Remote sensing estimation on regional continuous daily evapotranspiration based on Richards equation

Evapotranspiration (ET) is an important part of water cycle in nature, and the estimation of evapotranspiration on spatio-temporal scale has always been a hot issue. Remote sensing can estimate evapotranspiration on regional scale, but it is difficult to obtain evapotranspiration in continuous time series due to the limitation of satellite transit time. Soil moisture is an important controlling factor of evapotranspiration. Improving the remote sensing evapotranspiration model by combining soil moisture data is of great significance in improving the accuracy of remote sensing evapotranspiration estimation. However, most remote sensing methods give limited consideration to the characterization of soil moisture stress. This study used the evapotranspiration calculated by the vorticity correlation method as the actual evapotranspiration. combining with the single crop coefficient method recommended by FAO, the soil water content information was introduced into the Penman-Monteith formula to calculate the actual evapotranspiration. Based on Richards equation, the one-dimensional vertical soil water movement process under evaporation conditions was simulated to estimate the continuous daily evapotranspiration under soil water stress. Combining with remote sensing data, the regional scale evapotranspiration was estimated. The results show that the actual daily evapotranspiration calculated by the vorticity correlation method has a strong correlation with the potential daily evapotranspiration calculated by P-M formula, with the correlation coefficient of 0.918. With the introduction of soil water content information, the P-M formula improves the estimation accuracy of daily evapotranspiration significantly, and the RMSE reaches 0.133 mm/d. The estimated daily evapotranspiration under soil water stress based on Richards equation is close to the measured value, with the RMSE of 0.288 mm/d. The high value of daily evapotranspiration affected by the topography of the study area is concentrated in the water area and cultivated land area in the middle of the study area. The average daily evapotranspiration under different soil use types is water area > cultivated land > woodland > grassland > unused land, and the results on the regional scale show similar change with that measured in the the station in time series. This study provides basic information for understanding the influence mechanism of soil moisture on evapotranspiration and estimating regional evapotranspiration.