The Penman-Monteith Equation As a Method to Initialize Canopy Temperature in the Two-Source Energy Balance Model and Comparison to Other Methods.

A two source energy balance (TSEB) model can calculate the energy balance of the soil and canopy separately, which is useful in partitioning evapotranspiration (ET) into the evaporation (E) and transpiration (T) components. One common TSEB formulation is driven by a measurement of surface brightness temperature (Tb). Because Tb commonly includes both canopy (Tc) and soil (Ts) component temperatures for partial canopy cover, the TSEB requires an initial calculation of Tc, and then solves the energy balance by iteration. The initial calculation of Tc commonly uses the Priestley-Taylor (PT) equation, which has resulted in reasonable agreement between calculated and measured ET for most ranges of vapor pressure deficit. However, in recent studies where separate measurements of E and T were obtained, the PT equation overestimated initial Tc, resulting in incorrect partitioning of soil and canopy fluxes, where E was overestimated (RMSE/MBE of 2.8/1.8 mm) and T was underestimated (RMSE/MBE of 4.1/-3.9 mm). Substitution of the Penman-Monteith (PM) equation, where the bulk canopy resistance term was allowed to vary during iteration, greatly reduced error in E (RMSE/MBE of 1.5/0.79 mm) and T (RMSE/MBE of 1.3/0.76 mm) calculations compared with the PT equation, especially when vapor pressure deficit exceeded 3.5 kPa. Comparison of the PT and PM formulations used in the TSEB will be discussed in light of their physical assumptions.