Evapotranspiration can be measured using lysimeters, which are closed systems that monitor the changing weight of water due to inflows or outflows in the system. Precipitation and irrigation adds water and weight. Evapotranspiration and drainage decrease wa- ter and weight. Lysimeters are expensive and technically demand- ing to manage, so they generally are used for research of regional crop coefficients.

Scientific methodology using empirical equations and climatic parameters as input variables can be used to estimate ET.

designated as ETR. Crop coefficients or plant factors adjust the ref- erence ET to estimate the water needs of specific plants or crops.

be noted that the crop coefficient for most agricultural crops varies through the season with the crop’s size and growth stage.

Crop coefficients are generally measured in lysimeters and are available in the U.S. for landscape plants, turf grasses and most crops. The reference ET is multiplied by the crop coefficient to obtain ET for the crop, which is often abbreviated as ETc

ET can also be estimated, and irrigations scheduled, by monitor- ing soil moisture in the rootzone. Most soil moisture measuring

A commonly used equation is the ASCE standardized Penman- Monteith equation. Climatic inputs are temperature, relative humidity, solar radiation and wind speed. Those inputs allow for the estimation of reference evapotranspiration, which is designed to estimate the ET from well-watered cool-season grass and is abbreviated as ETo

. When alfalfa is used as the reference crop, it is

instruments are a proxy for soil moisture; in other words, the instrument measures some parameter other than measuring soil water content directly. Regardless, soil moisture instrumentation can be quite useful in understanding ET. Irrigation and precipita- tion increases soil moisture, while ET decreases soil moisture in the rootzone.

Weather station networks in many states are used to geographi- cally monitor climatic parameters including those required for estimating ET. The CIMIS weather station network (www.cimis. in California is quite well known as is COAgMet ( in Colorado and AZMET ( in Arizona.

. It should

A water balance can apply to an agricultural field, a golf course or park landscape, or a whole river system. The closed system, regardless of its geographic scale, can be thought of as a flower- pot. As shown in the graphic, water comes into the pot by irriga- tion or precipitation, and leaves the system as ET, or, if the inputs exceed the capability of the soil to absorb and hold water, as deep percolation, or runoff, often called irrigation return flows. Change in soil moisture storage in the plant rootzone can be predicted and verified by measurements.

As water shortages become more prevalent, water use efficiency is critical. The understanding of evapotranspiration and the over- all water balance of a system becomes increasingly important to both professionals and practitioners in the industry.

This graphic illustrates pressurized irrigation and the associated lesser return flows afforded by the higher irrigation efficiency of drip or sprinkler irrigation.

Dr. Stephen Smith is a consulting agricul- tural engineer in Colorado and was presi- dent of the Irrigation Association in 2009.

Dr. Tom Trout is recently retired as research leader with the USDA ARS Water Management Research Unit in Colorado. 17

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