ISHS Acta Horticulturae 278: Symposium on Scheduling of Irrigation for Vegetable Crops under Field Condition
TWO METHODS OF DETERMINING ATMOSPHERIC EVAPORATIVE DEMAND FOR PURPOSES OF SCHEDULING IRRIGATION |
| Authors: | W.H. van Zyl, J.M. de Jager |
| Abstract:
An agrometeorological approach to irrigation scheduling requires accurate determination of atmospheric evaporative demand, AED. Two methods of achieving this were investigated. The accuracy of the methods was tested against lysimeter measurements. Firstly the product of the Penman-Monteith equation and a new crop evaporation coefficient, kc, consisting of the sum of crop coefficients for plant evaporation, kv, and soil evaporation, ks, were proved to be accurate. The index of agreement and the mean absolute difference was 0.91 and 0.41 mm d-1. Secondly an alternative measure of the aerodynamic component (AC) of the Penman-Monteith equation, using a carborundum atmometer, was shown to provide accurate estimates of AED. The empirical equation for estimating AC in the Penman-Monteith equation utilizing a carborundum atmometer, screened from long and short wave radiation, was AC = 0.008 E + 0.06 (mm h-1) The correlation coefficient found was 0.95. E denotes evaporation from the carborundum disc in divisions. Estimated AED was obtained from the sum of the energy component in the Penman-Monteith equation and AC. The index of agreement and mean difference between estimated and measured AED was 0.95 and 0.03 mm h-1 respectively during 1985. Corresponding values for 1986 were 0.92 and 0.00 mm h-1. This technique offers a considerable advantage because it eliminates measurement of relative humidity and windspeed. | |
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