Evaluating the Impact of Daily Net Radiation Models on Grass and Alfalfa-Reference Evapotranspiration Using the Penman-Monteith Equation in a Subhumid and Semiarid Climate
Publication: Journal of Irrigation and Drainage Engineering
Volume 137, Issue 2
Abstract
Net radiation is the main driving force of evapotranspiration (ET) and is a key input variable to the Penman-type combination and energy balance equations. However, is not commonly measured. This paper analyzes the impact of 19 net radiation models that differ in model structure and intricacy on estimated grass and alfalfa-reference ET ( and , respectively) and investigates how climate, season and cloud cover influence the impact of the models on and . Datasets from two locations (Clay Center, Nebraska, subhumid; and Davis, California, a Mediterranean-type semiarid climate) were used. values computed from the 19 models were used in the standardized ASCE-EWRI Penman-Monteith equation to estimate and on a daily time step. The influence of seasons on the estimation of and on estimated and was investigated in winter (November–March) and summer (May–September) months. To analyze the influence of clouds on the impact of models, relative shortwave radiation was used as a means to express the cloudiness of the days as: for completely cloudy days; for partially cloudy days; and for clear sky days. The performances of models showed variations at the same location and between the locations for the same model based on methods used to calculate various model parameters. The most significant impact of on estimated and was related to the methods used to calculate atmospheric emissivity rather than methods used to calculate clear sky solar radiation or cloud adjustment factor . models that used average air temperature to compute and an estimated resulted in good performances at both locations. Empirical models that assumed showed poor to average performances at both locations. While model performances varied based on methods used to calculate , , and , there were significant seasonal variations in performances of models that calculated as a function of actual vapor pressure of the air . The seasonal variations in performances of these models were greater under subhumid climate at Clay Center than in semiarid climate at Davis, Calif. The models that calculated as a function of performed better under completely cloudy days than on other days, more so at Clay Center. Methods used to calculate have a significant impact on the model performance, especially in unstable climatic conditions such as at Clay Center where there are frequent and rapid changes in climatic variables in a given day and throughout the year. The results of this study can be used as a reference tool to provide practical information on which method to select based on the data availability for reliable estimates of daily relative to the ASCE-EWRI method in subhumid and semiarid climates similar to Clay Center, Neb. and Davis, Calif.
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© 2011 ASCE.
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Received: Mar 23, 2010
Accepted: Jul 13, 2010
Published online: Jul 16, 2010
Published in print: Feb 2011
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