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L. Pochop
R. Burman
March 1987

Agricultural Engineering Department
University of Wyoming
Laramie, Wyoming


Submitted to

Wyoming Water Development Commission
Cheyenne, Wyoming


Wyoming Water Research Center
University of Wyoming

Contents of this publication have been reviewed only for editorial and grammatical correctness, not for technical accuracy. The material presented herein resulted from objective research sponsored by the Wyoming Water Research Center, however views presented reflect neither a consensus of opinion nor the views and policies of the Water Research Center or the University of Wyoming. Explicit findings and implicit interpretations of this document are the sole responsibility of the author(s).


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.

Chapter 1 - Introduction
Chapter 2 - Climatological Data
Chapter 3 - Evapotranspiration Measurements
Chapter 4 - Procedures for Estimating Evapotranspiration
Chapter 5 - Basin Estimates
Chapter 6 - Summary
Literature Cited
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Appendix G
Appendix H

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