 4.7

Silty Clay Loam, Silty Clay 3.2 3.5 3.9 4.2 4.5

different soil types and root-zonedepths are shown in table 6. Irriga-tion intervals in excess of five dayswill likely result in ASW levels below65 percent during the peak wateruse period, which adversely affectstuber yield and quality.

Furrow Irrigation Furrow irrigationof potatoes does not produce thetuber quality obtainable with other

types of irrigation, even with bestachievable management practices.Water is required to traverse the fieldby overland flow in the furrow. Thetime required for water to reach theend of the furrow leads to greaterwater application at the inflow endcompared to the outflow end,resulting from differences in infiltra-tion opportunity time. Furthermore,infiltration is highly variable, with

15

Figure 14. Available soil moisture throughout the growing season for potatoes undercenter-pivot irrigation for: (a) 100% and (b) 85% available soil moisture atthe beginning of the peak-use period.

applications to individual plantsranging from half to twice the fieldaverage (Trout et al., 1994). Thus,furrow irrigation cannot achieve thedegree of uniform water applicationneeded to produce consistently highquality tubers on a commercial field-scale basis.

A common furrow irrigationpractice for potatoes is to irrigatealternate furrows on each succes-sive irrigation in an attempt toovercome some of the difficulty inapplying small irrigation depths.Consequently, only about 15 percentof the soil surface is wetted andwater is expected to move upwardlaterally to wet the whole root zone.In the absence of a clay soil ordense soil layers, gravity causeswater to move faster downward thanlaterally. Thus, attempts to com-pletely wet the root zone to the top ofthe hill are usually unsuccessful andresult in excessive deep percolationlosses. The lateral water distributionproblem results in significant varia-tion in soil water contents, varyingwidely near the furrow and remainingdry on hilltops.

A consequence of the non-uniform water distribution betweenand along furrows is the widevariation in nitrogen availability dueto both dry soil regions and leachinglosses. This tends to further reducetuber quality under furrow irrigationand also reduces nutrient useefficiency.

These limitations have causedmany producers to abandon furrowirrigation in favor of sprinkler irriga-tion. A common approach is to utilizea completely portable sprinklerirrigation system for potatoes, whichcan be moved around the farmaccording to crop rotation, and usingfurrow irrigation for the other rowcrops. The advantages of highergross income and reduced risk withbetter tuber quality, and higher water

16

and nutrient use efficiency withsprinkler irrigation, usually justify theuse of sprinklers for potato produc-tion. The ability to inject fertilizersand pesticides through sprinklersystems provides another significantadvantage over furrow irrigation.

SummaryThe primary goal of potato

irrigation management is to minimizesoil water fluctuations and maintainavailable soil water within theoptimum range of 65-85 percent.Irrigation systems best suited to thistask are those that are capable oflight, uniform, and frequent waterapplications. An effective irrigationmanagement program must includeregular quantitative monitoring of soilwater availability, and schedulingirrigations according to crop wateruse, soil water holding capacity andcrop-rooting depth. Potatoes aremore sensitive to water stress thanmost other crops, have relativelyshallow root systems, and arecommonly grown on coarse-texturedsoils. These conditions dictateutilization of a quantitative potatoirrigation management program forconsistent, optimum economicreturn.

Literature CitedJensen, M.E., R.D. Burman, andR.G. Allen. 1990. Evapotranspirationand Irrigation Water Requirements.ASCE Manuals and Reports onEngineering Practice No. 70. Ameri-can Society of Civil Engineers, NewYork, NY.

Curwen, D. 1993. Water Manage-ment. In: R.C. Rowe (ed.) PotatoHealth Management. AmericanPhytopathological Society, St. Paul,MN. pp 67-75.

Stark, J.C. 1996. Information man-agement systems for on-farm potatoresearch. Potato research andextension proposals for cooperativeaction. Univ. of Idaho, College ofAgric. pp. 88-95.

Sterling, R., and W.H. Neibling.1994. Final Report of the WaterConservation Task Force. IdahoDepartment of Water Resources,Boise, ID.

Trout, T.J., D.C. Kincaid, and D.T.Westermann. 1994. Comparison ofRusset Burbank Yield and QualityUnder Furrow and Sprinkler Irriga-tion. American Potato Journal71:15-28.

Wright, J.L., and J.C. Stark. 1990.Potato. In: Stewart, B.A., and D.R.Nielsen (eds.) Irrigation of Agricul-tural Crops. Monograph No. 30.American Society of Agronomy,Madison, WI pp. 859-888.

Additional Sourcesof InformationAshley, R.O., W.H. Neibling, andB.A. King. 1996. Irrigation Schedul-ing Using Water-Use Tables. CIS1039. University of Idaho College ofAgriculture.

Clapp, R.B., and G.M. Hornberger.1978. Empirical Equations for SomeSoil Hydraulic Properties. WaterResources Research 1464:601-604.

Kleinkopf, G.E., D.T. Westermann,and R.B. Dwelle. 1981. Dry MatterProduction and Nitrogen Utilizationby Six Potato Cultivars. AgronomyJournal 73:799-802.

Ley, T.W., R.G. Stevens, R.R.Topielec, and W. H. Neibling. 1994.Soil Water Monitoring and Measure-ment. PNW 475. University of IdahoCollege of Agriculture.

Ojala, J.C., J.C. Stark, and G.E.Kleinkopf. 1990. Influence of Irriga-tion and Nitrogen Management onPotato Yield and Quality. AmericanPotato Journal 67:29-43.

AuthorsBradley A. King is an irrigationresearch engineer in the Biologicaland Agricultural Engineering Depart-ment and Jeffrey C. Stark is aresearch agronomist in the Plant,Soil, and Entomological SciencesDepartment, both at the University ofIdaho Aberdeen Research & Exten-sion Center.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with theU.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The Univer-sity of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, age, gender, disability, or

status as a Vietnam-era veteran, as required by state and federal laws.

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