Achieving compromise between efficiency, equity and soil protection: A Tunisian case study.
An integrated development program, which includes many soil protection techniques and water harvesting systems, is being implemented in the upstream of Merguellil watershed in order to reduce erosion and improve local population income. This program has modified the spatial distribution of water resources at the watershed scale, increasing its availability in the upstream and reducing it in the plain, located at downstream, where a large area is irrigated from a groundwater aquifer recharged directly from upstream runoff. As a consequence, the economic income derived from irrigated agriculture has been strongly affected, leading to the need to target the level of soil erosion reduction. By coupling a hydrological model to an economic optimization model MOTAD Target, this work has simulated the impact of different scenarios of reducing soil erosion in the upstream on the improvement of local population income and the evolution of economic return of farmers in the plain. The simulated scenarios are to reduce erosion by 25%, 50% and 75%. The results showed that all simulated scenarios lead to additional benefits for population in the upstream area. In contrast, they result in losses of economic income for farmers in the downstream area (plain). These losses become highly significant beyond the scenario of reducing erosion by 50%. The global economic income generated throughout the watershed (upstream and downstream) also decreases with the level of soil protection. This means that economic gains obtained in the upstream can only partially compensate the lost benefits in the downstream, due to the diminution of groundwater recharge. Our simulations suggest targeting the reduction of soil erosion and show that, in the case of Merguellil watershed, the optimal compromise solution between the three objectives (economic, social and environmental) is situated in the range of scenarios corresponding to a reduction of erosion by 25% and 50%.