The 2,265 km² Andarax river basin located in Southern Spain is one of the most arid regions in Europe with a mean annual precipitation of 250-350 mm, which mainly falls (70%) in autumn and winter. The terrain changes from sea level at the coast to more than 2,500 m in the Sierra Nevada Mountains. Most of the precipitation falling in the mountainous areas, where the unit Triassic schists dominates, is converted into overland flow, which runs directly to the rivers. In areas with limestone most of the precipitation infiltrates and is subsequently routed through fractures before discharging into the Andarax river. When the river reaches the medium and lower-laying areas most of the water infiltrates into the highly permeable Detritic aquifer. River discharge into the Mediterranean Sea only occurs at rare occasions and for high rainfalls. The total recharge within the catchment determines the water availability in the delta region. Due to the high evapotranspiration in semi-arid or arid regions groundwater recharge can be as low as 1 % of the precipitation. Thus it is essential to accurately predict the seasonal and regional distribution of actual evapotranspiration (ET) within the river catchment as this it will lead to better estimate of the groundwater recharge and hereby of the groundwater availability in the delta region.  

The hydrological behaviour of the Andarax river basin is simulated by the MIKE SHE code, which is a physically based, distributed and integrated hydrological model. In the first scenario we only use traditional meteorological data and standard values for the vegetation characteristics. The traditional meteorological data are rather sparse for the Andarax river basin and to improve the estimation of evapotranspiration we use an energy-based two-layer SVAT model and apply remote sensing derived variables as input data. Specifically surface temperature, global radiation, albedo and leaf area index (LAI) are derived from remote sensing images.

We compare the two model simulations and focus in particular on the temporal and spatial distribution of evapotranspiration and recharge, and river discharges.