CUP+ (Daily Soil Water Balance Program) PROGRAMME DU BILAN QUOTIDIEN SOL-EAU

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1 ICID 21 st International Congress on Irrigation and Drainage, ICID 21 st Otober Congress, 2011, Tehran, Tehran, Otober Iran 2011 CUP+ (Daily Soil Water Balane Program) PROGRAMME DU BILAN QUOTIDIEN SOL-EAU Morteza N. Orang 1, J. Sott Matya 2, and Rihard L. Snyder 3 ABSTRACT A user-friendly Mirosoft Exel appliation program Consumptive Use Program+ or CUP+ was developed to help growers and water agenies determine referene evapotranspiration (ET o ), rop oeffiient (K ) values, rop evapotranspiration (ET ), and evapotranspiration from applied water (ET aw ), whih provides an estimate of the net irrigation water diversion needed to produe a rop. The appliation also an be used to study the impat of limate hange on evapotranspiration and irrigation water needs. CUP+ omputes referene evapotranspiration (ET o ) from daily solar radiation, maximum and minimum temperature, dew point temperature, and wind speed using the daily Penman-Monteith equation. In addition, the program uses a urve fitting tehnique to derive one year of daily weather data from the monthly data and to estimate daily ET o. It also uses daily rainfall data to estimate bare soil evaporation as a funtion of mean of ET o and wetting frequeny in days. A bare soil K value is alulated to estimate the off-season evapotranspiration and as a baseline for in-season K alulations. CUP+ aounts for the influene of orhard over rops on K values and it aounts for immaturity effets on K values for tree and vine rops. Further, the program omputes and applies all ET o and K values on a daily basis to determine rop water requirements by month, by season, by year. The water balane model is similar to that used in the Simulation of ET of Applied Water (SIMETAW) appliation program, whih was also developed as a ooperative effort between the University of California (UC) and the Department of Water Resoures (DWR). The appliation outputs a wide range of tables and harts that are useful for irrigation planning. Key words: Referene evapotranspiration, Consumptive Use Program +, SIMETAW, K values, subtropial rops 1 Senior Land & Water Use Sientist, Calif. Dept. of water Resoures., P.O. Box , Saramento, CA Land and Water Use Program Manager, Calif. Dept. of Water Resoures., P.O. Box , Saramento, CA Extension Biometeorologist, Dept. of Land, Air and Water Res., Univ. of California, One Shields Ave. Davis, CA

2 International Commission on Irrigation and Drainage RESUME Un Programme d appliation Mirosoft Exel Programme de l eau onsommée + (CUP +) qui s adapte failement aux usagers fut développé pour aider les ultivateurs et les agenes d'eau à déterminer l'évapotranspiration de référene (ET o ), les valeurs du oeffiient ultural (K ), l'évapotranspiration d un ouvert (ET ), et l'évapotranspiration de l'eau appliquée (ET aw ), pour leur permettre d évaluer la dérivation de l eau d'irrigation nette pour produire des ultures. Cette appliation peut aussi utiliser pour étudier l impat du hangement limattique sur l'évapotranspiration et les besoins en eau d'irrigation. Le CUP + permet de aluler l'évapotranspiration de référene (ET o ) de la radiation solaire quotidienne et de la température maximale et minimale, la température du point de rosée et la vitesse du vent utilisant l'équation quotidienne de Penman-Monteith. De plus, en utilisant une tehnique d ajustement d une ourbe, le programme permet aussi de réupérer les données météorologiques annuelles de haque jour des données mensuelles et d évaluer ET o haque jour. Il utilise également les données de préipitation quotidienne pour évaluer l'évaporation à partir du sol nu omme une fontion de la moyenne d'et o et de la fréquene d humidifiation des jours. La valeur du sol nu K est alulée pour évaluer l'évapotranspiration hors saison et omme un diagnosti pour le alul saisonnier du K. Le CUP + représente l'influene des ultures de verger sur les valeurs de K et il représente les effets d'immaturité sur les valeurs de K des arbres et des vignes. De plus, le programme alule et applique toutes les valeurs d ET o et de K tous les jours pour déterminer des besoins en eau de ulture à partir des données mensuelle, saisonnière et annuelle. Le modèle du bilan en eau est similaire au modèle utilisé dans le Programme d appliation de simulation d'et de l'eau appliquée (SIMETAW), lequel a été développé onjointement par l'université de la Californie (UC) et le Département des Ressoures en eau (DWR). Un grand nombre de tables et de diagrammes sont disponibles qui peuvent être utilisés pour la planifiation d'irrigation. Mots lés: Evapotranspiration de référene, Programme de l eau onsommée +, SIMETAW, valeurs du oeffiient ultural K, ultures subtropiales Referene Evapotranspiration 1. INTRODUCTION Referene evapotranspiration (ET o ) is estimated from daily weather data using a modified version of the Penman-Monteith equation (Allen et al., 1998; Allen et al., 2005). The equation is: ET o = ( Rn G) + γ u2 ( es ea ) T γ ( u ) 2 (1) where is the slope of the saturation vapor pressure as a funtion of the mean daily air temperature urve (kpa oc -1 ), R n and G are the net radiation and soil heat flux density in MJ m -2 d -1, γ is the psyhrometri onstant (kpa oc -1 ), T is the daily mean temperature ( o C), u 2 is the mean wind speed in m s -1, e s is the saturation vapor pressure (kpa) alulated from the 410

3 mean air temperature ( o C) for the day, and e a is the atual vapor pressure (kpa) alulated from the mean dew point temperature ( o C) for the day. The oeffiient onverts the R n G term from MJ m -2 d -1 to mm d -1 and the oeffiient 900 ombines together several onstants and overts units of the aerodynami omponent to mm d -1. The produt 0.34 u 2, in the denominator, is an estimate of the ratio of the 0.12-m tall anopy surfae resistane (r =70 s m -1 ) to the aerodynami resistane (ra=205/u2 s m -1 ). It is assumed that the temperature, humidity and wind speed are measured between 1.5 and 2.0 m above the grass-overed soil surfae. If only temperature data are available, then CUP+ alulates ET o using the Hargreaves-Samani equation (Hargreaves and Samani, 1982; Hargreaves and Samani, 1985): ET o = (T +17.8) R a (T d ) 1/2 (2) where T is the monthly mean temperature ( 0 C), R a is the extraterrestrial solar radiation (mm/ month) and T d is the differene between the mean minimum and mean maximum temperatures for the month ( o C). The alulation of extraterrestrial radiation and other parameters in the Penman-Monteith and Hargreaves-Samani equations are desribed in Allen et al. (1998) and Allen et al. (2005). If pan data are used in CUP+, then the appliation automatially estimates daily ET o rates using a feth value (i.e., upwind distane of grass around the pan). The new method in the CUP+ estimates ET o from Epan data without the need for wind speed and relative humidity data. 2. Crop Coeffiients and Evapotranspiration Field and Row Crops Field and row rop K values are alulated using a method similar to that desribed by Doorenbos and Pruitt (1977) and Allen et al. (1998). A generalized urve is shown in Fig. 1. In their method, the season is separated into initial (date A-B), rapid (date B-C), midseason (date C-D), and late season (date D-E) growth periods. K values are denoted K A, K B, K C, K D and K E at the ends of the A, B, C, D, and E growth dates, respetively. During initial growth, the K values are at a onstant value, so K A = K B. During the rapid growth period, when the anopy inreases from about 10% to 75% ground over, the K value inreases linearly from K B to K C. The K values are also at a onstant value during midseason, so K C = K D. During late-season, the K values derease linearly from K D to K E at the end of the season. 411

4 International Commission on Irrigation and Drainage % 100% 50% 20% 1.00 Crop Coeffiient (K ) Initial Rapid Mid-season Late-season A B C D E Growth Date Fig. 1. Hypothetial rop oeffiient urve for field and row rops using perentage of the season to delineate growth dates. The dashed line is for fresh market rops with no lateseason K drop (i.e., there is no date D). Doorenbos and Pruitt (1977) provide estimated number of days for eah of the four growth periods to help identify the end dates of growth periods. Beause there are limate and varietal differenes, however, and beause it is diffiult for growers to know when the infletion points our, irrigators often find this onfusing. To simplify this problem, perentages of the season from planting to eah infletion point rather than days in growth periods are used (Fig. 2). Irrigation planners need only enter the planting and end dates and the intermediate dates are determined from the perentages, whih are easily stored in a omputer program % % 35% Crop Coeffiient (K ) Leaf out 70% C g Leaf drop Rapid Mid-season Late-season B C D E Growth Date Fig. 2. Hypothetial rop oeffiient urve for deiduous tree and vine rops using perentage of the season to delineate growth dates. There is no initial growth period, so the season starts at leaf out on date B. 412

5 During initial growth of field and row rops, the default K value (K 1) is used for K A and K B unless it is overridden by entering a K based on rainfall or irrigation frequeny. If a soil wetting based K 1 is desired, the irrigation or rainfall frequeny is entered in the Input_Output worksheet. The values for K C = K D depend on the differene in (1) light intereption, (2) rop morphology effets on turbulene, and (3) physiologial differenes between the rop and referene rop. Some field rops are harvested before senesene, and there is no late season drop in K (for example, silage orn and fresh market tomatoes). Relatively onstant annual K values are possible for some rops (for example, turfgrass and pasture) with little loss in auray. Some field rops and landsape plants (type-2 rops) have fixed K values all through the year. However, if the signifiant rainfall frequeny is suffiient to have a higher K for bare soil than for the seleted rop, then the higher bare soil K should be used. CUP permits entry of monthly mean rainfall frequeny data. If entered, daily K values for bare soil evaporation are omputed for the entire year. The higher of the fixed rop K or the bare soil K is used to estimate ET for the rop. If no rainfall frequeny data are entered, then the fixed rop K is used. Deiduous Crops 3. Tree and Vine Crop K Values Deiduous tree and vine rops, without a over rop, have K urves that are similar to field and row rops but without the initial growth period (Fig. 3). Default K B, K C = K D = K 2 and K E = K 3 values are given in the Crop Referenes worksheet of the CUP+. The season begins with rapid growth at leaf out when the K inreases from K B to K C. The midseason period begins at approximately 70% ground over. Then, unless the rop is immature, the K is fixed between dates C and D, whih orresponds to the onset of senesene. For immature rops, the anopy over may be less than 70% during the midseason period. If so, the K will inrease from K C up to the K D as the anopy over inreases, so the CUP+ program aounts for K hanges of immature tree and vine rops. During late season, the K dereases from K D to K E, whih ours when the transpiration is near zero. Initially, the K value for deiduous trees and vines (K B) is seleted from a table of default values. However, the ET is mainly soil evaporation at leaf out, so CUP+ ontains the methodology to determine a orreted K B based on the bare soil evaporation. Immature deiduous tree and vine rops use less water than mature rops. The following equation is used to adjust the mature K values (K m ) as a funtion of perentage ground over (C g ). Cg π Cg π If sin 1.0 then K K = Km sin 70 2 =K m or else (3)

6 International Commission on Irrigation and Drainage Subtropial Crops For mature subtropial orhards (for example, itrus), using a fixed K during the season provides aeptable ET estimates. If higher, however, the bare soil K is used for the orhard K. For an immature orhard, the mature K values (K m ) are adjusted for their perentage ground over (Cg) using the following riteria. Cg π If sin 1.0 then K =K m or else 70 2 Cover Crop Corretions K Cg π = Km sin 70 2 (4) With a over rop, the K values for orhards and vines are higher. When a over rop is present, 0.35 is added to the lean-ultivated K. However, the K is not allowed to exeed 1.20 or to fall below CUP+ allows the beginning and end dates to be entered for two periods when a over rop is present in an orhard or vineyard. Estimating Bare Soil K Values A soil evaporation K value, based on ET o and rainfall frequeny is needed as a minimum (base line) for estimating ET. It is also useful to determine the K value during initial growth of field and row rops (K 1= K A= K B), based on irrigation frequeny, and the starting K for deiduous tree and vine rops (K 1 = K B). The K values used to estimate bare soil evaporation are based on a two-stage soil evaporation method reported by Stroosnjider (1987) and refined by Snyder et al. (2000). The method provides a K values as a funtion of ET o rate and wetting frequeny that are similar to those published in Doorenbos and Pruitt (1977). Figure 3 shows a bare soil K urve as a funtion of the square root of the umulative referene evapotranspiration (CET o ) y = 2.54x R 2 = 0.99 K, Crop Coeffiient (CETo) 0.5 (mm) 0.5 Fig. 3. Bare soil rop oeffient urve as a funtion of the square root of umulative ET o (CET o ). 414

7 The number of days per month of signiifiant rainfall is input into CUP+, and the program omputes an estimate of the days between signifiant rainfall events assuming that the rainfall events are evenly distributed through out the month. Daily preipitation is onsidered signifiant when Ps > 2xET o. A smooth urve fit of the monthly values of days between rainfall is omputed. CUP+ also omputes the monthly mean of daily ET o rate and an annual smooth urve fit of daily ET o values. It uses the daily days between rainfall (D br ) and daily estimate of ET o to alulate a orresponding 0.5 ( CET o ) = DBR ETo (5) for eah day. The equation K = 2.54 CET o (6) is used to estimate the bare soil K on eah day. During the off-season, the bare-soil K value is used to estimate the ET. During the season, the bigger of the bare-soil K or the K based on the rop K values is used to alulate the rop evapotranspiration as ET = ET K o (7) Figure 4 presents an example where the bare-soil K (dark line) was higher than the rop K (olored line) during part of the season. The olored line in Figure 4 shows a K urve for a rop that had frequent irrigation between planting that inreased the K value during initial growth. In all ases, the higher of the bare-soil and rop K is used to determine the ET on eah day. Figure 4 shows that the K values for the tomatoes have been adjusted for wetting frequeny from irrigation and rainfall during that period. 415

8 International Commission on Irrigation and Drainage B. Soil K Initial Stage Rapid Growth Mid-Season Late Season Crop Coeffiient Value Ot-95 De-95 Feb-96 Mar-96 May-96 Jul-96 Aug-96 Ot-96 De-96 Jan-97 Mar-97 Apr-97 Jun-97 Aug-97 Sep-97 Fig. 4. Daily alulated bare soil and rop oeffiient values with different olored lines for eah growth period for urrently entered daily weather and rop/soil information during the growing season and off-season. Evapotranspiration of Applied Water (ET aw ) ET aw is the sum of the net irrigation appliations to a rop during its growing season, where eah net irrigation appliation (NA) is equal to the produt of the gross appliation (GA) and an appliation effiieny fration (AE), i.e., NA = GA x AE. The gross appliation is equivalent to the applied water, and the appliation effiieny is the fration of GA that ontributes to rop evapotranspiration (ET ). Three possible methods to determine ET aw are explained below using the example of a tomato rop grown in in the Saramento San Joaquin River Delta. The ET o, ET, and K values for two sample years are shown in Figure

9 10 SA Tomatoes ETo ET K 2.00 Evapotranspiration (mm) Crop Coeffiient (K) /1/ /1/1921 2/1/1922 4/1/1922 6/1/1922 8/1/ /1/ /1/1922 2/1/1923 4/1/1923 6/1/1923 8/1/1923 Fig. 5. Crop (ET ) and referene (ET o ) evapotranspiration and rop oeffiient fators for a tomato rop grown in the Saramento-San Joaquin Rivers Delta. For all surfaes exept open water and riparian vegetation, daily water balane alulations start with the soil water ontent on the previous day (SWC o ). Then the water losses to ET are subtrated to determine the soil water ontent on the urrent day as SWC 1 =SWC o ET. This is the soil water depletion adjusted for ET. Next, any effetive seepage (E spg ) ontribution to the water balane is omputed by omparing the seepage (S pg ) with SWC 1. If S pg < SWC 1, then E spg = S pg, otherwise, E spg = SWC 1. Then the soil water ontent based on ET and effetive seepage is alulated as SWC 2 = SWC 1 + E spg. Then the soil water ontent is adjusted for effetive rainfall by omparing the preipitation (P) with SWC 2. If P < SWC 2, then the effetive rainfall is alulated as E r = P. Otherwise, E r = SWC 2. Then the soil water ontent based on ET, effetive seepage, and effetive rainfall is alulated as SWC 3 = SWC 2 + E r. Therefore, the final estimate of soil water ontent without onsidering irrigation is given in terms of the daily hange in soil water ontent (D sw ) as SWC = SWC D = SWC ( ET E E 3 o sw o spg r Irrigation is applied whenever the soil water ontent on a given day would fall below the management allowable depletion (MAD) set for that date. The net appliation (NA) amount is the depth of water needed to raise the soil water ontent (SWC 3 ) bak to field apaity (FC) on the irrigation date. On eah irrigation date, the NA is equal to SWC3, so the atual soil water ontent on eah day of the season is alulated as ) (8) SWC = SWCo Dsw + NA (9) where SWC o is the soil water ontent on the previous day, NA is the net appliation, whih is zero on non-irrigation days, and D sw is the daily hange in soil water ontent expressed as 417

10 International Commission on Irrigation and Drainage D sw = ET E E (10) spg r By definition, ET aw is the amount of applied irrigation water that ontributes to ET ; therefore, ET aw is the sum of the net irrigation appliations during a ropping season. The ET aw for n irrigation events is therefore alulated as ET + aw = NA1 + NA2 + NA n (11) Alternatively, ET aw an be alulated as the seasonal total evapotranspiration (CET ) minus the umulative effetive seepage ontribution (CE spg ) minus the umulative effetive rainfall ontribution (CE r ) minus the differene in soil water ontent ( WC) from the beginning to the end of the season (Figure 6). The umulative D sw urve (CD sw ) is, by definition, equal to CD sw = CET CE spg CE r (12) Therefore, the ET aw an also be expressed as ET aw = CD sw WC (13) Fig. 6 illustrates how one an determine ET aw from CET, CE spg, CE r, CD sw, and SW. The SW is unknown until the end of the season, however, so it annot be omputed until the end of a ropping season using this method. The ET aw an be omputed from the net appliations after the last NA is applied. This is the method used to determine the ET aw in CUP+. The other method to estimate ET aw uses the umulative daily hange in soil water ontent and the differene between the initial and final soil water ontent ( SW) as ET aw = CD sw WC = n ( CET CEspg CEr ) WC = i= 1 NA i (14) Thus, ET aw an be determined by (1) omputing the season aumulation of daily hanges in soil water ontent and subtrating WC, (2) alulating the seasonal umulative ET and subtrating the umulative Es and Er and the WC, or (3) summing the net irrigation appliations that our during the season (Fig. 6). 418

11 SA Tomatoes 800 WC = differene between initial and final soil water ontent 683 mm Soil Water Content (mm) CET CDsw CEspg CEr ETaw mm 508 mm WC = 121 mm 387 mm 152 mm 0 23 mm 31-De 31-Mar 30-Jun 30-Sep 31-De Fig. 6. A plot of CET, CEspg, CEr, and CDsw versus time for a tomato rop from the Saramento San Joaquin Rivers Delta. ET aw an be alulated as the daily evapotranspiration (DET ) minus the estimated daily effetive seepage ontribution (DE spg ) minus the daily estimated effetive rainfall ontribution (DEr) minus the differene in soil water ontent (DWC) from the beginning to the end of the season. The figure below shows the omparison of the umulative daily ET aw values with the umulative net appliation (Cum. NA) for tomatoes over the period of one year. 419

12 International Commission on Irrigation and Drainage CET CEr CEsdf Cum. NA CDETaw Soil Water Content (mm) Jan-96 Feb-96 Mar-96 Apr-96 May-96 Jun-96 Jul-96 Aug-96 Sep-96 Ot-96 Nov-96 De-96 Jan-97 Feb-97 Mar-97 Apr-97 May-97 Jun-97 Jul-97 Time (day) Fig. 7. A plot of CET, CEsdf, CEr, CDsw, Cum. NA, and CET aw Vs time using data from Jan. 1 to Jul. 31 of the following year Pp NA FC PWP SWC SWCx ET Soil Water Content (mm/mm) ET (mm) and Preipitation (mm) 0 0 Jan-96 Feb-96 Mar-96 Apr-96 May-96 Jun-96 Jul-96 Aug-96 Sep-96 Ot-96 Nov-96 Time (days) De-96 Jan-97 Feb-97 Mar-97 Apr-97 May-97 Jun-97 Jul-97 Fig. 8. Flutuations in soil water ontent (SWC) between field apaity (FC) and maximum soil water ontent (SWCx) over the period of one year. 420

13 The CUP+ program also plots daily alulated water balane for rops using daily weather data. The plot shows flutuations in soil water ontent between field apaity and the maximum depletion during the off-season and between field apaity and maximum soil water ontent during the growing season. The plot also shows the daily values for rop evapotranspiration (ET ) and rainfall. Irrigation events are given when the maximum soil water depletion exeeds the maximum soil water ontent (Figure 8). REFERENCES Allen, R.G., Pereira, L.S., Raes, D., and Smith, M., Crop evapotranspiration: Guidelines for omputing rop water requirements. FAO Irrigation and Drainage Paper 56, FAO, Rome. Allen, R. G., Walter, I. A., Elliott, R. L., Howell, T. A., Itenfisu, D., Jensen, M. E. and Snyder, R. L. (2005) The ASCE Standardized Referene Evapotranspiration Equation. Tehnial Committee report to the Environmental and Water Resoures Institute of the Amerian Soiety of Civil Engineers from the Task Committee on Standardization of Referene Evapotranspiration. 173 p. Doorenbos, J. and Pruitt, W.O Rev. Crop water requirements. FAO Irrig. and Drain. Paper 24, FAO of the United Nations, Rome, p Hargreaves GH, Samani ZA Estimating potential evapotranspiration. Teh. Note, J. Irrig. and Drain. Engng., ASCE, 108(3): Hargreaves GH, Samani ZA Referene rop evapotranspiration from temperature. Appl. Engng. in Agri. 1(2): Snyder, R.L., Bali, K., Ventura, F, and Gomez-MaPherson, H Estimating evaporation from bare or nearly bare soil, J. Irrigation and Drainage Division, Amerian Soiety of Civil Engineers, 126(6): Stroosnijder, L., Soil evaporation: test of a pratial approah under semi-arid onditions, Netherlands J. Agri. Si., 35:

IRRIGATION SCHEDULING OF ALFALFA USING EVAPOTRANSPIRATION. Richard L. Snyder and Khaled M. Bali 1 ABSTRACT

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