Understanding Plant Water Use. Evapotranspiration (ET)

ET Facts
What is ET?
How does ET occur?
How is ET Estimated?
What is Reference ET?
How to Use ET Provided on CoAgMet

More than 99.9% of the water used by an irrigated crop or turf is drawn through the roots and transpires through the leaves. Only small amount (0.1%) of the water taken up by plants is actually used to produce plant tissue.

The overall amount of ET for a crop or landscape over an entire growing season is about the same as the seasonal water requirement.

Keeping track of ET in your local area can help you more efficiently irrigate your crops and landscapes.

What is ET?

Evapotranspiration (ET) is a term used to describe the water consumed by plants over a period of time. Evapotranspiration is the water loss occurring from the processes of evaporation and transpiration. Evaporation occurs when water changes to vapor on either soil or plant surfaces. Transpiration refers to the water lost through the leaves of plants.

How does ET occur?

For irrigated crops and landscapes that provide complete ground cover for most of the growing season, the majority of the seasonal ET is from transpiration. Transpiration losses are usually high and are directly linked to plant growth and productivity. This is because the pathway for transpiration in plants is the same one that allows for plant intake of carbon dioxide. Both exchange processes occur through pores called stomates on the leaf surface. Stomates are fully open when plants receive enough water through the soil and when both transpiration and photosynthesis are occurring at maximum rates. If soil water becomes limiting, stomates begin to close causing a decrease in transpiration and photosynthesis.

How is ET Estimated?

The most important factors needed to estimate ET are: 1) the local weather conditions and 2) the cropping system for which estimates are needed (type of crop, planting date, crop development).

Local weather conditions are important because ET is driven by weather factors that determine the drying power of the air. We can accurately predict ET losses in a given area from the measurements of four local weather variables:

1. solar radiation
2. temperature
3. humidity
4. wind

What is Reference ET?

The term "reference" refers to the ET equations calibrated to estimate the water use of a well-watered alfalfa or grass field under a set of local weather conditions. The reference ET value provided in CoAgMet outputs are computed using an Alfalfa reference equation (1982 Kimberly Penman). To use ETref values for other crops the ETref values must be adjusted by using a crop coefficient or multiplier. The daily ET for a specific crop is the product of the ETref multiplied by the crop coefficient for that same date.

Reference ET values are for conditions where soil moisture is not limiting (greater than 50% field capacity). If moisture does become limiting, a soil coefficient value 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: Irrigation Scheduling: The Water Balance Approach (Fact Sheet number 4.707; http://www.ext.colostate.edu/pubs/crops/04707.html).

How to Use ET Provided on CoAgMet

Consider the soil water in your field or landscape as a bank account. In the soil, daily ET amounts are withdrawals from the account of soil storage. Any rainfall or irrigation is a deposit to soil storage. When an initial soil water value is determined, the water balance can be estimated by subtracting ET daily or over a period of time. Should the water balance calculations project soil water to drop below some minimum level, the irrigation is needed. The following is an example of a soil water balance:

Several worksheets, spreadsheets and computer programs are available to make using this type of irrigation scheduling easier. You can find some of these offered at no cost online at:
http://ccc.atmos.colostate.edu/~crop/
http://www.oznet.ksu.edu/irrigate/Software/Irrsoftware.htm

ET can also be used to estimate the next irrigation by comparing to system capacity. For example, your system irrigated a net application of 1.5 inches to your field four days ago. The average ET rate since this irrigation has been 0.25 inches / day and there has been no rain. It will take two days to irrigate this field and you are anticipating a similar ET rate during that time. Therefore, you need to begin irrigating today to maintain your soil moisture and keep up with crop demand. ({4 days since last irrigation + 2 days to irrigate field} x 0.25 inches /day = 1.5 inches).

The CoAgMet outputs provide reference (ETref) and daily ET values for nine potential crops. The crop ET values are calculated for you automatically by multiplying the ETref and a crop coefficient, determined by using the default or user entered planting date. For crops not provided in CoAgMet outputs, consult with your local Natural Resources Conservation Office (NRCS) or water conservancy district for crop coefficients appropriate for your area. The provided ETref can then be used by multiplying it by the appropriate coefficient.

ET-based scheduling using the water balance approach should be verified periodically by checking soil moisture in the field with a probe or shovel. Refer to the fact sheet, Estimating Soil Moisture, 4.700 (http://www.ext.colostate.edu/pubs/crops/04700.html) for help in this process.