Aerodynamic Surface Temperature Modeling Under Various Surface Conditions.

Most remote sensing models use the energy balance (EB) equation in the estimation of evapotranspiration. The process is such that the various components of the EB equation i.e. net radiation (R_n), Soil heat flux (G), and the sensible heat flux (H) are estimated using remotely sensed surface radiances, and latent heat flux (LE) is then determined as a residual. The challenge with the H estimation is the difficulty in measuring or estimating the aerodynamic temperature (T_o) as is required in the bulk resistance equation. Many models have thus used radiometric surface temperature (T_s) in its place, but that has resulted in errors under some surface and atmospheric conditions. Attempts have been made to model T_o, based on several parameters which include T_s, wind speed (u), leaf area index (LAI) and aerodynamic resistance to heat transfer (r_ah). Such models have however varied according to crop type, and possibly stage of growth, and soil water content regimes. The objective of this study is to determine how crop type, stage of growth, and stress conditions influence the modeling of T_o, so that if possible a more robust model could be developed. Thermistor-based air temperature probes will be used to measure aerodynamic temperature within the canopy in corn, sunflower, and alfalfa fields in Rocky Ford and Greeley, Colorado during the 2012 cropping season. Measurements of T_o will be related to specific parameters to enable modeling using a statistical procedure. The parameters include T_s, LAI, r_ah which are determined from remote sensing images, other parameters are wind speed and T_a which are both obtained either from infield or the nearest weather station. The modeled T_o will be used to determine H and ET, and the two will be validated using a precision weighing lysimeter, an eddy covariance and Bowen Ratio-Energy Balance systems.