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United States Department of Agriculture

Agricultural Research Service

Title: Derivation of Alfalfa and Grass Reference Evapotranspiration

Author
item Wright, James

Submitted to: International Evapotranspiration Irrigation Scheduling Conference
Publication Type: Proceedings
Publication Acceptance Date: November 1, 1996
Publication Date: N/A

Interpretive Summary: In arid and semi-arid climates, irrigation is often an important part of agricultural crop production and of maintaining landscaping. The rate of water use by irrigated crops and landscaping needs to be known so that irrigations can be efficiently scheduled to meet plant needs while avoiding excessive or wasteful water applications. It is usually impractical to directly measure the use of water by field crops or landscapes. Consequently, considerable research during the last several years has been aimed at developing mathematical relationships for calculating crop water use from meteorological data which is or can become available. Some of the recently developed methods require a reference evapotranspiration to characterize the drying power of the air mass moving over the land surface. The reference evapotranspiration characterizes the rate at which water evaporates from the plant leaves and the soil surface in response to climatic conditions. In the reported research, we were able to develop procedures for calculating either an alfalfa-based or a grass-based reference ET using daily meteorological measurements of air temperature, humidity, windspeed and solar radiation. The grass-based reference evaporation results are new for the Southern Idaho region and may be transferable to other similar regions. They will facilitate national and world-wide efforts to develop standardized procedures for computing climatically based reference evapotranspiration values applicable for use in methods of computing water-use in agricultural and urban irrigated situations.

Technical Abstract: The reference evapotranspiration (ET) for a specified reference crop characterizes the rate at which water, when readily available within the root zone, is evaporated from the plant and soil surfaces in response to climatic conditions. Crop ET (Etc) for a given crop can be computed from the reference ET using an appropriate ET crop coefficient (Kc). Reference ET's reported here were derived from meteorological and weighing lysimeter data obtained from 1968 through 1991 at the USDA-ARS, ET research site in southern Idaho (Kimberly). By 1982, the Penman method had been modified to provide an alfalfa-based reference ET (ETr), often referred to as the 1982 Kimberly-Penman method, for use with irrigated crops in arid regions. Now, because of world-wide interest in using a reference ET based on a clipped grass surface (ETo), wind functions, similar in nature to those used to compute Etr, have been derived to compute daily grass reference ET (ETrg), for 'Fawn' tall fescue grass, using the same meteorological data as used to compute Etr. On a seasonal basis, ETrg was 83% of ETr.

Last Modified: 8/19/2014
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