INFLUENCES OF ANGSTROM'S COEFFICIENTS ON ESTIMATE OF SOLAR RADIATION AND ReFERENCE EVAPOTRANSPIRATION BY PENMAN-MONTEITH EQUATION

摘要

In the estimation of evapotranspiration based on the reference evapotranspiration (ET0) and crop coefficients, net radiation Rn derived from extraterrestrial radiation and actual duration of sunshine is the most important parameter in FAO-56 Penman-Monteith equation.Since the hypothesis of uniform albedo of 0.23 can never be realized in the measurement of net radiation, solar radiation Rs is more convincible than net radiation Rn in the evaluation of the Angstrom's coefficients as and b8 on calculation of radiation and ET0. In order to evaluate influence of as and bs values on estimate of solar radiation and hence the ET0 based on solar radiation measurement, case study was carried out in north China, two regimes of as and bs values including the FAO recommended one and the local calibrated one were used. Results show that calibrated values of as and bs yield more reasonable results of Rs, Rn and ET0 when it compared with results. derived from the measured solar radiation. Further study was conducted under different relative sunshine hour n/N, it indicates that errors of Rs, Rn and ET0 increased greatly with the reduction of n/N, and the calibrated values of as and bs performs better than the recommended one. In case of n/N≥0.67, both Angstrom's coefficients regimes perform well in the determination of Rs, Rn and ET0, but difference between as and bs regimes become increasingly remarkable with the reduction of n/N. Therefore calibration of as and bs is quite essential for the calculation of radiation and ET0 with measurement of solar radiation but not net radiation,especially in the case of lower sunshine duration.