On the potential of MetOp ASCAT-derived soil wetness indices as a new aperture for hydrological monitoring and prediction: a field evaluation over Luxembourg

In situ soil moisture data from the Bibeschbach experimental catchment in Luxembourg are used to evaluate relative surface soil moisture observed with the MetOp-A Advanced Scatterometer (ASCAT). Filtered and bias-corrected surface soil wetness indices (SWIs) derived from coarse-resolution (25km) C-band scatterometer observations are shown to be highly correlated (r=0.86) with catchment-averaged soil moisture measured in the field. The combination of ASCAT and ENVISAT Advanced Synthetic Aperture Radar (ASAR) data sets yields high-resolution (1km) relative surface soil moisture that is equally well correlated with in situ measurements. It is concluded that for soil moisture monitoring applications at a catchment scale, the two soil moisture products are equivalent. The best correlation between the SWI derived from ASCAT and ASCAT-ASAR with in situ soil moisture observations at ca. 5cm depth is obtained with a characteristic time length parameter T equal to 288 h. These results suggest that satellite-derived surface soil wetness may serve as proxy for soil storage that enables the monitoring of abrupt switches in river system dynamics to appear when an effective field capacity is exceeded and rapid subsurface stormflow is initiated. In catchments where soil moisture is the main controlling factor of rapid subsurface flow, MetOp ASCAT-derived SWI has the potential to monitor how a river system approaches a critical threshold. Copyright (c) 2011 John Wiley & Sons, Ltd.