Hydrological effects of the westward expansion of mediterranean climate and revegetation in Atlantic-Mediterranean transitional headwaters

Abstract

In this study, a complete hydro-climatic characterisation of six mid-mountain headwater catchments in a transitional Atlantic-Mediterranean ecotone in the Western Iberian Range (Spain) is performed. Three catchments exhibit bioclimatic characteristics that are Atlantic type (Oja, Najerilla and Albercos rivers) and the other three are Mediterranean (Cidacos, Linares and Añamaza). The analyses include, for each catchment: (i) trends in river discharges, precipitation and temperature (1965-2015); (ii) assessment of discharge response to precipitation variability at different accumulated time scales; (iii) changes in land use between 1978 and 2010; and (iv) trends in vegetation activity between 1982 and 2015. The results show: (1) a generalised decrease in annual river discharges, especially during spring and summer; (2) a decrease in precipitation, especially in catchments of the Atlantic domain; (3) a significant and widespread increase in annual, spring and summer temperatures; (4) a fast response of river discharges to precipitation dynamics in Atlantic catchments, and a more sustained (over time scales) response in Mediterranean ones; (5) a much closer climate-discharge relationship during winter, which disappears during summer months in all catchments; and (6) an increase in vegetation activity and forest cover in all catchments. These results point to a westward extension of Mediterranean climatic features into the Atlantic region, reflected in reduced spring precipitation (leading to lower discharges) and increased autumn precipitation in Atlantic catchments. In eastern catchments, Mediterranean conditions strengthen, with higher autumn precipitation and rising spring and summer temperatures, further reducing discharges. Although an increase in vegetation likely increases water consumption and evapotranspiration, the relatively short study period does not yet confirm a clear relationship with river discharge trends.