In the semi-arid Brazilian Northeast, the exploitation of alluvial aquifers for irrigation and domestic supply to rural communities over the last 10 years has upset the traditional mechanisms of water resources management. In the Forquilha watershed (221km2; 5°17''S, 39°30''W), the two main water resources are reservoirs (with a capacity exceeding 0.9-6.7x106m3), used for domestic water supply only, and an alluvial aquifer (2.3x106m3), used for irrigation and domestic water supply. From 1998 to 2006, the irrigated area with alluvial groundwater increased from 0 to 75ha, and the fraction of population supplied through domestic water networks, using reservoirs and the aquifer, increased from 1% to 70%. Based on physical and socioeconomic issues, three main water territories have been defined ("Aquifer", "Reservoirs", and "Disperse Habitat"). Considering the next 30 years with a realistic population growth, three hypotheses regarding irrigated area (i.e., 0, 75, or 150ha), and several possible water-management scenarios, hydrological balance models were built and used to simulate the different impacts on water resource availability and salinity. Simulation results showed that, in all cases, releases from the upstream main reservoir are necessary to keep reservoir salinity below 0.7gL-1 and for guaranteeing domestic needs in the whole watershed. As a consequence, a management approach that takes into account the interrelations among the three territories is necessary. Moreover, the simulations showed that the area of irrigated fields cannot exceed the current extent (75ha), or serious restrictions on water availability and salinity will take place. Moreover, important socioeconomic problems are expected, including a high cost of palliative water supply with tank trucks from external sources.
