Theoretical analysis of a remotely measurable soil evaporation transfer coefficient

Reducing soil evaporation (E) is an important way to increase water use efficiency for agriculture and sustainable water use. To achieve this goal, an accurate estimate of soil evaporation at the macro-scale level is necessary. By including the surface temperature of dry soil, the three temperatures model (3T model) was proposed as an estimate of E, and its temperature term was defined as the soil evaporation transfer coefficient (ha). Although it has been primarily shown that ha determines the boundaries of E, further studies of its properties are necessary because ha has the application potential for remote sensing. The objectives of this study are to (1) investigate the relationships between ha and those parameters that are important for E estimation but are not able to be remotely measured (aerodynamic resistance, soil surface resistance (rs), and cumulative evaporation (Ec)); and (2) discuss the possibility for remote sensing application. Two experiments were carried out for these purposes: one in an open field and the other one in a closed growth chamber at constant temperature, humidity, and radiation. The following results were obtained: (1) Given the boundaries of 0 ≤ ha ≤ 1, E was determined between its maximum value (potential evaporation rate) and minimum value (zero); (2) During the period when evaporation is controlled by rs, a linear relation between ha and log (rs) is observed with a coefficient of determination r2 = 0.76. Because ha and rs are independent variables with significant differences in properties and magnitudes, these results indicate that ha and rs are well related to each other; (3) During stage 1 of evaporation, cumulative evaporation (Ec) increased with time while ha remained constant. Afterwards, ha linearly increased with Ec. The relationships among ha, Ec, ra, and rs would be useful for estimation of evapotranspiration by remote sensing.