On the relationship between ERS-1 SAR/backscatter and surface/sub-surface soil moisture variations in vertisols

The quantitative estimation of soil moisture is usually obtained by employing an empirical relationship that converts the radar-measured backscattering coefficient (sigma(o)) into volumetric soil moisture content (theta(v)). Owing to the wide range of soil conditions and the complex interactions between microwaves and matter in the natural environment, existing relationships are usually site specific and have a limited range of validity. In view of this, the present study aims at realising the potential of ERS-1 SAR data towards estimating the spatial variations of soil moisture in the heavy textured soils of East Godavari District (A.P.), India. A comparison between ERS-1 SAR measured soil moisture estimates and field observations indicates that in non-vegetated smooth terrain of vertisols with relatively higher moisture content (more than or nearly equal to field capacity), the relationship between sigma(o) and theta(v) is almost linear up to 0-10 cm depth. However, a similar terrain with low moisture content (less than field capacity) has not been found suitable for soil moisture estimation using ERS-1 SAR data. While the results indicate the sensitivity of SAR data to soil moisture variations, surface moisture estimates which have been computed from SAR data have been found inadequate to extrapolate further to measure the soil moisture status beyond 20 cm depth. With regard to profile moisture estimation, it has been observed that, between the neighbouring soil layers, correlation between SAP-observed moisture content and in situ measured soil moisture status deteriorates as the distance between layers increases. (C) 1998 Published by Elsevier Science Ltd.