**5. Conclusions**

The regional modeling approach we have proposed here provides a base for exploiting simple models such as Palmer water balance in a physical consistent manner: (1) without the need to apply bias corrections and (2) reducing uncertainty by eliminating additional sources of errors which are brought through coupling RCMs with complex hydrological models. Of course, these advantages come with a cost: information provided by our proposed methodology is basin-averaged and cannot account for details at the sub-basin level which could be essential for some specific application.

Our methodological approach is based on two pillars: the Palmer water balance model validated and applied at catchment level and the multimodel ensemble of regional climate experiments (which provides physically consistent information under climate change scenarios). The RCM ensemble analyzed in this chapter manages to reproduce relatively well the observed components of water cycle such as potential evapotranspiration and precipitation when multimodel averages of regional climate results are used over the river basins. These results provide a certain level of confidence when analyzing future evolution of precipitation, potential evapotranspiration, potential runoff and related indices in the area of interests under climate change scenarios.

The correlations between observed streamflow at the observation stations in each basin and PDSI-related indices show that the PDSI represents reasonably well the local water balance and drought-related processes taking place in the catchments (**Figure 3**). This allows us to use the basin-averaged PDSI computed with multimodel ensemble data for the assessment of the climate change impact on drought over the selected basins under the moderate and worst-case concentration scenarios. Spatial average procedure applied here to gridded PDSI provides robust results.

The results from the case studies based on the ensemble means of PDSI suggest that depending on the specific climate scenario and catchment, droughts that in the Palmer classification were deemed as incipient, mild or severe toward the end of the twentieth century, will become a normal summer feature toward the end of the twenty-first century in the mid and lower Danube basin. The tendency toward drought is present under the conditions of a reduction of runoff, mostly in summer, revealing the important role of potential evapotranspiration increase and precipitation decrease in the drought-related processes over the mid-latitude areas. However, the analysis of individual evolution of PDSI in the five numerical experiments, under both climate scenarios, reveals uncertainties associated with the identified signal of enhanced aridity at the basin level. The largest (lowest) uncertainty is found for the Somes (Arges) river basin. More studies that couple local climatic information with their hydrologic impact are needed to provide the background for the assessment of water resources under climate change conditions in terms of adaptation planning and sustainable development.
