Evapotranspiration and Its Importance in Precision Irrigation Water Management (FS-2025-0792)

Evapotranspiration (ET) represents the total water transferred from land to the atmosphere through two processes: evaporation and transpiration. Evaporation is the process through which water transforms from a liquid into a gas in the atmosphere. Transpiration is the process through which water is absorbed by the plant roots, transported through the plant, and released into the atmosphere through the stomata in the form of water vapor (Figure 1).

In agricultural systems, understanding ET is fundamental because it represents the actual water requirement of the plants. Water is essential for plant functions, including photosynthesis and nutrient transport. Crops need an optimum amount of water to maintain growth and function. If the water loss through ET exceeds the water available to the plant in the root zone, the plant may experience water stress, and this may hinder its growth. Conversely, overcompensating for ET and supplying excess water may lead to nutrient leaching and loss in soil fertility over time. ET rate is affected by a variety of climate factors, soil properties, crop characteristics, and management practices (Table 1).

The most accurate method of measuring ET in the field is with weighing lysimeters, also called floating lysimeters (Figure 2). Typically, weighing lysimeters are constructed from large containers filled with either undisturbed soil cores or reconstructed soil cores representing the original characteristics of a soil of interest. Scientists can calculate the loss of water from the system by measuring changes in the mass of the soil core over time. Since the soil itself should remain the same weight, any changes in mass are due to changes in the water content of the soil. Whether undisturbed or reconstructed, the soil core sample is taken from the crop root zone to accurately measure soil water relevant to plant uptake.

Similarly, we can estimate ET using eddy covariance towers, which are field-based systems that directly measure the exchange of water vapor between the field and the atmosphere. Sensors affixed to metal structures typically 2-10 m tall to collect temperature, humidity, and wind speed data, which is then used to estimate the amount of water that has evaporated from the area.

ET can also be estimated using the Penman-Monteith equation for irrigation scheduling at the field scale as recommended in the Food and Agriculture Organization’s Irrigation and Drainage Paper No. 56 (Allen et al.,1998). The Penman-Monteith equation is used to compute a quantity known as reference evapotranspiration (ET_o), which is the evapotranspiration of a reference crop, such as alfalfa or well-watered grass. Inputs for the equation include air temperature, relative humidity, wind speed, and solar radiation. Once the reference evapotranspiration (ET_o) is computed, it can be converted into crop evapotranspiration. Crop evapotranspiration (ET_c) is the water loss from a specific crop (e.g., corn, soybean, etc.) under optimal conditions of ample water supply and well-fertilized, disease-free soil. The following equation is used to convert reference ET_o to ET_c:

ET_c = K_c * ET_o (Eq. 1)

where ET_c is crop ET and K_c is the crop coefficient, or the factor by which the presence of a specific crop changes how much water is lost from the system. K_c values are typically found in reference tables for different crops and their growth stages. K_c changes depending on local weather conditions, soil evaporation, and crop growth stage. In the early stages of crop growth, K_c is low because the plants are small and do not cover much of the soil surface. As the crop progresses through vegetative growth, K_c increases and then peaks when the crop canopy has fully developed. Then, if the crop is allowed to reach maturity, such as is the case for corn, soybean, or fruits such as melons, the K_c value decreases as the crop leaves wither and the crop canopy becomes sparse.

In the single crop coefficient approach (using only K_c in Equation 1), K_c represents both the effect of crop transpiration and soil evaporation. However, the soil evaporation rate is subject to change based on the amount of sunlight, the amount of rainfall, and the applied irrigation. Therefore, K_c expresses a time-average effect of crop evapotranspiration and is commonly used in systems with longer irrigation intervals (seven to ten days), such as furrow, basin, or surface irrigation.

To calculate the irrigation requirements for high-frequency irrigation systems such as drip and center pivot, it is better to use the dual crop coefficient. In this method, plant transpiration and soil evaporation are represented by separate coefficients: K_cb for plant transpiration and K_e for soil evaporation. K_c in the dual coefficient method is described as:

K_c = K_cb + K_e (Eq. 2)

where K_cb is the basal crop coefficient (the single value crop coefficient found in reference tables, K_c in Equation 1), and K_e is the soil water evaporation coefficient as determined by soil water balance. The value of K_e is 1 when the surface is wet from irrigation or rainfall and decreases to zero as the surface dries.

Here is an example of calculating the ET_c of the field corn during the middle of the growing season.

Given: 

• ET_o = 5.88 mm/day
• K_c (mid-season corn) = 1.20 (Allen et al.,1998) 

Using Equation (1): 

ET_c= ET_o* K_c 

ET_c = 5.88*1.20 mm/day 

ET_c = 7.05 mm/day 

Therefore, the daily crop water requirement is 7.05 mm per day for a corn crop in the middle of the growing season.

To maintain healthy crop growth, the irrigation schedule should replenish the water lost through ET_c.

STEP 1: Determine Irrigation Interval

Given:

• No significant rainfall during the period.
• The soil has an available water-holding capacity (AWC) of 50 mm in the root zone (e.g., for a medium-textured soil).
• The allowable depletion is 50%, so we can use up to 25 mm before irrigating.

Based on calculated ET_c:

Irrigation Interval = Allowable depletion ÷ ET_c = 25 mm ÷ 7.05 mm/day = 3.54 = 4 days

Therefore, irrigation should be applied every 4 days during the mid-growing season

STEP 2: Calculate irrigation Depth

Irrigation Depth = 7.05 mm (daily crop water requirement) *4 days = 28.2 mm total.

Convert millimeters to inches: 28.2 mm ÷ 25.4 =1.11 inches

1.11 inches of water should be applied every 4 days

Evapotranspiration reflects the actual water demand of crops through the combination of evaporation and transpiration processes. By estimating the crop ET, we can determine the amount of water required to meet the needs of the crop without causing water stress or waste.

Effective irrigation scheduling based on ET_c enables farmers to:

• Apply the right amount of water at the right time
• Conserve valuable water resources
• Enhance crop yield and quality
• Adapt to increasingly variable climatic conditions.

As climatic instability and water shortages become more problematic, incorporating ET-based irrigation techniques will be essential to maintaining food security, minimizing environmental effects, and encouraging effective water management in agriculture.

This publication is supported by the funding from USDA Natural Resources Conservation Service.

Hemendra Kumar, PhD. 
Precision Agriculture Extension Specialist, 
Precision Agriculture Lab, 
College of Agriculture and Natural Resources, 
University of Maryland 
hemendra@umd.edu

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