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Evapotranspiration-based irrigation scheduling or water-balance method

The status of the soil water for an irrigated crop needs monitoring regularly to assist the irrigation manager in making irrigation decisions. Typically, irrigation scheduling can be done in two ways. One is by directly monitoring soil-water by using soil moisture sensors. The other way is to use weather data to account for soil-water in the rooting depth by soil-water balance approach. This method is usually referred to as weather-based or evapotranspiration (ETc) - based irrigation scheduling or water balance method.

How to use the water balance method

soil water components illustration
Figure 1. Soil water balance components.

Estimating soil water using the water-balance approach is done by accounting for all the incoming and outgoing water from the soil root zone (Figure 1). Major inputs include precipitation (P) or rainfall and irrigation (Irr). Outputs include ETc, runoff (R) and deep percolation (DP). Daily soil water depletion in the rooting zone is calculated using the equation below:

DDETc - P - Irr + R + DP       (Equation 1)

Where Dc stands for soil water deficit (net irrigation requirement) in the rooting zone on current day, Dp is the previous day soil moisture deficit, ETc is crop evapotranspiration on the current day, P is precipitation for the current day, Irr is the irrigation amount for the current day, R is the surface runoff and DP is the deep percolation.

Since it is very difficult to estimate R and DP in the field, these variables can be accounted for by setting Dc to zero whenever water additions (P and Irr) to the root zone are greater than water subtractions (Dp + ETc). Using these assumptions, equation 1 can be simplified to:

DDETc - P - Irr     (Equation 2)

Estimating initial soil moisture and soil water deficit

Before beginning the water balance calculations, you should know the initial soil moisture. You can estimate initial soil moisture using gravimetric soil water sampling, the hand-feel method or soil moisture sensors. From the initial soil moisture content, soil water depletion/deficit (Dc) for the successive days can be estimated using equation 2.

The estimated soil water deficit (Dc) from the water balance equation is then compared with maximum allowable depletion (MAD) - which is usually 50% of total available water (TAW) in the root zone - to make irrigation decisions. Remember that TAW = Available water holding capacity (AWC) X rooting depth.

Plants start to experience water stress once the soil water deficit/depletion in the root zone is greater than the root zone MAD. Generally, irrigation should be initiated when Dc approaches MAD. However, if the irrigation system has limited capacity, then the irrigator should not wait for Dc to reach MAD and should irrigate more frequently. Discussions about MAD, AWC and TAW are available in Basics of irrigation scheduling. More information about MAD strategies and pumping capacity can be found in Irrigation management strategies

Estimating crop water use or crop evapotranspiration (ETc)

Evapotranspiration (ETc) is the biggest subtraction from the water balance equation (Equation 2). The ETc changes throughout the growing season due to weather variations and crop development.

Crop water use or ETc depends on many factors. These include

  • Crop type.
  • Growth stage.
  • Climatic conditions (parameters that have a major effect on a crop's daily water use include the maximum and minimum temperatures, solar radiation, humidity and wind).
  • Management and environmental conditions.
  • Soil moisture, etc.
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Acknowledgements

The author wishes to thank former University of Minnesota colleagues Joshua Stamper and Jerry Wright for their previous development efforts in earlier iterations of this content. The author would also like to thank Luke Stuewe and Jeppe Kjaersgaard from Minnesota Department of Agriculture for adjusting Manual 70 Kc values for use in Minnesota.

Vasudha Sharma, Extension irrigation specialist

Reviewed in 2019

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