Rainfall in Iberian transnational basins: a drier future for the Douro, Tagus and Guadiana?

Water scarcity is critical in both Portugal and Spain; therefore, assessing future changes in rainfall for this region is vital. We analyse rainfall projections from climate models in the CMIP5 ensemble for the transnational basins of the Douro, Tagus and Guadiana with the aim of estimating future impacts on water resources. Two downscaling methods (change factor and a variation of empirical quantile mapping) and two ways of analysing future rainfall changes (differences between 30 years periods and trends in transient rainfall) are used. For the 2050s, most models project a reduction in rainfall for all months and for both methods, but there is significant spread between models. Almost all significant seasonal trends identified from 1961 to 2100 are negative. For annual rainfall, only 3 (2) models show no significant trends for the Douro/Tagus (Guadiana), while the rest show negative trends up to −6 % per decade. Reductions in rainfall are projected for spring and autumn by almost all models, both downscaling methods and both ways of analysing changes. This increases the confidence in the projection of the lengthening of the dry season which could have serious impacts for agriculture, water supply and forest fires in the region. A considerable part of the climate model disagreement in the projection of future rainfall changes for the 2050s is shown to be due to the use of 30 year intervals, leading to the conclusion that such intervals are too short to be used under conditions of high inter-annual variability as found in the Iberian Peninsula.