AREA-AVERAGE ESTIMATES OF EVAPORATION, WETNESS INDICATORS AND TOP-SOIL MOISTURE DURING 2 GOLDEN DAYS IN EFEDA

Surface fluxes and soil moisture were measured in situ during the Echi val Field Experiment in Desertification-threatened Areas (EFEDA), exec uted in Castilla la Mancha, Central Spain. Although the observation ne twork had a high density (20 flux towers and 46 soil moisture plots), the area-average values at a scale of 100 km for a Mediterranean agric ultural landscape were difficult to deduce. An attempt with remote sen sing data has been made to study the possibility to extrapolate in sit u measurements to a coarser scale. A remote sensing flux algorithm was applied to estimate (1) actual evaporation, (2) evaporative fraction, (3) bulk surface resistance and (4) top soil moisture from spectral r adiances at different spatial scales. The spatial variability and area -average values were computed for two different golden days (June 12 a nd 29). The evaporative fraction (latent heat flux/net available energ y) estimated from remote sensing was within the error bounds of the va lues derived from instrumented flux towers in 85% of the cases compare d. The bulk surface resistance derived from the remote sensing flux al gorithm could be related successfully to in situ near-surface soil moi sture measurements conducted with Time Domain Reflectometers (TDR). Un fortunately, the relationship between resistance and soil moisture is shown to be space and time dependent. Nevertheless, after calibration with field scale measurements, instantaneous relationships between res istance and top soil moisture could be applied to estimate soil moistu re at unsampled locations. Thereafter, the area-average top soil moist ure could be assessed. It was concluded that an arithmetic means of th e distributed field measurements of evaporation (n = 13) and soil mois ture (n = 46) gives st wrong indication of the area-average values at a scale of 100 km, and that weighting factors for the areal integratio n can be derived from remote sensing data. During a 17-day drying peri od in the Special Observation Period, top soil moisture in Castilla la Mancha at a scale of 100 km decreased from 0.16 to 0.10 cm(3) cm(-3) resulting in an increase of the areal bulk surface resistance from 661 to 1166 s m(-1). The associated daily evaporation at this scale decre ased from 2.0 to 1.3 mm d(-1). (C) 1997 Elsevier Science B.V.