Evapotranspiration Equations Now Available on Irrigation Association Web Site

ET is the loss of water from the soil both by evaporation and by transpiration from the plants growing thereon. ET equations help predict and estimate the amount of water that is used in the ET process. The equations range from simple estimates, using only temperature, to very complex equations that involve energy balance of the water movement. The more complex equations measure the influence of solar irradiance, air temperature, humidity and wind speed on the rate of ET. This is calculated as a volume of water per time period per unit area and is generally expressed as inches per day or mm per day.

In most cases, ET information is calculated for a reference crop. The two reference crops most frequently used in the U.S. are full-cover alfalfa (about 2 ft. tall) and clipped, cool-season grass (about 4 to 6 in. tall). In either case, the reference crop must be modified by a crop coefficient (Kc) to more accurately reflect the amount of water actually used by individual plants or crop types. Each type of vegetation has its own crop coefficient curve that is based upon the reference ET equation being used, the reference crop, the specific crop variety, and stage of growth. Therefore, it is critical that users of reference crop procedures and their respective crop coefficients understand that these coefficient curves may not directly transfer between the reference procedures.

The IA’s list includes ET equations, reference crop coefficients, and agency and contact personnel for state and regional ET information. According to the IA, the list serves to minimize confusion over ET equations that arises from the lack of standard equations between states. The IA and The American Society of Civil Engineers are currently working on establishing a standardized ET equation.

The IA plans to revise and update the list as needed to maintain its accuracy. The IA has also requested help from people within the industry who can assist in making the ET information better understood and more readily available.

For more information please visit www.irrigation.org/ia/about/wtrmgmt.html. Click on the "ET Connection" link to view the ET equation list.