Hourly and Daytime Evapotranspiration from Grassland Using Radiometric Surface Temperatures

Estimates of evapotranspiration (ET) are needed for many applications in agriculture, hydrology and meteorology because ground-based measurement techniques of ET and variables controlling it, such as canopy density (i.e., leaf area index LAI), soil water availability, and surface temperature (Ts) are inadequate over large or heterogeneous areas. Remote sensing can be a handy source for such variables at a reasonable resolution. Soil moisture availability is a key variable, as it exerts control over the ratio between actual and potential ET. Although soil moisture sensing is progressing rapidly, remotely sensed soil moisture content data are not always available or accurate, especially for dense vegetation. Moreover, remotely sensed soil moisture does not represent the entire soil water profile (root zone) that controls ET. Therefore, a method is needed to find ET directly from Tsr, without requiring soil water availability. In this study, we propose a procedure to estimate ET using Tsr. The method uses a dimensionless temperature DT, defined as (Tsa - Ta)/(Tmax - Ta), where Tsa is aerodynamic surface temperature, Ta is the air temperature and Tmax is the surface temperature that would occur if all the net radiation (Rn - G) was converted to sensible heat flux (H) and no evaporation occurred. The aerodynamic surface temperature is the temperature that gives the correct value of H at a clearly specified value of the scalar roughness length, zoh, based on Monin-Obukhov Similarity (MOS) theory in the surface sublayer. Radiometric surface temperature is converted into aerodynamic surface temperature using an Analytical Land-Atmosphere Radiometer Model (ALARM). Instantaneous (or hourly) ET was extrapolated to daily ET by assuming a constant evaporative fraction (EF = ET/Rn). This approach has been tested on data taken at two grassland sites. The results demonstrate that, for grassland, the model gives good estimates of ET when Ta and Tsr are available. The method presented has the conceptual advantage that it produces EF, which is a key variable to characterize the hydrology of a site, directly in terms of surface and air temperatures.