Soil Moisture Mapping from ASAR Imagery of the Mulargia basin

The state of the soil moisture is a key variable controlling surface water and energy balances. High resolution data of the ASAR (advanced synthetic aperture radar) sensor aboard European Space Agency's Envisat satellite offers the opportunity for monitoring surface soil moisture at high resolution (up to 30 m), which is suitable for distributed mapping within the small scales of typical Mediterranean basins. These basins are characterized by strong topography and high spatial variability of physiographic properties, and only high spatial resolution satellite images allow to estimate adequately soil moisture spatial variability. ASAR-based soil moisture mapping of the Mulargia basin (area of about 65 sq.km) are collected for 2003-2006 years. In Mediterranean basins, such as the Mulargia basin, characterized by water-limited conditions, even though there is no universal relationship between vegetation and soil patterns in water-limited conditions some relationship between soil water storage capacity and vegetation type and density can be found: for instance, typically an increase of woody vegetation dimension and canopy density when moving from uplands of a hillslope (with thin coarse textured soils) to alluvial fans (with deep soils of finer texture). We investigated the relationships between soil moisture spatial variability, soil depth and vegetation distribution, which impact strongly soil, vegetation and atmosphere interactions. For the case study ASAR products at single and double polarization are tested and validated. For validating radar soil moisture estimates, spatially distributed soil moisture ground-truth data have also been collected over the whole basin through the TDR technique and the gravimetric method, in days with available radar images. Results shows: 1) the high resolution ASAR imagery accuracy for producing maps of surface soil moisture patterns at the catchment scale and their reliability for different seasons (wet vs dry), and 2) a surprising strong relationships between soil moisture spatial variability, soil depth (alluvial vs uplands) and vegetation spatial distribution (grass vs woody vegetation) with contrasting wetter alluvial valleys and drier uplands.