IWRA Proceedings

AbstractIn semi-arid areas, hydrological impact of water and soil conservation works (WSCW) has most often been studied at the local level. Studies on the regional scale are rare. In central Tunisia, the Merguellil catchment (1183 km2) defined by the big El Haouareb dam has been subject to WSCW for several decades. They consist of contour ridges and small earth dams collecting hillslope runoff and wadi flow respectively. 97% of the surface areas equipped with WSCW between 1989 and 2005 are located in the lower area of the Merguellil catchment, downstream the Skhira upper subcatchment (189 km2). In this lower area, 32% of the surface area is covered by WSCW. A local and a regional approach were developed. The first one performed water budgets at the reservoir scale. In average, one third of the flow collected by earth dams is lost through evaporation and 12% are pumped for additional irrigation. This calculation is in good agreement with field surveys. Simulations with a coupled rainfall-runoff/water balance model indicated that estimated water withdrawal corresponds to a 50% water shortage risk. In contour ridged hillslopes, the whole runoff collected in the ditches is probably evapotranspirated without any agricultural yield increase. The second approach characterized the hydrological changes induced by WSCW at the Merguellil catchment scale. The space-time variability of the rainfall-runoff relationship was analysed, comparing the runoff responses of the Skhira upper subcatchment and of the lower area. A non-parametric test, based on a resampling approach, was applied to the lower area runoff simulated with the GR4J daily rainfall-runoff model. Results indicate that between periods 1989-1996 and 1997-2005 the runoff produced by rainfall below 40 mm was reduced by over 70% in the lower area, a significant change at the 95% confidence level. This drop in runoff was estimated at about 45% when considering all rain depths and at about 30% when including the Skhira upper subcatchment in the analysis. No runoff change was found for rains above 40 mm in the lower area, nor for any rain depth range in the Skhira upper subcatchment. Possible sources for the runoff reduction (climate, land use/land cover, or water exchanges with the aquifer) were considered. The WSCW appear as the most likely cause. Their impact on the water resources produced and stored in the Merguellil catchment was assessed from the local and regional approaches. Expressed as percentage of total runoff produced in the catchment, infiltrated and pumped volumes changed from 54% to 45% and from 18% to 14% respectively. Evaporation changed from 27% to 41%. These changes are equivalent to a global 19% water resources decrease in the Merguellil catchment. Consequences for the regional economy could become deleterious, should WSCW development continue in the Merguellil catchment. The El Haouareb dam located at its outlet is the main recharge point for the overexploited Kairouan aquifer which is the major water resources for this populated area.