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Chapter 6

The primary purpose of this study has been to collect consumptive use data and to assess models for estimating evapotranspiration in the Green River Basin of Wyoming. The basin is a headwaters basin of the Colorado River and is subject to the terms of the Upper Colorado River Basin Compact of 1948. The study has been concerned mainly with water use as it relates to the compact. The main concern, therefore, has been to develop water use information concerning crop water use requirements.

Measurements of water use and climatic data were collected to provide a data base for defining evapotranspiration rates and model assessment. Water use measurements consisted of lysimeter and evaporation pan data. Fourteen water balance lysimeters were installed in the basin and operated for a three year period, with the first year considered as a start-up period. The 14 lysimeters consisted of 8 with mountain meadow vegetation, the main crop grown in the basin; 3 with alfalfa, the other major crop in the basin; and 3 with alta fescue as a reference crop. The major water use measurement obtained was that of maximum water use, with the intent of simulating water use conditions under intensive irrigation practices. Weekly water use measurements were taken, but most analyses were completed using monthly summaries of the water use rates since this was an assessment study where annual totals, and not short term variations, were of concern. Three evaporation pans were operated to obtain measurements of free water evaporation rates and to provide a second reference for the crop water use rates. Seven automated weather stations were operated with stations located to provide as complete coverage of the basin as possible. The stations measured weather parameters not available through other sources and provided data required for the most complex of the evapotranspiration formulas. Measured water use rates for alfalfa and mountain meadows were high and were close to the evaporation rates from Class A evaporation pans.

Eight evapotranspiration formulas, or variations thereof, and one evaporation formula were evaluated for their ability to predict water use rates in the Green River Basin. The evapotranspiration formulas included the original Blaney-Criddle, the SCS Blaney-Criddle, the original Jensen-Haise, the modified Jensen-Haise, the ASCE Penman, the FAO Blaney-Criddle, the FAO Radiation, and the FAO Penman. The Kohler-Nordenson-Fox formula was the evaporation formula considered. Analyses indicated a need for calibrating all formulas for Green River Basin conditions. Upon calibration, the SCS Blaney-Criddle, the modified Jensen-Haise, the ASCE Penman, and the FAO formulas all performed about the same in terms of predicting variations in water use across the basin. The other formulas, for one reason or another, did not perform as well as these equations. The calibrations consisted of developing crop coefficients for each equation, except in the case of the SCS Blaney-Criddle for which the temperature coefficient was also calibrated. The Kohler-Nordenson-Fox equation was compared with pan data, with estimated values comparing closely with measured pan evaporation. Calibrated versions of the temperature based formulas were found to perform as well as the more data-intensive formulas for the monthly and/or annual estimates required for water use assessment studies. The transfer of the calibrated models to other basins was shown to be only moderately successful , although differences between data taken in the Green River Basin and other sources may have contributed to the transferability question.

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