What is ET?

Description of ET

Evapotranspiration = (ET)

ET is the abbreviation used for the word evapotranspiration. Evapotranspiration is the combination of two separate processes whereby water is lost from the soil surface by evaporation or used by plants called transpiration. The evaporation and transpiration occur simultaneously and is measured in millimeters or inches of water per time period which could be hourly, daily, weekly, monthly or yearly.

ET is calculated by measuring the energy from various sources that impact plant growth. These energy sources are solar radiation, wind, air temperature and humidity and are measured by various instruments on weather stations.

ET is calculated for an actively growing reference crop that is well watered and managed. The reference crops are either grass or alfalfa. The grass reference is for a cool season grass that is 12cm (@ 5 in.) tall and the evapotranspiration is expressed as ET_O. Alfalfa reference is ET_R for a uniform crop that is 50cm (@ 20 in.) tall. In Colorado, both references are acceptable and used. Turf managers prefer the grass reference, while agricultural producers prefer to use the alfalfa reference.

There are numerous ET equations. The equation that has proven to be the most effective in calculating reference ET is the ASCE Penman Monteith equation for cool season grass at 12 cm. The 1982 Kimberly Penman equation for alfalfa has likewise proven to be effective. All of the participating ET networks, (Denver Water, Colorado Springs Utilities, NCWCD and CoAgMet) use the Penman-Monteith equation for grass.

CoAgMet and Northern Colorado Water Conservancy District provide alfalfa reference ET using the 1982 Kimberly Penman for agricultural producers.

Since either choice is a reference, the end user should modify the reference ET to match the needs of the particular crop and situation. The modifications are done with crop coefficients (Kc). It is important to know for which reference crop the Kc is to be used, since there is about a 25-30% difference in the amount of ET between grass and alfalfa. Alfalfa being a larger plant requires more water. The crop coefficients should be expressed as Kc_O for using with grass reference and Kc_R for using with alfalfa reference.

For cool season Kentucky Bluegrass at various mowing heights the following tables list crop coefficients that should be used with the appropriate reference ET.

ET is the abbreviation used for the word evapotranspiration. Evapotranspiration is the combination of two separate processes whereby water is lost from the soil surface by evaporation or used by plants called transpiration. The evaporation and transpiration occur simultaneously and is measured in millimeters or inches of water per time period which could be hourly, daily, weekly, monthly or yearly.

ET is calculated by measuring the energy from various sources that impact plant growth. These energy sources are solar radiation, wind, air temperature and humidity and are measured by various instruments on weather stations.

ET is calculated for an actively growing reference crop that is well watered and managed. The reference crops are either grass or alfalfa. The grass reference is for a cool season grass that is 12cm (@ 5 in.) tall and the evapotranspiration is expressed as ETO. Alfalfa reference is ETR for a uniform crop that is 50cm (@ 20 in.) tall. In Colorado, both references are acceptable and used. Turf managers prefer the grass reference, while agricultural producers prefer to use the alfalfa reference.

Using ET for Irrigation Scheduling