Understanding and Accounting for Crop Water Use
Water use is the driving force for plant growth and models using weather variables can estimate this use.
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Figure 1. Transpiration accounts for the majority of ET. |
A familiar fact about western US irrigation is that agricultural crops account for 80 to 90 % of all water use. Two main factors account for this apparent anomaly. First, the amount of land area devoted to irrigated crops in this region is much higher than any other enterprise or activity requiring water. Second, actively growing plants use a lot of water. The purpose of this article is to first explain why plants use so much water and then to describe how we account for the amount of water crops are actually using.
The water requirement of irrigated crops varies widely depending on a number of factors. Crude studies conducted in the early 1900’s using a diverse array of crops revealed that the amount of water used to produce a pound of dry matter varied from 300 to 1000 pounds. The plant tissue associated with each pound of plant dry matter contains only a little over four pounds of water. This amounts to less than 0.1 % of the total water requirement, assuming the best-case scenario of 300 pounds of water use per pound of dry matter. So where does the rest of the water go? The best answer is, “…into thin air.” In other words, more than 99.9% of the total water requirement of an irrigated crop is consumed by evaporation (from water occurring on either soil or crop surfaces) and transpiration. Transpiration refers to the evaporation that occurs from water on internal plant surfaces. The combined water loss from the processes of evaporation and transpiration is called evapotranspiration or ET. The cumulative amount of ET for a crop over an entire growing season is roughly equivalent to that crop’s seasonal water requirement.
For irrigated crops that reach complete ground cover for most of the growing season, most of the seasonal ET is from transpiration (Figure 1). Transpiration water losses from a crop that completely covers the ground are similar in magnitude to observed evaporation from the surface of an open water body of comparable area. Although transpiration losses are high, they are directly linked to crop growth and, therefore, yield. This is because the pathway for transpiration water losses in plants is the same one that allows for plant uptake of carbon dioxide, which is the raw material for photosynthesis. Both exchange processes occur through pores called stomates on the leaf surface. When soil water is not limiting, which is usually the case under irrigated conditions, stomates are fully open. When this condition exists, both transpiration and photosynthesis are occurring at maximum rates allowed by current conditions both internal and external to the plant. If soil water becomes limiting, stomates begin to close, limiting both transpiration water losses and photosynthesis.
A key ingredient of irrigation water management is the ability to estimate the magnitude of ET losses for any given set of conditions. The most important factors that have to be accounted for are: 1) the local weather conditions and 2) the cropping system for which estimates are needed (type of crop, planting date, etc.). Local weather conditions are important because ET is driven by weather factors that determine the drying power of the air. A branch of science known as agricultural meteorology has provided good insight into the variables that drive evaporation of water from soil and crop surfaces. We can accurately predict ET losses in a given area from measurements of four local weather variables; solar radiation, temperature, humidity, and wind. To be useful, these measurements have to be made under a standardized set of conditions. By convention, the variables are measured using instrumentation of specific design located within large areas devoted to stands of irrigated grass or alfalfa. The data from these measurements are then used in specially calibrated equations that accurately predict the daily rate of ET for these standardized conditions. The values obtained from this process provide standardized measurements of ET that are referred to as reference ET. The term, reference, refers to standardization of the entire process including type of crop used under the weather-monitoring instrumentation, the weather variables measured, and the calculations performed. When all these factors are accounted for, the ET of the reference crop, which is designated as reference ET, can be estimated with great accuracy. In most cases, reference ET values are generated on a daily basis. The specific calculations used are from a set of calculations known as combination equations. The common name of Penman is often used to refer to the equations used.
Reference ET (ETref) values apply to a specific reference crop grown (usually alfalfa or grass) under a set of local weather conditions. To be useful for other crops within the area in which the reference values were obtained, ETref values have to be adapted to fit these other crops. This is accomplished by adjusting the ETref values by use of a crop coefficient. Locally adapted crop coefficients are available for most kinds of crops that are likely to be grown in a given area. These coefficients provide daily adjustments to the ETref values generated each day throughout the growing season. In practice, the coefficient is used as a multiplier such that the actual daily ET for a given crop on a specific day of the season is the product of the ETref obtained for that date times the crop coefficient for that same date. The procedures described here are for use under conditions where soil moisture is not limiting. If moisture does become limiting, an additional adjustment factor, called the soil coefficient, can be applied in addition to the crop coefficient. A discussion of how to use the soil coefficient factor can be found in the fact sheet publication mentioned below.
Most states, including Colorado, have a network of weather stations that provide localized reference ET values. The network for Colorado is called the CoAgMet network, and is accessible through the web pages of the Colorado Climate Center (web address: www.coagmet.com). This network provides local reference ET values on a daily basis throughout the growing season for most of Colorado. Crop coefficients for specific crops can be obtained through local Cooperative Extension offices or from a CSU Cooperative Extension Fact Sheet titled, Irrigation Scheduling: The Water Balance Approach (Fact Sheet number 4.707), which can be accessed on line at through CSU’s Cooperative Extension web pages: (http://www.ext.colostate.edu/PUBS/crops/04707.html).
