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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

On the importance of downscaling methods to study the impact of climate change on Mediterranean basins

Pere Quintana Seguí (Observatori de l'Ebre (Universitat Ramon Llull - CSIC)); Aurélien Ribes, Eric Martin, Florence Habets, Julien Boé

Studies of the impact of climate change on water resources usually follow a top to bottom approach: a scenario of emissions is used to run a GCM simulation, which is downscaled and bias-corrected. Then, this data is used to force a hydrological model. Seldom, impact studies take into account all relevant uncertainties. In fact, many published studies only use one climate model and one downscaling technique. In this study, the outputs of an atmosphere-ocean regional climate model are downscaled and bias-corrected using three different techniques: a statistical method based on weather regimes, a quantile-mapping method and the method of the anomaly. These data are used to force a distributed hydrological model to simulate the French Mediterranean basins, which are characterized by water scarcity and an increasing human pressure, which cause a demand in assessments on the impact of climate change hydrological systems.
The purpose of the study is mainly methodological: the evaluation of the uncertainty related to the downscaling and bias-correction step. The periods chosen to compare the changes are the end of the 20th century (1970-2000) and the middle of the 21st century (2035-2065). The study shows that the three methods qualitatively produce similar anomalies of the mean annual precipitation, but, quantitatively, there are important differences, mainly in terms of spatial patterns.
The study also shows that there are important differences in the anomalies of temperature. These uncertainties are amplified by the hydrological model. In some basins, the simulations do not agree in the sign of the anomalies and, in many others, the differences in amplitude of the anomaly are very important. Therefore, the uncertainty related to the downscaling and bias-correction of the climate simulation must be taken into account in order to better estimate the impact of climate change, with its uncertainty, on a specific basin. The study also shows that according to the RCM simulation used and to the periods studied, there might be significant increases of winter precipitation on the Cévennes region of the Massif Central, which is already affected by flash floods, and significant decreases of summer precipitation in most of the region. This will cause a decrease in the average discharge in the middle of the 21st in most of the gauging stations studied, specially in summer. Winter and, maybe spring, in some areas, are the exception, as discharge may increase in some basins.