Using Simplified Thermal Inertia to Determine the Theoretical Dry Line in Feature Space for Evapotranspiration Retrieval

With the development of quantitative remote sensing, regional evapotranspiration (ET) modeling based on the feature space has made substantial progresses. Among those feature space based evapotranspiration models, accurate determination of the theoretical dry/wet lines remains a challenging task. This paper reports the development of a new method, named DDTI (Determination of Dry line by Thermal Inertia), which determines the theoretical dry line based on the relationship between the thermal inertia and the soil moisture. The Simplified Thermal Inertia value estimated in the North China Plain is consistent with the value measured in the laboratory. Two evaluation methods, which are based on the comparison of the locations of the theoretical dry line determined by two models (DDTI and the heat energy balance method) and the comparison of the Evaporative Fraction between the estimates from the two models and the in situ measurements, were used to assess the performance of the new method DDTI. The location of the theoretical dry line determined by DDTI is more accurate than that determined by the heat energy balance method. When compared with the in situ measurement of Evaporative Fraction (EF) at Yucheng Experimental Station, the ET model based on DDTI reproduces the pixel scale EF with an RMSE (Root Mean Square Error) of 0.095, which is much lower than that based on the heat energy balance method with an RMSE of 0.224. Also, the bias between the DDTI method and the in situ measurements is 0.069, lower than the bias of the heat energy balance method, which is 0.168. OPEN ACCESS