Synergistic use of ENVISAT ASAR Global Mode Soil Moisture Products in the Okavango Delta: Runoff & Wetland Monitoring

The Okavango Delta of northern Botswana is a fast-changing system of canals and floodplains which serves as an important wetland habitat. The area of the wetland is highly dependent on local source of precipitation as well as on external inflow from the upper Okavango River. The Advanced Synthetic Aperture Radar (ASAR) is an active remote sensing instrument onboard ENVISAT platform operating at C-band. The data from the ASAR Global ScanSAR Mode (GM) have amply demonstrated the ability for inland wetland monitoring as well as for near surface soil moisture derivation. The processing chain for ENVISAT derived soil moisture was setup within the ESA Tiger DUE Innovator project SHARE for hydrometeorological applications in the Southern African Development Community (SADC). The ASAR GM provides up to weekly samples of the Okavango delta with 1 km spatial resolution. The extent of the Okavango Delta wetlands is derived from the ENVISAT ASAR GM data applying threshold of absolute backscatter values. The relations of the wetland size, river discharge, and the relative mean soil moisture in the upper Okavango catchment are studied. Correlation above 0.9 can be observed between the relative mean soil moisture and river discharge. High dependence of the wetland extent on the relative mean soil moisture in the upper Okavango is also clearly evident. With this work we demonstrate that the relative soil moisture derived from the ENVISAT ASAR GM data can be clearly related to the river discharge measurements in subtropic environments. Additionally, we show the ability of ENVISAT ASAR Global Mode to monitor dynamics of wetland areas as a response to the relative soil moisture in the upper Okavango catchment. This allows for prediction of the wetland extent up to six months in advance. An incorporation of spatially improved soil moisture and wetland products may improve prediction models for the wetland region.