Area-average estimates of evaporation, wetness indicators and top soil moisture during two golden days in EFEDA

Surface fluxes and soil moisture were measured in situ during the Echival Field Experiment in Desertification-threatened Areas (EFEDA), executed in Castilla la Mancha, Central Spain. Although the observation network 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 agricultural landscape were difficult to deduce. An attempt with remote sensing data has been made to study the possibility to extrapolate in situ 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 radiances at different spatial scales. The spatial variability and area-average values were computed for two different golden days (June 12 and 29). The evaporative fraction (latent heat flux/net available energy) estimated from remote sensing was within the error bounds of the values derived from instrumented flux towers in 85% of the cases compared. The bulk surface resistance derived from the remote sensing flux algorithm could be related successfully to in situ near-surface soil moisture measurements conducted with Time Domain Reflectometers (TDR). Unfortunately, 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 resistance and top soil moisture could be applied to estimate soil moisture at unsampled locations. Thereafter, the area-average top soil moisture could be assessed. It was concluded that an arithmetic means of the distributed field measurements of evaporation (n = 13) and soil moisture (n = 46) gives st wrong indication of the area-average values at a scale of 100 km, and that weighting factors for the areal integration can be derived from remote sensing data. During a 17-day drying period 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 decreased from 2.0 to 1.3 mm d(-1). (C) 1997 Elsevier Science B.V.