Abstract
Abstract. In this study the climatic and hydrological trends across 88 sub-basins of the Ebro River basin were analyzed for the period 1950–2006. A new database of climate information and river flows for the entire basin facilitated a spatially distributed assessment of climate-runoff relationships. It constitutes the first assessment of water yield evolution across the whole Ebro basin, a very representative example of large Mediterranean rivers. The results revealed a marked decrease in river discharges in most of the sub-basins. Moreover, a number of changes in the seasonality of the river regime was found, resulting from dam regulation and a decrease in snowpack in the headwaters. Significant and positive trends in temperature were observed across most of the basin, whereas most of the precipitation series showed negative coefficients, although the decrease in magnitude was low. The time evolution of the residuals from empirical models that relate climate and runoff in each sub-basin provided evidence that climate alone does not explain the observed decrease in river discharge. Thus, changes in water yield are associated with an increase in evapotranspiration rates in natural vegetation, growth of which has expanded as a consequence of land abandonment in areas where agricultural activities and livestock pressure have decreased. In the lowlands of the basin the decrease in water yield has been exacerbated by increased water consumption for domestic, industrial and agricultural uses. Climate projections for the end of the 21st century suggest a reduced capacity for runoff generation because of increasing temperature and less precipitation. Thus, the maintenance of water supply under conditions of increasing demand presents a challenging issue requiring appropriate coordination amongst politicians and managers.
Highlights
Mountains play a critical role in water resources availability in lowland areas of drainage basins
Based on estimates of the evolution of the spring normalized difference vegetation index (NDVI), derived from NOAA satellite imagery (8 km spatial resolution) since 1981, more than 45% of the surface area has been affected by a statistically significant increase in the vegetation index, which is related to the increase in vegetation cover and its physiological activity (Nogues et al, 2010)
Runoff generation in the Pyrenees is decreasing, but the pressure on its water resources has increased as its contribution to the Ebro River has gained in importance in recent decades
Summary
Mountains play a critical role in water resources availability in lowland areas of drainage basins. Water management in the Mediterranean region is difficult as water is a scarce resource but a key element in ensuring the development of irrigation agriculture, increased populations, improved living standards, industry and tourism activities (Cudennec et al, 2007). In this context water resources generated in mountains are under increasing pressure because of constantly increasing demand. Increasing efforts are being made to quantify and understand the consequences of environmental change for the hydrological response of mountain areas, and to assess its impact in the lowlands This may improve the capacity to optimize water use and enhance preparedness for future challenges in water management. The results are discussed in the context of climatic and environmental projections for the region in coming decades
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