Human impacts on components of hydrological balance in an alluvial aquifer in the semiarid Northeast, Brazil

The exploitation of an alluvial aquifer (2 x 10(6) m(3)) has been developed since 1998 in the Valley of Forquilha (Quixeramobim, State of Ceara). For this purpose, 165 wells were drilled along the 23 km of the valley that supplies 500 families and their farms. Monthly monitoring of piezometric and electrical conductivity (2000-2003) show seasonal variations in the water volume (35%) and in the mean value of the conductivity (800-1200 muS cm(-1)). A conceptual hydrogeological model was developed. Analysis of the data and simulations highlight that the recharge of the aquifer is mainly due to infiltration from the river in the rainy season, which is of the order of 1% of the rain over the catchment area (195 km(2)). The abstractions increase the recharge between 30 and 60%. The model makes it possible to propose scenarios of sustainable exploitation of the water resource in the catchment. For the period between 1970 and 1988, other simulations show that it would have been possible to maintain irrigated cultures on 75 ha for 80% of the time. During the remaining 20%, the water level is too low, and farmers would have to reduce the irrigated area.