Satellite Remote Sensing and Energy Balance Modelling for Water Balance Assessment in (Semi-) Arid Regions

Abstract

The terms of the water balance in semi-arid regions may be monitored using different types of remotely sensed information from satellites. Such an integrated approach focusses on the possibilities of monitoring the soil moisture status and evapotranspiration over time from the combination of (a) thermal infrared, (b) visible and near infrared and (c) passive microwave remote sensing.Application of such an approach is expected to give a better insight into the spatial and temporal variability of soil moisture content and evapotranspiration, and therefore may contribute to a better understanding of large scale recharge phenomena in semi-arid regions. Actual recharge depends on subsurface hydrogeological conditions for percolation to underlying aquifers, and may vary between localized concentrated percolation on the one hand and more regional diffuse percolation on the other. Thus a subsurface scaling problem comes to the fore which is beyond the remote sensing approach described in this paper which focusses on the average surface conditions at pixel scale, as a possible boundary condition for recharge.The surface energy balance may be modelled using thermal infrared surface temperature observations and large scale near surface meteorological information, which allows the evapotranspiration rate to be estimated and the soil moisture status to be inferred together with stress conditions of the vegetation. This requires a remotely sensed estimate of the vegetation cover and biomass which may be derived from visible and NIR signatures. Separately, the soil moisture status of the top soil may be derived from passive microwave signatures. This paper illustrates the approach of using all these data together and describes some aspects of an energy and mass balance model to be used in the data analysis for scarcely vegetated areas, such as in semi-arid regions. 89 I. Simmers (ed.), Estimation of Natural Groundwater Recharge, 89-116.