Driving hydrological models with climate data is a tough challenge – whether the data are from the observational record or climate models. One reason for this is that many hydrological models require long daily time-series of precipitation and evaporation. The scarcity of appropriate observed data in many parts of the MENA region is therefore a potential constraint for the development of such models. Although climate models have the capacity to produce daily time-series for the whole region, the results of impacts studies driven directly by model output would be prejudiced by model error – particularly in precipitation, which is one of the most difficult variables to simulate. This chapter describes how these problems can be addressed by using a simple statistical rainfall model (weather generator) in conjunction with a regional climate model. This enables climate model bias to be corrected, observed monthly data to be disaggregated and the length of a precipitation time-series to be extended.