A quantitative approach to the Pruitt and Doorenbos version of the Penman equation |
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| Authors: | Albert Weiss |
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| Institution: | (1) Agricultural Meteorology, University of Nebraska, Panhandle Station, 4502 Avenue I, 69361 Scottsbluff, NE, USA |
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| Abstract: | Summary The Pruitt and Doorenbos version of the Penman equation developed from information given in Appendix II of FAO Irrigation and Drainage Paper No. 24 is calculated mainly from tables and is based on measurements made over a grass surface. Procedures are presented here to quantify these relationships, such as the calculation of net radiation, and to extend this approach to measurements made over alfalfa (Medicago sativa, L.). The latter was achieved by using a wind function that takes into account the height of the alfalfa and the use of mean rather than maximum relative humidity to calculate the correction factor used to take into account day and night weather conditions on calculated reference crop evapotranspiration. Using the above procedures, calculated values of evapotranspiration overestimated measured values from alfalfa by 13%. From data collected with an automated weather station near Broadwater, Nebraska, a much better agreement was obtained between these general procedures and a Penman equation with a locally derived- wind function. With greater utilization of low cost, automated weather stations for agricultural use, the procedures given for calculating reference crop evapotranspiration can easily be implemented in irrigation scheduling programs.Published as paper no. 6865, Journal Series, Nebraska Agricultural Experiment Station |
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