Estimating the surface water and radiation balance in an upland area from space

The Synthetic Aperture Radar (SAR) carried aboard ERS-1 and-2 has been used to estimate soil moisture in a naturally vegetated area of upland moor in Lancashire, north-west England. This holds promise for the determination of catchment-scale evapotranspiration rates using direct measurements of changes in soil moisture content. Combining such measurements as these with observations of net radiation from the NOAA AVHRR passive radiometer has led to the development of a method, based upon the techniques of Price (IEEE Transactions on Geoscience and Remote Sensing, 28 , 940-948, 1990) combined with some new methodologies, whereby the sensible heat flux can be found by the closure of the energy balance equation. A review of the methodology developed is presented together with the potential for refining the system using further instruments. There are limitations of the procedure due to the restricted temporal coverage of the sensors concerned, and their inability to penetrate the surface to investigate soil-water-vegetation processes within the root zone. It is believed, however, that models can be constructed to account for these deficiencies to enable retrieval of an entire surface energy and water budget. These would have a plethora of applications, both direct and as inputs to atmospheric and hydrological models.