We characterize the precipitation and groundwater in a mountainous (peaks slightly above 3000 m a.s.l.), semi-arid river basin in SE Spain in terms of the isotopes 18O and 2H. This basin, with an extension of about 7000 km2, is an ideal site for such a study because fronts from the Atlantic and the Mediterranean converge here. Much of the land is farmed and irrigated both by groundwater and runoff water collected in reservoirs. A total of approximately 100 water samples from precipitation and 300 from groundwater have been analysed. To sample precipitation we set up a network of 39 stations at different altitudes (800-1700 m a.s.l.), with which we were able to collect the rain and snowfall from 29 separate events between July 2005 and April 2007 and take monthly samples during the periods of maximum recharge of the aquifers. To characterize the groundwater we set up a control network of 43 points (23 springs and 20 wells) to sample every 3 months the main aquifers and both the thermal and non-thermal groundwater. We also sampled two shallow-water sites (a reservoir and a river). The isotope composition of the precipitation forms a local meteoric water line (LMWL) characterized by the equation δD= 7.72δ18O + 9.90, with mean values for δ18O and δD of -10.28‰ and -69.33‰, respectively, and 12.9‰ for the d-excess value. To correlate the isotope composition of the rainfall water with groundwater we calculated the weighted local meteoric water line (WLMWL), characterized by the equation δD = 7.40δ18O + 7.24, which takes into account the quantity of water precipitated during each event. These values of (dδD/dδ18O)<8 and d-excess (δD-8δ18O)<10 in each curve bear witness to the 'amount effect', an effect which is more manifest between May and September, when the ground temperature is higher. Other effects noted in the basin were those of altitude and the continental influence. The isotopic compositions of the groundwater are represented by the equation δD = 4.79δ18O + 18.64. The groundwater is richer in heavy isotopes than the rainfall, with mean values of -8.48‰ for δ18O and -59.27‰ for δD. The isotope enrichment processes detected include a higher rate of evaporation from detrital aquifers than from carbonate ones, the effects of recharging aquifers from irrigation return flow and/or from reservoirs' leakage and enrichment in δ18O from thermal water. Copyright © 2010 John Wiley & Sons, Ltd.