Effect of Cloud Cover on Temporal Upscaling of Instantaneous Evapotranspiration

Abstract

Studying the effect of cloud cover on the temporal upscaling of instantaneous evapotranspiration (ET) is significant in the effort toward a more accurate and widely applied upscaling method to obtain the exact ET on a daily or longer time scale, thereby benefiting the practical applications. In this article, the authors concentrated on the effects of cloud cover in different amounts and for varying time durations, with three commonly used upscaling approaches including the constant evaporative fraction (EF) method, the constant reference evaporative fraction (EFr) method, and the constant global solar radiation (Rg) method. Transient cloud cover and persistent cloud cover were defined according to the occurrence time, namely, the cloud that appeared 1 h before or after the upscaling moment and the cloud lasting the whole day except during the upscaling time, respectively. The different cloud cover amounts were indicated by the different losses of downwelling shortwave irradiance. The instantaneous fluxes were simulated from the atmosphere-land exchange (ALEX) model, which was driven by the meteorology measurements at the Yucheng station in China. The results showed that (1) the cloud caused the deterioration of the underestimation or overestimation of the daily ET upscaling in comparison with the results of clear days. Specifically, the persistent cloud cover had a more significant effect on the three upscaling methods; for the transient cloud cover, the upscaling results had larger deviations when the cloud appeared before the upscaling moments than when it appeared after them; (2) the effects on the upscaling factors and the upscaling results both increased proportionally with the growth of the cloud cover; and (3) the constant EFr method performed best for both clear and cloudy situations, with a minimal bias less than 4.7 W/m2 (5.5%) and a root-mean-square error (RMSE) less than 8.9 W/m2 (2.6%); the EF method was most severely affected, with a bias up to 24.1 W/m2 (28.3%) and an RMSE up to 24.9 W/m2 (57.7%); the Rg method had an intermediate accuracy with a bias less than 2.9 W/m2 (24.6%) and an RMSE less than 2.3 W/m2 (47.1%); and (4) all three approaches were influenced most significantly around noontime.