Soil moisture variability: a comparison between detailed field measurements and remote sensing measurement techniques

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Abstract
Surface soil moisture, defined here as the volumetric soil water content in the top 10 cm of the soil, shows a great deal of variability. The variance observed within a square metre can be as large as for a whole field. For that reason alone, point measurements in low quantities cannot be representative for the average soil moisture of a whole field. Soil moisture estimated by remote sensing techniques, especially by microwave sensors, might give adequate information on areally-averaged soil moisture. If these techniques are to be worthwhile in hydrological modelling, it will be necessary to link the various spatial resolutions of the different instruments to each other and to point-scale measurements. Despite differences in instrument configuration it is shown that it is possible, under certain circumstances, to downscale high resolution areal soil moisture estimates to low resolution soil moisture estimates using data from HAPEX-Sahel'92 and Washita'94.