Simultaneous Measurements of Energy Balance Components at cm to km Scales: Lessons Learned in the Middle Rio Grande Valley.

Knowledge of the energy balance at the soil surface and canopy height is needed for the noninvasive characterization of biophysical processes in the root zone. Ratios between latent and sensible heat fluxes such as Bowen Ratio or Evaporative Fraction provide information on root zone soil moisture content. Surface temperatures combined with the energy balance components provide information on the thermal soil regime. In the Middle Rio Grande Valley we have evaluated simultaneous ground measurements and remote sensing estimates of soil, latent, and sensible heat fluxes, and net radiation at a wide range of scales. The remote sensing estimates of the energy balance components have been obtained using the Surface Energy Balance for Algorithm for Land (SEBAL) at scales varying from 900 m² in a Landsat pixel to 62500 m² and 1 km² in a MODIS pixel. The scale of the ground measurements varied from about 10 cm² for the soil heat flux, to about 10 m² for the net radiation, to about 4500 m² for the footprints of the sensible and latent heat fluxes measured with eddy-covariance, and finally to typical scintillometer footprints of 1-4 km² for sensible heat fluxes. The objective of this study is to derive best estimates of evapotranspiration rates from simultaneous ground measurements and remote sensing observations at widely different scales.