A thermal sharpening algorithm (TsHARP) providing fine resolution land surface temperature (LST) to the Two-Source-
Model (TSM) for mapping evapotranspiration (ET) was applied over two agricultural regions in the U.S. One site is a
rainfed corn and soybean production region in central Iowa, while the other is an irrigated agricultural area in the Texas
High Plains. Application of TsHARP to coarse (1 km) resolution thermal data over the rainfed agricultural area is found
to produce reliable fine/within-field (60 m) resolution ET estimates, while in contrast, the TsHARP algorithm applied to
the irrigated area does not perform as well, possibly due to significant sub-pixel moisture variations from irrigation. As a
result, there may be little benefit in applying TsHARP for generating TSM-derived 60 m ET maps for the irrigated
compared to the rainfed region. Consequently, reliable estimation of fine/within-field ET and crop stress still requires
fine native resolution thermal imagery in areas with significant sub-pixel moisture variations.
Radiometric surface temperature (Ts) is commonly used as a surrogate for aerodynamic temperature (To) in computing the sensible heat flux term (H) in the energy balance. However, these temperatures may differ by several degrees, leading to possible errors (especially for large H) and their relationship is not well known. Previous researchers have established empirical and semi-empirical parameterizations of the radiometric roughness length (zor) or some related form (e.g., kBr−1 = ln[zom/zor], where zom is the momentum roughness length). In this paper, we estimated To - Ta (where Ta is air temperature at 2 m height) and zor using large, precision weighing lysimeters planted with irrigated alfalfa, irrigated and dryland cotton, and dryland grain sorghum. Ts was measured by infrared thermometers mounted over the lysimeters. No apparent relations were found between (To − Ta) and (Ts - Ta) or between zor (in the kBr−1 form) and meteorological variables or leaf area index (LAI). The kBr−1 parameter appeared to be most influenced by the different surface roughness of each crop type. Using constant kBr-1 values established for each type of surface, the energy balance model showed reasonable agreement with H and LE derived from lysimeter measurements.