Irrigation Scheduling: The Water Balance Approach Fact Sheet No. 4.707 Crop Series|Irrigation by A. A. Andales, J. L. Chávez, T. A. Bauder* The water requirement of a crop must that can be used by the plants. The AWC of Quick Facts be satisfied to achieve potential yields. The soil is typically expressed in terms of inches crop water requirement is also called crop of water per inch of soil depth. General values • The water balance approach evapotranspiration and is usually represented of AWC are provided in Table 1. Available to irrigation scheduling as ET . Evapotranspiration is a combination water capacity values for specific soils can be c keeps track of the soil of two processes – evaporation of water obtained from county soil surveys or online water deficit by accounting from the ground surface or wet surfaces of at http://websoilsurvey.nrcs.usda.gov/app/. for all water additions and plants; and transpiration of water through the stomata of leaves. The water requirement can subtractions from the soil be supplied by stored soil water, precipitation, Water Balance Accounting root zone. and irrigation. Irrigation is required when As the crop grows and extracts water • Crop water consumption or ET (crop water demand) exceeds the supply from the soil to satisfy its ET requirement, c c evapotranspiration accounts of water from soil water and precipitation. As the stored soil water is gradually depleted. for the biggest subtraction ET varies with plant development stage and In general, the net irrigation requirement c of water from the root zone weather conditions, both the amount and is the amount of water required to refill the timing of irrigation are important. The water root zone soil water content back up to field while precipitation and balance (accounting) method of irrigation capacity. This amount, which is the difference irrigation provide the major scheduling is one method of estimating the between field capacity and current soil water additions. level, corresponds to the soil water deficit required amount and timing of irrigation • Crop evapotranspiration for crops. This method can be used if initial (D). The irrigation manager can keep track of can be obtained from soil water content in the root zone, ET , D, which gives the net amount of irrigation c the Colorado Agricultural precipitation, and the available water capacity water to apply. On a daily basis, D can be of the soil are known. estimated using the following accounting Meteorological Network The soil in the root zone has an upper equation for the soil root zone: (CoAgMet) or by using as well as a lower limit of storing water that atmometers. Dc=Dp+ETc -P-Irr-U+SRO+DP [1] can be used by crops. The upper limit is where D is the soil water deficit (net • The soil in the root zone has called the field capacity (FC), which is the c irrigation requirement) in the root zone an upper as well as a lower amount of water that can be held by the soil on the current day, D is the soil water limit of storing water that can against gravity after being saturated and p deficit on the previous day,ET c is the crop be used by crops. drained; typically attained after 1 day of rain evapotranspiration rate for the current day, P or irrigation for sandy soils and from two is the gross precipitation for the current day, • As the crop grows and to three days for heavier-textured soils that Irr is the net irrigation amount infiltrated extracts water from the soil contain more silt and clay. The lower limit is into the soil for the current day, U is upflux of to satisfy its ET requirement, called permanent wilting point (PWP), which c shallow ground water into the root zone, SRO the stored soil water is is the amount of water remaining in the soil is surface runoff, and DP is deep percolation gradually depleted. when the plant permanently wilts because or drainage. it can no longer extract water. The available The last three variables in equation 1 • Atmometers are designed to water capacity (AWC), or total available (U, SRO, DP) are difficult to estimate in the simulate the water use of a water, of the soil is the amount of water field. In many situations, the water table well-watered reference crop. between these two limits (AWC = FC - PWP) is significantly deeper than the root zone and is the maximum amount of soil water and U is zero. Also, SRO and DP can be accounted for in a simple way by setting D © Colorado State University c Extension. 11/93. Revised 1/15. *A.A. Andales, Colorado State University Extension to zero whenever water additions (P and Irr) irrigation specialist and assistant professor, soil and www.ext.colostate.edu to the root zone are greater than Dp + ETc. crop sciences; J.L. Chávez, Extension irrigation Using these assumptions, equation 1 can be specialist, assistant professor, civil and environmental simplified to: engineering; T.A. Bauder, Extension water quality specialist, soil and crop sciences. 1/2015 Dc= Dp+ETc-P-Irr Table 1. Soil texture and plant available water capacity (AWC) In irrigation practice, only a percentage Soil Texture Available water capacity of AWC is allowed to be depleted because Low High Average plants start to experience water stress even – inch of water / inch of soil – before soil water is depleted down to PWP. Therefore, a management allowed depletion Coarse sands 0.05 0.07 0.06 (MAD, %) of the AWC must be specified. Fine sands 0.07 0.08 0.08 Refer to fact sheet number 4.715 (Crop Loamy sands 0.07 0.10 0.08 Water Use and Growth Stages) for values of Sandy loams 0.10 0.13 0.12 MAD for selected crops. Ranges of rooting Fine sandy loams 0.13 0.17 0.15 depth for selected crops are given in Table Sandy clay loams 0.13 0.18 0.16 2. The rooting depth and MAD for a crop will change with developmental stage. The Loams 0.18 0.21 0.20 MAD can be expressed in terms of depth Silt loams 0.17 0.21 0.19 of water (dMAD; inches of water) using the Silty clay loams 0.13 0.17 0.15 following equation. Clay loam 0.13 0.17 0.15 MAD dMAD= 100 *AWC*Drz [3] Silty clay 0.13 0.14 0.13 where MAD is management allowed Clay 0.11 0.13 0.12 depletion (%), AWC is available water capacity of the root zone (inch of water per inch of soil), and D is depth of root zone (if D is negative, then set it to 0.0) [2] equation 2 is the same as the actual deficit rz c (inches; see Table 2). in the field (soil water content readings Take note that Dc is set equal to zero if its The value of d can be used as a guide using soil moisture sensors). Remember MAD value becomes negative. This will occur if for deciding when to irrigate. Typically, that Dc is the difference between field precipitation and/or irrigation exceed (Dp + irrigation water should be applied when the capacity and current soil water content. ETc) and means that water added to the root soil water deficit (D ) approaches d , or Therefore, the actual deficit in the field can c MAD zone already exceeds field capacity within when D ≥ d . To minimize water stress be determined by subtracting the current c MAD the plant root zone. Any excess water in on the crop, D should be kept less than soil water content from the field capacity of c the root zone is assumed to be lost through d . If the irrigation system has enough the root zone. If D from equation 2 is very MAD SRO or DP. c capacity, then the irrigator can wait until D different from the observed deficit, then use The amounts of water used in the approaches d before starting to irrigate. the observed deficit as the D value for the MAD equations are typically expressed in depths c The net irrigation amount equal to D can next day. These corrections are necessary to c of water per unit area (e.g., inches of be applied to bring the soil water deficit to compensate for uncertainties in the water water per acre). Equation 2 is a simplified zero. Otherwise, if the irrigation system balance variables. Field measurements of version of the soil water balance with has limited capacity (maximum irrigation current soil water content can be performed several underlying assumptions. First, any amount is less than d ), then the irrigator using the gravimetric method (weighing MAD water additions (P or Irr) are assumed to should not wait for D to approach d , but of soil samples before and after drying) or c MAD readily infiltrate into the soil surface and should irrigate more frequently to ensure using soil water sensors like gypsum blocks the rates of P or Irr are assumed to be less that D does not exceed d . However, than the long term steady state infiltration (resistance method). MAD rate of the soil. Actually, some water is lost to surface runoff if precipitation or Table 2. Irrigation management depths (Drz) in inches for selected crops. (Assumes rooting is not restricted by compaction or shallow soils.) (Bauder and Schneekloth, 2006) irrigation rates exceed the soil infiltration rate. Thus, equation 2 will under-estimate Irrigation management depths the soil water deficit or the net irrigation Annual crop Seedling Vegetative Flowering Mature requirement if P or Irr rates are higher - Soil depth or Drz (inches) – than the soil infiltration rate.