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Improving temporal extrapolation for daily evapotranspiration using radiation measurements

Abstract Accuracy needs to be improved when extrapolating instantaneous remote sensing evapotranspiration (ET) values for daily ET estimates. Daily net radiation ( R n 24 ) is a critical variable for this extrapolation. We expand the de Bruin and Sticker formula for R n 24 in the evaporative fraction method by incorporating ground radiation measurements. In addition, an improved daily transmittance formula that considers atmospheric conditions and solar declination is proposed for R n 24 calculation. The coefficient for daily net longwave radiation ( R nl 24 ) in R n 24 is regressed from the ground radiation measurements and the improved daily transmittance values. A comparative study was conducted in the Haihe River basin, China. Experimental results show that the improved method yields a root mean square error of 0.97 mm for daily ET and only about 2% underestimation for actual seasonal ET in the study area. These results are better than the values obtained by traditional methods.