Richard G. Allen

Chapter 8. Water Requirements

Evapotranspiration (ET) calculation guidelines are based on the crop coefficient-reference evapotranspiration method (Kc ETref). Equations for the ASCE-EWRI standardized Penman-Monteith method are provided for grass and alfalfa references, where the grass reference standardization follows the FAO Penman-Monteith procedure. Linearized FAO-style crop coefficients from FAO-56 and curvilinear coefficients from Wright are presented as both mean and as dual (basal) crop coefficients. ET coefficients for landscape utilize a decoupled procedure similar to that summarized by the Irrigation Association Water Management Committee. Guidelines for calculating irrigation water requirements and peak system design rates are described.

Adjusting Wind Speed Measured over Variable Height Alfalfa for Use in the ASCE Standardized Penman-Monteith Equation

The ASCE Standardized Reference Evapotranspiration Equation expects the weather station wind speed data to represent that occurring at a height of 2 m over and downwind of a smooth measurement surface such as clipped grass. The Task Committee on the Standardized Equation provided guidance for adjusting wind speed measured at height other than 2 m, or, for situations when the wind speed is measured over and downwind of 0.5 m alfalfa. The latter adjustment attempts to account for the effects of both grass and alfalfa crop characteristics (height, roughness) on the wind profile. A more physically representative approach to adjust wind speeds at various heights and various weather measurement surface conditions to equivalent wind speed at 2 m height over clipped grass is tested. Wind speeds were simultaneously measured during the 2008 growing season at 2-m and 3-m heights above ground surface over variable height alfalfa at two Colorado Agricultural Meteorological Network (CoAgMet) electronic weather stations and at the research alfalfa lysimeter installation at the Colorado State University Arkansas Valley Research Center. These wind speed measurements were adjusted to estimated wind speed at 2 m over grass, and compared.

Estimation of Evaporation and Evapotranspiration during Nongrowing Seasons Using a Dual Crop Coefficient

Evapotranspiration and net irrigation water requirement estimates were updated in 2007 by Allen and Robison (2007; Allen et al., 2007) for agricultural areas in Idaho. ET calculation procedures were employed that use an updated procedure to calculate crop coefficients that considers the impact of surface wetting by irrigation and precipitation on total evapotranspiration. ET was calculated for daily, monthly and annual timesteps for 123 weather station locations across Idaho for complete, available periods of record. ET estimates were made for all times during the calendar year including winter to provide design and operation information for managing land application of agriculture, food processing and other waste streams and to provide full-calendar year estimates of E and ET for hydrologic studies and to estimate beginning of growing season soil water content.

Translating Wind Speed Measured over Alfalfa for Use in the ASCE Standardized Reference ET Equation

Weather station wind speed data for use in the ASCE Standardized Reference Evapotranspiration Equation are expected to represent that occurring at a height of 2 m over and downwind of a smooth measurement surface such as clipped grass. Many agricultural weather stations used for evapotranspiration estimation may not meet such siting conditions for all or part of the growing season. A physically-based approach to adjust wind speeds at various heights and various weather measurement surface conditions to equivalent wind speed at 2 m height over clipped grass is tested. Wind speeds were simultaneously measured during the 2008 and 2009 growing seasons at 2-m and 3-m heights above ground surface over and downwind of variable height alfalfa in several southeast Colorado locations. These wind speed measurements were translated to estimated wind speed at 2 m over grass, and compared. The effects on computed reference evapotranspiration are illustrated and discussed. The results have important consequences in determination of the State of Colorado’s compliance with the Arkansas River Compact.