This work presents hydrological simulations on the Ebro River basin (Spain), using both control (1961–1990) and future (2071–2100) climate scenarios, to investigate the effect of climate change on water resources. Using the Soil and Water Assessment Tool hydrological model, simulations were carried out in four subcatchments representative of typical situations within the basin. Model parameters were identified using sensitivity analysis and long-term calibration procedures, which enabled the historical behaviour of the catchments to be reproduced. Following validation, the parameters were used to simulate the effects of climate change on future streamflows. Bias-corrected daily time series of precipitation and mean temperature from two regional climate models (RCMs), using the same medium-high SRES A2 emissions scenario, were used as drivers of the hydrological simulations during the future scenarios. Important annual and seasonal differences in the projected future precipitation and air temperature fields were observed among the RCMs. However, the two models project an overall increase in the mean annual temperature accompanied by a reduction in the annual precipitation, with the strongest differences with respect to the control period observed during the summer season. When these changes were used to project future streamflows, a general decrease was observed in the streamflows at the outlet of the selected catchments. Changes in streamflows were in general agreement with the projections of daily precipitation and temperature fields, with the largest drop in predicted monthly streamflows for M. Zambrano-Bigiarini (*), B. Majone, and A. Bellin Department of Civil and Environmental Engineering, University of Trento, Via Mesiano 77, I-38123 Trento, Italy e-mail: mauricio.zambrano@ing.unitn.it C.I. Bovolo, S. Blenkinsop, and H.J. Fowler School of Civil Engineering and Geosciences, University of Newcastle upon Tyne, Newcastle NE1 7RU UK D. Barcelo and M. Petrovic (eds.), The Ebro River Basin, Hdb Env Chem (2011) 13: 47–75, DOI 10.1007/698_2010_85, # Springer-Verlag Berlin Heidelberg 2010, Published online: 1 September 2010 47 the subcatchments with the lowest aridity index, and seasonal differences that appears to be related to the elevation range of the subcatchments.