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 (ET_c) - 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 ET_c, runoff (R) and deep percolation (DP). Daily soil water depletion in the rooting zone is calculated using the equation below:

Dc - Dp = ET_c - 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, ET_c 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 + ET_c). Using these assumptions, equation 1 can be simplified to:

Dc = Dp + ET_c - 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 (ET_c) is the biggest subtraction from the water balance equation (Equation 2). The ET_c changes throughout the growing season due to weather variations and crop development.

Crop water use or ET_c 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.

How to estimate crop evapotranspiration

Crop Evapotranspiration (ET_c) can be estimated using a two-step approach to determine the crop-specific water use (ET_c) as per the following equation:

ET_c= ET_ref x K_c (Equation 3)

Where ET_ref is the reference evapotranspiration - which is the evapotranspiration from a reference crop (grass or alfalfa) - and K_c is the crop coefficient. K_c varies by crop development stage. Crop coefficient values for common crops grown in Minnesota are given in Tables 1 -3.

Alfalfa-reference and grass-reference evapotranspiration (ET_ref) are calculated using the standardized Penman-Monteith equation. Climate variables measured by a weather station are used as input data to calculate ET_ref. Climatic data included are air temperatures, solar radiation, humidity and wind.

Tools for ET_c information

Central Minnesota Agricultural Weather network - This website provides daily crop water use information for different cropping systems that can be used in water balance irrigation scheduling.
NDAWN Crop water use (NW Minnesota and North Dakota)
Daily ET estimates (for SE Minnesota and Wisconsin)

Table 1. Mean crop coefficients (Kc) of commonly grown crops in Minnesota for use with alfalfa reference ET.

Days after planting (DAP)	Corn	Beans	Potatoes	Spring grains¹	Sugarbeets
5	0.20	0.23	0.20	0.20	0.26
10	0.19	0.26	0.20	0.19	0.26
15	0.20	0.32	0.20	0.22	0.26
20	0.22	0.40	0.23	0.29	0.26
25	0.26	0.51	0.28	0.39	0.26
30	0.33	0.64	0.36	0.50	0.26
35	0.42	0.77	0.46	0.61	0.26
40	0.53	0.87	0.56	0.71	0.27
45	0.66	0.95	0.65	0.85	0.29
50	0.79	0.98	0.72	1.02	0.32
55	0.91	0.98	0.76	1.03	0.38
60	1.00	0.95	0.77	1.04	0.45
65	1.00	0.92	0.80	1.03	0.56
70	0.99	0.78	0.80	1.03	0.69
75	0.99	0.60	0.79	1.03	0.85
80	0.97	0.43	0.78	1.04	1.03
85	0.96	0.30	0.77	1.04	1.04
90	0.94	0.20	0.75	1.04	1.03
95	0.92	0.14	0.72	1.01	1.03
100	0.89	0.11	0.70	0.94	1.02
105	0.86	0.10	0.67	0.70	1.01
110	0.82	0.08	0.65	0.52	1.01
115	0.77		0.63	0.38	0.99
120	0.72		0.61	0.28	0.97
125	0.65		0.58	0.21	0.95
130	0.58		0.55	0.16	0.91
135	0.50		0.50	0.13	0.87
140	0.40		0.44	0.10	0.83
145	0.29		0.37		0.78
150	0.17		0.29		0.73
155			0.21		0.69
160			0.18		0.66
165					0.63
170					0.61
175					0.59
180					0.56

Source: K_cnumbers are adapted from ASCE manual 70 and were adjusted based on the following assumption:
Assumes full cover at 60, 54, 65, 60 and 80 DAP for corn, beans potatoes, spring grains and sugarbeets, respectively.
¹Spring grains include wheat and barley.

Table 2. Mean crop coefficients (Kc) for alfalfa during the establishment year for use with alfalfa reference ET. Assumes that there will be two cuttings during the establishment year.

Days since planting	Kc between planting and 1st cutting	Days since 1st or 2nd cutting	Kc during 1st or 2nd cuttings	Days since final cutting	Kc after final cutting
5	0.60	5	0.40	5	0.43
10	0.70	10	0.63	10	0.57
15	0.78	15	0.84	15	0.56
20	0.85	20	0.96	20	0.48
25	0.90	25	1.00	25	0.44
30	0.95	30	1.00	30	0.44
35	0.98	35	0.99	35	0.44
40	1.00	40	0.98	40	0.44
45	1.00	45	0.94	45	0.44
50	1.00	50	0.94
55	0.98	55	0.94
60	0.95	60	0.94
65	0.95	65	0.94
70	0.95
75	0.95
80	0.95

Source: Kc numbers are adapted from ASCE manual 70 and were adjusted based on the following assumption:
Assumes 60 days from planting to 1st cut and 45 days from 1st cut to 2nd cut.

Table 3. Mean crop coeffecients (Kc) for alfalfa during production or non-establishment years. Assumes as many cuttings as grower defines.

Days since 1st cutting	Kc during 1st cut regrowth	Days since intercutting	Kc during intercutting regrowth	Days since last cutting	Kc after last cutting
5	0.71	5	0.54	5	0.43
10	0.86	10	0.87	10	0.57
15	0.96	15	1	15	0.56
20	1	20	0.99	20	0.48
25	0.98	25	0.98	25	0.44
30	0.95	30	0.93	30	0.44
35	0.95	35	0.93	35	0.44
40	0.95	40	0.93	40	0.44
45	0.95	45	0.93	45	0.44

Source: Kc numbers are adapted from ASCE manual 70 and were adjusted based on the following assumptions:
Assumes 28 days to 1st cut, 28 days between cuttings, and 28 days from last cut to dormancy.

Using an ET gauge for estimating ETc

ET Gage atmometer — Figure 2. An ET Gage installed in the field.

Since there are not many weather stations available in Minnesota that provide ETc information, there are alternative tools to determine ET_ref. One such tool is the atmometer or ET Gage (Figure 2).

ET Gage is a device that simulates the crop water use from a plant canopy. It has a canvas covered ceramic evaporator plate at the top that allows water to evaporate the same way as a crop does.

There are different canvas covers available and can be changed for different plant surfaces. For example, #30 cover simulates grass reference evapotranspiration (ET_o) and #54 cover simulates alfalfa reference ET (ET_r).

The reservoir at the bottom of an ET Gage is filled with distilled water. For each inch of water used by the plant, the water level in the site tube (shown in Figure 2) drops by one inch. Reading a site tube is as easy as reading a rain gauge.

This device comes in two models, manual (Model A) and electronic (Model E). In the manual model (Model A), the site tube has to be read manually once or twice a week. However, the electronic Model E automatically senses each time there is a 0.01-inch evaporation from the ET Gage.

The cost of Model A is $355 and Model E is $700. Model E can be connected to any event data logger such as Irrometer watermark datalogger, Aquatrac etc.

For more information on ET Gages visit http://www.etgage.com/. To convert ET Gage reading to ET_c, again the two-step approach can be used as per equation 3 in the section above.

The ET_c information obtained from various sources (eg. Ag weather network, ETgage etc. as discussed above). It can then be used as an input for the water balance calculations (Equation 2).

Water balance irrigation scheduling tools

Irrigation management assistant (IMA) is automated irrigation software that works on the principles of water balance irrigation scheduling method as described in this article. At this time, the tool is available for Benton, Otter Tail, Becker, Hubbard, Wadena and Todd Counties of Minnesota.
Another water balance irrigation scheduling tool available is the Checkbook spreadsheet method.

Note: The recommended method is a combination of in-field monitoring by soil moisture sensors and a daily soil water accounting using weather data (water balance method).

Sources

Andales, A. A, J.L. Chavez and T.A. Bauder. 2015. Irrigation Scheduling: The Water Balance Approach. Colorado State University Extension. Fact Sheet No. 4.707.

ASCE-EWRI (Environmental and Water Resources Institute). 2005. The ASCE standardized reference evapotranspiration equation. Final Rep., Standardization of Reference Evapotranspiration Task Committee, Reston, VA.

ASCE Manuals and Reports on Engineering Practice No. 70. (2016). Evaporation, Evapotranspiration and Irrigation Water Requirements. Second edition. Edited by Marvin E. Jensen and Richard G. Allen.

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 K_c values for use in Minnesota.

Vasudha Sharma, Extension irrigation specialist

Reviewed in 2023