Evapotranspiration (ET) is a critical process that regulates the heat and water transfer between the land and atmosphere. Instantaneous satellite data can provide area-wide daily ET measurements. Surface energy budgets used in satellite-based calculations generally overpredict daily net radiation and the related ET. Our objective was to improve the accuracy of satellite-based daily evapotranspiration by correcting the overprediction of net radiation. To do so, we introduced a routine for calculating the downward longwave radiation on cloudy days. This new algorithm removed the upward bias in the predicted net radiation at seven ChinaFlux sites. Then, using previously developed methods for converting instantaneous measurement into daily averages, we found that evapotranspiration rates were predicted more accurately than existing methods at the same ChinaFlux sites. In addition, our evapotranspiration rates compared well with watershed ET and other calibrated and validated spatiotemporal gridded data products in China

Evapotranspiration (ET) is the major component of the hydrology cycle. Satellites provide a convenient way for gathering information to estimate regional ET. The most widely applied method for converting the instantaneous satellite measurement to daily scale assumes that evaporative fraction (EF), defined as the ratio of ET to the available energy, is constant during the daytime. However, this method was proved to underestimate the daily ET. This study implemented a theoretically improved EF algorithm to calculate daily ET with the decoupling factor method based on the Penman-Monteith and McNaughton-Jarvis equations. Seven improved algorithms were developed by assuming that various parameters remain constant during the day. The satellite-based ET estimates were compared with seven local flux tower measurements in China. The results showed that: (1) The original ET method calculated the daily evaporation more accurately than the other algorithms. However, the good fit was based on two compensating inaccuracies. Compared to the flux tower measurement, the original ET method underestimated the daily EF by 26% and overestimated the daily net radiation by 30%. (2) Six of the seven proposed algorithms underpredicted the daily ET by 30-60%, mainly due to the inaccurate daily net radiation. (3) The algorithm that assumed that the instantaneous decoupling parameter Ω* was equal to its daily value method calculated EF and ET with the relative errors of 8% and 10% when the inaccurate estimated daily net radiation was replaced by the observed flux tower data.