A review on evapotranspiration data assimilation based on hydrological models

Accurate estimation of evapotranspiration （ET）, especially at the regional scale, is an extensively investigated topic in the field of water science. The ability to obtain a con- tinuous time series of highly precise ET values is necessary for improving our knowledge of fundamental hydrological processes and for addressing various problems regarding the use of water. This objective can be achieved by means of ET data assimilation based on hydro- logical modeling. In this paper, a comprehensive review of ET data assimilation based on hydrological modeling is provided. The difficulties and bottlenecks of using ET, being a non-state variable, to construct data assimilation relationships are elaborated upon, with a discussion and analysis of the feasibility of assimilating ET into various hydrological models. Based on this, a new easy-to-operate ET assimilation scheme that includes a water circula- tion physical mechanism is proposed. The scheme was developed with an improved data assimilation system that uses a distributed time-variant gain model （DTVGM）, and the ET-soil humidity nonlinear time response relationship of this model. Moreover, the ET mechanism in the DTVGM was improved to perfect the ET data assimilation system. The new scheme may provide the best spatial and temporal characteristics for hydrological states, and may be ref- erenced for accurate estimation of regional evapotranspiration.