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WWRC 86-11
Development of Evapotranspiration Crop Coefficients, Climatological Data, and Evapotranspiration Models for the Upper Green River


Information on evapotranspiration in the Upper Green River Basin of Wyoming has been developed. The suitability of various evapotranspiration models for estimating water use of agricultural crops has been investigated. Measurements of water use and climatological data were taken in the Green River Basin to provide a source of data for calibration of evapotranspiration models.

Fourteen non-weighing water balance lysimeters were installed in the Basin during the fall of 1982 and spring of 1983. The lysimeters consisted of 3 with alfalfa, 3 with alta fescue, and 8 with mountain meadow vegetation. Weekly measurements of maximum water use were taken with these lysimeters during the 1984 and 1985 growing seasons. The alfalfa and mountain meadow lysimeters provided direct measurements for the primary vegetation in the Basin while the alta fescue served as a reference crop. In addition, three evaporation pans were operated to provide measurements of free water surface evaporation as well as provide another reference for crop water use rates. Seven automated weather stations were installed to give climatic data throughout the Basin.

Calibration of a number of evapotranspiration formulas, based on climatic data, was performed. The formulas ranged from those based only on temperature data to combination formulas which require temperature, radiation, wind, and humidity data. The ability of the models to estimate water use rates throughout the Basin was analyzed. Crop coefficients for alfalfa and mountain meadows were developed. Results indicated that no one equation provided the best results under all conditions. However, the temperature based equations did permit, with calibration, estimates of water use comparable to estimates obtained using the more complex equations.

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