Mapping land surface fluxes using microwave and optical remote sensing data under high vegetation cover conditions during SMEX02/SMACEX

A two-source (soil + vegetation) energy balance model using microwave-derived near-surface soil moisture has been successfully applied in areas of relatively low vegetation cover. The utility of this approach in areas with high vegetation cover is explored and compared with a two-source scheme using thermal infrared data. The investigation used data collected over areas of high corn and soybean cover in central Iowa during the Soil Moisture Experiment in 2002 (SMEX02) and the Soil Moisture Atmosphere Coupling Experiment (SMACEX). Maps of near-surface soil moisture data were obtained from the Polarimetric Scanning Radiometer (PSR) observations, which provided 800 m resolution for the regional area. Fractional vegetation cover and leaf area index were estimated from the Landsat data, which also provided surface temperature. These data, along with local meteorological data, provided inputs for a two-source model. The model output using microwave data were compared with tower-based flux measurements in the watershed area. The model computed reliable estimates of net radiation and soil heat flux, yielding a root-mean-square-difference (RMSD) of around 20 Wm^-2. However, the model generally underestimated latent heat flux, LE, and overestimated sensible heat flux, H resulting in RMSD values of ~70 Wm^-2 for LE and 45 Wm^-2 for H respectively. The larger discrepancies in heat fluxes are due in part to the mismatch in model output resolution (800 m) versus source area contributing to the tower fluxes (~100m). The comparison between the model output of surface temperature and Landsat indicates that the temperature estimation is reasonable agreement with a RMSD 1.4degC. A comparison of fluxes mapped over the regional area using the microwave-based and thermal-infrared based model is made and factors contributing to the differences between model outputs are discussed