Experimental determination of soil evaporation stages with soil surface temperature

Soil evaporation process can be divided into three stages: the constant-rate, the falling-rate, and the low-rate stages. Soil surface temperature is closely related to the soil evaporation rate and soil evaporation stage. The objectives of this study were: (i) to propose an approach to determine the soil evaporation stage by using infrared (IR) radiometers, the three-temperature (3T) model, and the Priestley-Taylor (PT) model; (ii) to test it on three soil types (coarse sand, sand, and clay), and (iii) to apply it using remote sensing. A drying experiment to address these objectives was continuously performed tor 539 h at constant air temperature (25°C) and relative humidity (50%). The results clearly indicated that the combination of the IR radiometers and the PT model had improved the performance of the 3T model. Soil evaporation stages could be determined with the soil evaporation transfer coefficient (h_a), a key parameter in the 3T model. At the constant-rate stage, h_a was a constant with low value, while cumulative evaporation (E_c) rapidly increased. At the falling-rate stage, h_a increased with E_c and the relationship between them was curvilinear. At the end of the falling-rate stage, h_a could be >0.90. Our results showed that the application of thermal imaging lor the estimation of h_a and the evaporation rate was promising for applying this technique in remote sensing.