Evapotranspiration over an Agricultural Region Using a Surface Flux/Temperature Model Based on NOAA-AVHRR Data

Abstract

The possibility of using infrared surface temperatures from satellites (NOAA, GOES) for inferring daily evaporation and soil moisture distribution over large areas (102 to 105 km2) has been extensively studied during the past few years. The methods are based upon analysis of the surface energy budget, but treating surface transfers as over bare soils. In this context, we have developed a methodology using infrared surface data (from NOAA-7) as input data, in a one-dimensional boundary layer/vegetation/soil model, including a parameterization of transfers within the canopy, based on the formalism of Deardorff which allows the use of a small number of mesoscale surface vegetation parameters. As shown from the model sensitivity tests, a single surface temperature measured near midday (provided by NOAA-7) is sufficient for obtaining the surface energy fluxes over dense vegetation and for deriving the only governing parameter that remains, the bulk canopy resistance to evaporation, a different concept from moisture availability used for bare soils. The objective of the model in predicting the area-averaged surface fluxes and canopy resistances over dense vegetation is analyzed in conjunction with experimental surface flux measurements for three cases with cloudless NOAA images over a flat monocultural region (the Beauce in France). In the absence of a current capability for routine daily soil moisture observation over an agricultural region, an area-averaged evaluation of the soil moisture can be derived from the canopy resistance obtained by this methodology, using an empirical expression relating this resistance to the root zone water content. Spatial gradient of water content between two areas of Beauce with different soil drainage properties is thus evaluated.