Date of Award:

5-2004

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Civil and Environmental Engineering

Department name when degree awarded

Irrigation Engineering

Committee Chair(s)

Wynn R. Walker

Committee

Wynn R. Walker

Committee

Richard G. Allen

Committee

James L. Wright

Committee

Lawrence E. Hipps

Committee

Christopher M.U. Neale

Abstract

This study evaluated the potential of using Evaporative Fraction (EF) and Reference Evapotranspiration Fraction (ETrF) as estimators of daytime and 24-hour evapotranspiration (ET). The crops under study were beans, potatoes, com, peas, and sugar beets grown at Kimberly, Idaho. The data set included hourly weather data and lysismeter ET measurements for each year and crop considered. The procedure included the calculation of the fractions mentioned using two approaches for ET (ASCE-EWRI and FAO 56-Penman-Monteith), to see which one best fit the lysimeter data. EF was calculated using net radiation measurements as well as net radiation predicted. Net radiation parameterization, by means of canopy albedo modeling, was made for the same row crops under study (except sugar beets). The albedo model is an empirical exponential model, based on sun altitude angle, which allows one to calculate canopy-soil albedo (αcs) for row crops. The modeled albedo provided good estimates of net radiation. Since no albedo measurements were available for the crops and years under study, it was not possible to validate the model with actual albedo measurements, but only with back-calculated albedo.

When comparing fractions for ET estimation, ETrF demonstrated to be a good estimator for potatoes and beans. ET0F performed well for potatoes and peas in daytime ET estimation. On the other hand, EF was very sensitive to advection, giving systematical underestimation of ET for all the crops. When considering 24-hour ET estimation, only ETrF performed well for potatoes, whereas the other fractions did not for any of the crops in this study.

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