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The potential of satellite passive microwave remote sensing for the monitoring of surface moisture in semi-arid areas is generally recognized. While the largely unknown behavior of a.o. the vegetation parameters in the radiative transfer algorithm requires the validation of the satellite derived surface moisture for a restricted calibration time period, it often proves difficult due to the lack of adequate surface moisture field measurements on the regional scale and/or for the corresponding lifetime of the space platform. The present study describes the indirect validation of surface moisture derived from historical Nimbus/SMMR data (1978-1987) collected over the Upper Guadiana Catchment in central Spain using recharge estimates from historical surface now measurements. The recharge estimates are used to calibrate a soil moisture balance model, which is applied in the absence of surface moisture measurements, if the followed approach is successful, satellite passive microwave inversion techniques may be used to assess recharge estimates. Since the recharge rate corresponds to the maximum abstraction yield without depletion of the groundwater reservoir, the approach may provide a tool for the sustainable development of areas suffering from the overexploitation of water resources.