Temporal upscaling methods for daily evapotranspiration estimation from remotely sensed instantaneous observations

Evapotranspiration (ET), including evaporation from soil surface and vegetation transpiration, is an important component for water and energy balances on the Earth's surface. The quantification of ET at daily or long time scales is significant in modeling the global hydrological cycle, studying climate change, and managing water resources. However, current remote sensing-based ET models can generally only provide snapshots of ET at the time of a satellite overpass and do not satisfy the expectations of hydrologists, irrigation engineers, and water resources managers concerned with practical applications. In this paper, a comprehensive overview of the methods for estimating daily ET from remotely sensed instantaneous observations is presented. These methods include the constant upscaling factor methods (constant evaporative fraction, constant decoupling factor, radiation-energy derived, constant reference evaporative fraction, and constant surface resistance) and data assimilation method. The commonly used approaches are compared with a discussion regarding the main merits, limitations, and accuracies. The problems and uncertainties of the temporal upscaling of ET, including the evaluation of model applicability, the daily variation of cloud, the spatial interpolation accuracy of meteorological parameters, nighttime ET, the uncertainties from the temporal upscaling methods and ET models, and the approaches of accuracy assessment, are discussed. To improve the accuracy of daily ET estimation from remotely sensed instantaneous observations, several suggestions for future research are proposed as follows: First, research on the continuous surface meteorological data at remote sensing image pixel scale should be enhanced because large-area applications of the temporal upscaling methods are hampered by the lack of appropriate ground-based observations and the spatial heterogeneity causes low accuracy of the spatial interpolation methods of meteorological parameters. Second, the accuracy of ET estimation using remotely sensed data can significantly affect the accuracy of the temporal upscaling of ET. As ET estimation models have not been perfected yet, the methods for the temporal upscaling of ET can be combined with those for ET estimation to reduce the influence of accumulated errors. Third, to weaken the influence of unstable upscaling factor during cloudy days, research on the relation between the constant upscaling factor and cloud (e.g., the appearing time, thickness, and duration of cloud) should be enhanced. Therefore, developing a robust method for the temporal upscaling of ET during cloudy days is vital. Fourth, research on the physical mechanisms of each commonly used method and development of an improved upscaling factor that can be independent of the variation in atmospheric variables and can incorporate the horizontal advection are essential. Fifth, numerous methods for the temporal upscaling of ET can only accurately provide daytime ET, whereas the daily ET is closely concerned with practical applications. Thus, research regarding the temporal upscaling methods should be enhanced in consideration of nighttime ET and its physical mechanisms. Finally, enhancing the research on the new technology and methods of accuracy assessment for ET can weaken the uncertainty of verification. © 2019, Science Press. All right reserved.