Accuracy needs to be improved when extrapolating instantaneous remote sensing evapotranspiration (ET) values for daily ET estimates. Daily net radiation () is a critical variable for this extrapolation. We expand the de Bruin and Sticker formula for 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 calculation. The coefficient for daily net longwave radiation () in 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.