G1994

Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients

This NebGuide describes the basic principles of the evapotranspiration (ET) process and presents practical information on estimating actual crop ET from reference ET and a crop coefficient and its use for irrigation management.

• Evapotranspiration Basics
• How Irrigation Affects ET
• Soil Evaporation and Plant Transpiration
• Factors Affecting the Transpiration Rate
• Estimating Evapotranspiration for Irrigation Management
• Crop Coefficient Concept
• Source of Crop Coefficient Data for Different Crops
• Source of Local Reference Evapotranspiration (ET_ref) Data

Evapotranspiration Basics

In general, evapotranspiration (ET) can be defined as the transfer of water in the form of water vapor from the soil surface, a body of water, and vegetative and other surfaces to the atmosphere. Water evaporates from any moist surface into the air unless the air is saturated. Evapotranspiration in agro-ecosystems is the sum of two processes:

1. transpiration, in which water entering the plant roots is carried to stems and leaves for building plant tissue via photosynthesis and then passed through the leaves into the atmosphere, and
   
   
2. evaporation, which is water evaporating from soil, water surfaces, or plant leaf surfaces holding water droplets from rain, irrigation, or dew formation.

Evapotranspiration is usually the largest component of the hydrologic cycle, given that most precipitation that falls on land is returned to the atmosphere. Globally, about 60 percent of annual precipitation falling over the land surface is consumed by ET. On average, ET consumes about 70 percent of the United States annual precipitation, including more than 90 percent of the precipitation in the western and midwestern United States and as much as 100 percent in some desert areas. In Nebraska, 90-93 percent of precipitation is used for ET.

Quantification of ET is used for many purposes, including crop production, water resources management, and environmental assessment. In agriculture, accurate quantification of ET is important for effective and efficient irrigation management. When evaporative demand exceeds precipitation, plant growth and quality may be adversely affected by soil water deficit. A large part of the irrigation water applied to agricultural lands (Figure 1) is consumed by evaporation and transpiration.

Figure 1. Evaporation and transpiration and the factors that impact these processes in a center pivot-irrigated corn field.

In field measurements it is difficult to separate evaporation from transpiration, and the two processes are usually considered together as ET. In some areas, irrigation is used to supplement natural precipitation and minimize potential losses in crop production due to water deficits.

In a given agricultural field (for example, a center pivot-irrigated corn field, Figure 1), many soil, plant, and management factors will affect the ET process. These factors include plant species, canopy characteristics, stand density (plant population), degree of surface cover, plant growth stage, irrigation regime (over irrigation can increase ET due to larger evaporation), tillage practice, planting date, maturity group of the plant variety/cultivar, and soil water availability. However, ET is primarily driven by climatic conditions, such as air temperature, solar radiation, relative humidity of air, and wind speed.

As the plants transpire water and evaporation occurs from the soil and plant surface, water moves to the surrounding atmosphere in the form of very small water vapor particles (shown by the small black points in Figure 1). The movement of this water vapor within, from or to a field is mainly determined by wind speed and direction although other climatic factors also can play a role. Evapotranspiration increases with increasing air temperature and solar radiation — the two primary drivers of ET. Wind speed usually causes ET to increase, but not always. Above a certain wind speed the plant leaf stomata — the small pores on the top and bottom leaf surfaces that regulate transpiration — close. This reduces transpiration and ET. Plants also may close their stomata during a windy period if they are not able to maintain the evaporative demand of the surrounding air. Wind also can cause mechanical damage (lodging) to plants which can reduce ET due to reduced leaf area. Similarly, hail can reduce leaf area and ET. Higher relative humidity reduces ET as the demand for water vapor by the atmosphere surrounding the leaf surface decreases. A reduction in relative humidity (dry air) increases ET because low humidity increases the vapor pressure deficit (an indication of the dryness of the air) between the vegetative surface and air. Higher transpiration and evaporation will need to occur to meet the evaporative demand of the air for moisture. On a rainy day, incoming solar radiation decreases, relative humidity increases, and air temperature usually decreases, resulting in decreased ET. However, depending on climatic conditions, actual crop water use usually increases in the days after a rain event due to increased availability of water in the soil surface and crop root zone.

In a given crop field, ET may not occur uniformly due to variations in crop germination, soil water availability, and other factors such as non-uniform water and nutrient applications and an uneven distribution of solar radiation within the canopy. Generally, the top leaves are more active in transpiration than the lower leaves because they receive more light. Also, the bottom leaves mature and age earlier and they may have lower transpiration rates than the greener and younger top leaves.

How Irrigation Affects ET

In water deficit settings, ET will be less than in fully irrigated conditions because deficit-irrigated plants cannot transpire water at the same rate as fully watered, healthy, and actively growing plants. Under the same microclimatic conditions irrigated crops will have higher ET rates than rainfed crops. Under rainfed or deficit irrigation, the plant leaf stomata will close when the soil cannot supply water at a sufficient rate, or the root system is not extensive and efficient enough to withdraw water from the soil system to meet the atmospheric demand. Rainfed crops usually will have deeper and more extensive root systems than irrigated crops and can withdraw water from deeper soil layers. However, in the absence of rain, when the available soil water is depleted, rainfed plants will experience wilting and ET will be reduced. Beyond a certain water stress threshold, crop yield will decrease. Under rainfed conditions, seasonal ET of a crop usually will be close or equal to the sum of the available water in the soil profile and rainfall.

Soil Evaporation and Plant Transpiration

Evaporation in the field can occur from the crop canopy, from the soil surface, or from a free-water surface. When the soil surface is at least partly bare, evaporation can occur directly from the soil. This can constitute a considerable proportion of the ET, in both irrigated and rainfed conditions. Evaporation is a process that, if uncontrolled, can cause considerable water losses. In Nebraska, depending on the soil, climate, crop growth stage, and management practices, soil evaporation can be up to 20-30 percent (or more) of the seasonal total ET for irrigated crops.

Water lost through soil evaporation is wasted and does not contribute to crop yield. Soil evaporation is highest in the early growing season and gradually decreases as the plants develop and the canopy closes. It is minimal when the canopy completely shades the soil surface. Even under a fully closed canopy, some soil evaporation occurs. Soil evaporation increases toward the end of the season when leaf senescence and aging begin and transpiration is reduced. Several management practices can reduce soil evaporation. Early planting, when feasible, can result in early canopy closure, which will reduce soil evaporation. No-till and reduced-till practices also can reduce soil evaporation as compared to conventional tillage practices.

Water has three main functions in plants.

1. Water cools and hydrates plants and is essential for the transport of nutrients from the root system to stems and leaves.
   
   
2. Water is necessary for the transpiration process. Transpiration is minimal early in the growing season when plants are small and do not require much water, but increases with greater leaf area until complete closure of the canopy occurs. For irrigated crops, transpiration is usually the dominant water consumer (i.e., 65-70 percent of the seasonal total ET). Studies have revealed that transpiration accounts for about 10 percent of the moisture in the atmosphere, with oceans, seas, and other bodies of water (lakes, rivers, streams) providing nearly 90 percent. (A small amount comes from sublimation when ice changes into water vapor without first becoming liquid).
   
   
3. Photosynthesis uses less than 1 percent of the total water absorbed by plants, and the rest is lost through transpiration. Considering water makes up 90 percent of the weight of most crops, the consumption of less than 1 percent of water may seem unexpected, but plants use their water for other purposes such as transpiration to build green biomass and grain.

Factors Affecting the Transpiration Rate

The transpiration rate of a given crop is influenced by many factors, including crop type, soil and plant management practices, irrigation regime, climatic conditions, plant growth stage, plant genetic characteristics, leaf orientation, and leaf age. Reducing soil evaporation can make more water available for plant transpiration and increases crop’s water use efficiency because transpiration and crop yield are linearly and strongly correlated. Irrigation management practices should be designed to reduce soil evaporation, and make most of the water available for plant transpiration to increase plant water use efficiency. Each plant type has a unique transpiration rate. For example, corn usually uses more water per growing season than soybeans under the same climatic conditions so the total transpiration rate for corn will be higher than that for soybeans under the same climatic conditions. Although water is consumed by plants via transpiration, ET rate (transpiration plus soil evaporation) should be considered in determining the total water use of a given plant to estimate irrigation requirements.

Estimating Evapotranspiration for Irrigation Management

The main purpose of effective irrigation management is to deliver the proper amount of water to the field at the correct time to meet crop water requirements. In areas where rainfall supplies some of the seasonal crop water requirement, irrigation should be used as a supplement. Several approaches can be used to manage irrigation. These approaches vary from simple soil water measurement to the more complicated observation and measurement of plant water status and/or plant physiological parameters (i.e., plant temperature, relative water content of leaf, stem diameter, etc.). A common irrigation management approach is to measure soil water moisture in the crop root zone.

Determination of soil water status to schedule irrigations probably dates back to the origin of irrigated agriculture when advanced sensors and instruments were not readily available. Sometimes, growers or consultants use the hand-feel” method to determine when to irrigate and how much water to apply. The hand-feel method is generally inaccurate and usually provides false information, leading to under or over estimations of irrigation amount. It is a qualitative rather than a quantitative indicator. Today, advanced, accurate, durable, economical, and scientifically based tools can be used for irrigation management. These tools are readily available and should be used instead of the hand-feel method for more accurate irrigation management decisions.

Another approach for irrigation management is to use ET information to determine crop water requirement (crop water use) and replenish the soil profile to meet the amount of ET that is consumed by plants in a given period. Currently, a common practice for estimating the actual crop ET rate (or crop water use rate) for a specific crop (ET_c) requires first calculating reference evapotranspiration (ET_ref) and then applying the crop coefficients (K_c) to estimate actual crop ET as:

ET_c = ET_ref x K_c

where ET_c is the actual crop ET (inches/day, inches/week, inches/month) and ET_ref is the alfalfa-reference ET (inches/day, inches/week, inches/month). The ET_ref can represent either grass or alfalfa-reference ET. Since alfalfa-reference ET is more commonly used the Midwest, including Nebraska, in this publication ET_ref represent alfalfa-reference ET. Grass-reference ET is commonly used in other states, especially in the eastern, southern, southeastern, and western U. S.

Alfalfa-reference ET is defined as the ET rate from an extensive, uniform surface of dense, actively growing alfalfa, about 20 inches tall and not short of soil water. The terms “reference ET” and “reference crop ET” often are used interchangeably and they both represent the same ET rate from a tall alfalfa surface.

Crop Coefficient Concept

While ET_ref accounts for variations in weather and is used as an indicator of atmospheric demand for water, K_c values account for the difference between ET_ref and ET_c and link them. K_c is the crop coefficient for a given crop and growth stage, and is usually determined experimentally. Each agronomic crop has a set of specific crop coefficients used to predict water use rates at different growth stages. An example of a K_c curve as a function of days or weeks after planting is shown in Figure 2.

Figure 2. Crop coefficients (Kc) vary according to plant growth stage [(Kc)ini: crop coefficient for initial growth stage; (Kc)dev: coefficient for plant development stage; (Kc)mid: coefficient for mid season; and (Kc)end: coefficient toward the end of the season].

In general, there are four main crop growth stages: initial, crop development, mid season, and late season. The length of each stage depends on the climate, latitude, elevation, planting date, crop type, maturity group of different varieties or cultivars, and management practices. Early in the growing season during the crop germination and establishment stage, most ET occurs as evaporation from the soil surface. As the crop canopy develops and covers the soil surface, evaporation from the soil surface decreases and transpiration increases. Early in the season when the plant is small, the water use rate and K_c value also are small (K_c initial stage). As the plant develops, the crop ET rate increases (Figure 2). For agronomic plants, the crop ET rate is at the maximum level when the plant is fully developed (K_c mid-season). (Note that Figure 2 is not drawn to scale and the K_c values can be greater than 1.0.) The ET rate decreases again toward the end of the season when the plant reaches physiological maturity (K_c end season). The plant growth stage (leaf area), soil water status, and climatic conditions drive the rate of crop ET.

Source of Crop Coefficient Data for Different Crops

K_c values for many crops are publicly available. One of the best sources for crop coefficients is the High Plains Regional Climate Center (HPRCC: http://www.hprcc.unl.edu/). The alfalfa-reference crop coefficients for six major agronomic crops are presented in Table I based on the plant growth stages. After determining ET_ref on a daily or weekly basis, the ET_ref value is multiplied with the proper K_c value to estimate actual crop water use. For example, if the ET_ref for July 20-24 is 1.20 inches for corn at the silking stage (K_c = 1.10 from Table I), the actual crop water use is 1.32 inches (ET_c = ET_ref x K_c = 1.20 x 1.10 = 1.32 inches]. If the application efficiency of a center pivot is considered to be 85 percent, then 1.55 inches needs to be applied [1.32 inches ÷ 0.85 = 1.55 inches] to account for losses from the irrigation application and to meet crop water requirements. Local field observations to determine the actual crop growth stage are best for choosing the proper K_c values from Table I.

Source of Local Reference Evapotranspiration (ET_ref) Data

Two main sources of daily ET_ref values are available for Nebraska. One of the best, with widespread sources is the High Plains Regional Climate Center (HPRCC, http://www.hprcc.unl.edu/). It operates about 60 automated weather stations in Nebraska and many other stations in North Dakota, Kansas, South Dakota, and Colorado as part of an automated weather network. The main climatic data reported by HPRCC include: maximum and minimum air temperature, maximum and minimum relative humidity, solar radiation, rainfall, wind speed and direction, growing degree days, soil temperature, and ET_ref, on a daily and hourly basis. Users can subscribe to an HPRCC service to receive daily ET_ref information from the weather station closest to their fields for which the ET_c estimates are being made.

Another source for ET_ref is an atmometer (ET_gage™). Information about operational characteristics, installation, maintenance, data interpretation, and how it can be used to measure ET_ref, and its use for irrigation management are described in detail in the UNL Extension NebGuide, Using Modified Atmometers (ET_gage™) for Irrigation Management (G1579); (http://www.ianrpubs.unl.edu/epublic/live/g1579/build/g1579.pdf).

As part of a large effort to implement newer tools and technologies for irrigation management, the Nebraska Agricultural Water Management Demonstration Network (NAWMDN: water.unl.edu/cropswater/nawmdn) captures weekly totals from ET_gages at more than 200 locations scattered across Nebraska. On the NAWMDN Web site users can view ET_ref data from the ET_gage location closest to their fields to estimate crop water use for their locations.

This publication has been peer reviewed.