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W. Wagner, C. Pathe, M. Doubkova, D. Sabel, A. Bartsch, S. Hasenauer, G. Blöschl, K. Scipal, J. Martínez-Fernández, A. Löw:
"Temporal stability of soil moisture and radar backscatter observed by the Advanced Synthetic Aperture Radar (ASAR)";
Sensors, 8 (2008), 2; 1174 - 1197.

The high spatio-temporal variability of soil moisture is the result of atmospheric
forcing and redistribution processes related to terrain, soil, and vegetation characteristics.
Despite this high variability, many field studies have shown that in the temporal domain
soil moisture measured at specific locations is correlated to the mean soil moisture content
over an area. Since the measurements taken by Synthetic Aperture Radar (SAR)
instruments are very sensitive to soil moisture it is hypothesized that the temporally stable
soil moisture patterns are reflected in the radar backscatter measurements. To verify this
hypothesis 73 Wide Swath (WS) images have been acquired by the ENVISAT Advanced
Synthetic Aperture Radar (ASAR) over the REMEDHUS soil moisture network located in
the Duero basin, Spain. It is found that a time-invariant linear relationship is well suited for
relating local scale (pixel) and regional scale (50 km) backscatter. The observed linear
model coefficients can be estimated by considering the scattering properties of the terrain
and vegetation and the soil moisture scaling properties. For both linear model coefficients,
the relative error between observed and modelled values is less than 5 % and the
coefficient of determination (R2) is 86 %. The results are of relevance for interpreting and
downscaling coarse resolution soil moisture data retrieved from active (METOP ASCAT)
and passive (SMOS, AMSR-E) instruments.

Soil moisture, SAR, backscatter, scaling, temporal stability