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SHARE - Product Presentation
Soil moisture maps covering the entire southern African subcontinent with 1km resolution are now available from ENVISAT. The TIGER project SHARE is the first project to demonstrate that spaceborne Synthetic Aperture Radar (SAR) instruments can deliver soil moisture data of high spatial (1 km) and temporal (< 1 week) resolution. The relative soil moisture values are retrieved from ScanSAR data collected by the ENVISAT ASAR instrument operating in Global Mode. The applied change detection algorithm accounts for spatial differences in vegetation cover and surface roughness. This approach is based on year long experience with ERS scatterometer data, which serve already as input for global operational soil moisture derivation on 50 km resolution since 2002 covering 1992 until now. The new 1km dataset from ENVISAT also allows enhanced interpretation of the coarse ERS product. The analyses of the relationship between local and regional measurements can be used for evaluation on varying scales. First analyses of monthly composites over the upper Okavango River basin demonstrate that the near surface relative soil moisture shows consistency with other hydrological observations. River discharge measurements correlate well with average basin soil moisture (R²=0.96) with respect to system response time (3 months) (Figure 1).
The relative soil moisture values are location specific. Backscatter measurements are scaled between minimum and maximum values observed at the 1km grid point over the entire period of data availability. The absolute difference between these limits varies according to signal sensitivity.
Monthly composites of relative soil moisture are available on country level for evaluation by current and potential users for January 2005 until December 2006. Sample data sets cover Angola (Figure 2), Botswana, Namibia and South Africa. Relative soil moisture maps from single acquisitions are available on province level: Huila (Angola), Hereroland (Namibia), Kgalagadi (Botswana) and Free State (RSA). On average 100 maps are provided for the two year period for each province. All maps can be visualized with Google Earth and evaluated via a web interface:
The actual database covers all continental SADC countries and is updated regularly.
Georeferenced datasets can be requested via the SHARE webpage.
Figure 1: Mean monthly relative soil moisture in the Angola Highlands and mean monthly discharge of the Okavango River at Mohembo (source: HOORC), January 2005 - June 2006.
Figure 2: Monthly composites of the ENVISAT ASAR Global Mode derived relative soil (1km) for Angola, January - December 2006