Crop Water Use

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Crop Insights* written by Derrel Martin¹, Suat Irmak¹, William Kranz¹, Simon van Donk¹, Tyler Smith¹ and John Shanahan²

Overview

Knowledge of factors governing crop water use is vital to developing effective irrigation management strategies.
Crop water use, often referred to as evapotranspiration or ET is comprised of 2 components: 1) soil evaporation (E) and 2) crop transpiration (T).
Factors affecting ET include weather (i.e. solar radiation, temperature, wind, humidity, etc.) and management variables (i.e., soil water, residue, planting date and density, etc.).
The ratio of E to T varies as the crop matures; E comprises the greatest proportion for young crops, but E decreases and T increases as the crop grows.
Presence of crop residue on soil surface can greatly reduce E during early growth, saving irrigation water.
Use of an ET model is critical to effective irrigation management and more efficient use of water resources.

Introduction

Management of irrigation systems requires an understanding of crop water use, including:

How crops use water and what factors affect water use.
How to estimate crop water use rates and manage the irrigation system accordingly.
How crop residue may reduce irrigation needs.

Evapotranspiration (ET)

Evapotranspiration (ET) is the conversion of liquid water in soils or plants, as well as water on soil and plant surfaces, into water vapor. Water vapor travels from soil surfaces, water bodies, plant vegetation and other surfaces into the atmosphere. There are 2 components to ET: 1) evaporation from the soil, bodies of water, or plant leaves or residue and 2) transpiration from plants.

During transpiration, plants take up water from the soil and transport water to the leaves. Small openings in the leaves (stomata) allow water vapor to pass from the plant into the atmosphere. Transpiration cools plants and helps maintain photosynthesis and growth, leading to a direct relationship between transpiration and yield. Separately measuring evaporation from transpiration is difficult, so the processes are usually measured and/or computed as a combined flux (ET).

Factors That Affect Evapotranspiration

Transpiration cools plants and thus, depends on climatic conditions - primarily air temperature, relative humidity and solar radiation. Transpiration increases when air temperature and solar radiation increase. Conversely, high humidity levels suppress transpiration. Wind enhances the transport of water vapor from plant leaves into the atmosphere; therefore, ET increases as wind intensifies. However, stomata may close for high winds, and transpiration may decrease.

Other factors affecting ET include plant species, growth stage and relative maturity; planting date and density; canopy characteristics and surface cover; soil water status and irrigation regime; and tillage practices and crop residue levels.

Effect of Residue - Crop residue can have a significant effect on evaporation of water from the soil surface. A University of Nebraska study found that plots with residue removed required 1.5 to 2.5 inches more of irrigation water to achieve the same yield as plots with residue on the surface. In addition, at the end of the growing season, plots with residue contained 1.5 inches more water in the top 4 feet of soil than plots with no residue cover, i.e., bare plots. Thus, residue on the soil surface could have saved 3 to 4 inches of irrigation water compared to bare soil. Water conservation from residue cover diminishes for very arid areas where the time between wetting events is long.

Change in Soil Evaporation vs.
Crop Transpiration During Season

The ratio of evaporation to transpiration changes as crops grow and shade more of the soil. When crops are small, the portion of crop ET (ETc) due to transpiration is minimal relative to soil evaporation. The surface area of leaves is small, and more of the soil surface is exposed (Figure 1).

**Figure 1.** Sources of ET early in the growing season; more water leaves the soil through evaporation compared to the small amount transpired by the small plants.

When crops reach full canopy, the soil is completely shaded, and evaporation from soil is minimal (Figure 2).

**Figure 2.** Sources of ET in the middle of the growing season; leaf area is now much larger than the exposed soil surface, and transpiration accounts for 90-98% of ET.

Calculating Evapotranspiration

Managing irrigation to meet crop water needs requires information about the amount of water used by the crop over a period. Water use of a crop is difficult to measure; therefore, crop ET is often estimated from the ET of a reference crop (ETr), typically grass or alfalfa. Computation of ETr utilizes meteorological instruments described by the High Plains Regional Climate Center (HPRCC). Calculation of ETc is possible once ETr is known by using the equation:

Kc is the “crop coefficient” or “conversion factor” for the specific crop. Crop coefficients are determined through research comparing water use of a crop to the reference crop. Crop coefficients also depend on the growth stage of the crop.

The High Plains Regional Climate Center (HPRCC) maintains and monitors weather stations throughout the region. The stations provide data for calculating ETr. Some stations measure only high and low air temperature and precipitation while other stations monitor more variables that influence ET: air temperature, solar radiation, relative humidity and wind speed.

Measuring all variables required for computing ETr can be complicated and expensive. An atmometer is a simpler method for estimating reference crop ET. Information about atmometers is available in the UNL Extension publication (NebGuide) G1579, Using Modified Atmometers for Irrigation Management.

Once the reference crop ET has been measured or calculated, a table of crop coefficients can be used to calculate the ET for the crop of interest. For example, if an ETgage® is being used, the change in the water level in the sight gauge is 2.10 inches and the crop of interest is a field of soybeans at the full pod stage, then the crop water use would be 2.31 inches. Using the Kc found in Table 1 and the previous equation:

Water use can be computed for daily or weekly intervals. Table 1 lists Kc values for corn, soybean and wheat for the HPRCC method. The UNL CropWatch provides ETc estimates for these and other crops based on the emergence date and growing degree days required for maturity.

Table 1. HPRCC crop coefficients for corn, soybean and wheat.

Crop coefficients depend on the ETr method. There are many methods for determining ETr. Some crop coefficients are for dry soil surfaces while others represent average irrigation and precipitation frequencies that include wet soil evaporation. In addition, some crop curves are for a grass reference crop while others use alfalfa. It is essential to match crop coefficients to the source of reference crop ET. For example, values from Table 1 are appropriate for the ETgage and HPRCC systems but not other ETr sources. Growers should obtain local information for their area from other weather data networks available across the Corn Belt.

Seasonal Evapotranspiration

Daily ET varies throughout the growing season, but a trend evolves when averaging over multiple years. ET varies with the crop. Figure 3 shows that the peak ET for corn occurs earlier than for soybeans. The growing season ETc depends on local climatic conditions. Figure 4 shows how ET varies across Nebraska. Values are highest in the Southwest where lower humidities and higher air temperatures are typical.

**Figure 3.** Average daily ET for corn and soybeans in central Nebraska.

Map: Average seasonal ET for corn in Nebraska — **Figure 4.** Average seasonal ET for corn in Nebraska.

Summary

ET consists of water evaporation from the soil surface as well as transpiration from crops.
Irrigation scheduling decisions require knowledge of the previous ETr (i.e., since the scheduling exercise).
Calculating ETc requires weather data for conditions that affect the rate of ETr.
After obtaining reference crop ET data, water use of the specific crop can be calculated with a crop coefficient.

For example, a producer wants to estimate how much water his corn crop used last week. The corn is in the 10-leaf stage of growth, and an ETgage near the field shows reference crop water use of 2.4 inches last week. Estimating the water use of corn relies on the crop coefficient in Table 1. The result is: