Runoff as an additional water resource in a semiarid karstic aquifer

(eng) In a semi-arid limestone area, finding additional water resources is a priority. Moreover in the Sierra de Gador-Campo de Dalias, the local economy is booming and mainly based on groundwater resources. In common with other semi-arid to arid regions in particular along both sides of the Mediterranean are facing the same difficulties in balancing economic and natural resources. Aquifers are often overexploited and the demand is still growing. Hence, for sustainable development, water resources must be evaluated. Long term estimates are generally based on global assessment of diffuse recharge. This is somehow paradoxal as extreme events are recognized to preferentially recharge the aquifer. We show that the selectivity of deuterium excess to monthly precipitation volume allowed estimating the return period of precipitation leading to aquifer recharge at 0.9-4.9 years. Still moderate rainfall, which occurs more frequently, represents c. 60-90% of the total precipitation volume. This thesis aims to contribute to the understanding of the groundwater recharge system with particular attention to runoff processes and a focus on runoff as a component of water balance in semi arid areas. Runoff is usually regarded as negligible in the recharge system of the semi-arid aquifers. Generally rare and discontinuous, the runoff process is difficult to monitor. The approach used is to first evaluate the main factors explaining the occurrence of runoff reaching the bottom of the hillslope for several representative sites in the study area (0.3-20 ha). It was demonstrated that runoff events depend on thresholds i.e. rainfall depth per event, the antecedent precipitation index over 20 days (API20) and soil surface characteristics. Then, a simple rainfall/runoff model was adapted and validated at the hillslope scale. The expert-based model allowed acceptable results for predicting runoff at this scale. Finally, the hillslope model was extrapolated to the catchment scale (26 km²) in order to estimate the impact of runoff as an additional source of surface and groundwater. The comparison between recharge determined by means of tracer techniques reported in the literature for the same catchment and concentrated recharge from our model results revealed that runoff that infiltrates in ephemeral stream contributes to a non negligible volume of about 10% of the total recharge.