Unravelling the role of vegetation on the different trends between climatic and hydrologic drought in headwater catchments of Spain
The availability of water resources is a major challenge of the Mediterranean region. Intense land use transformation has impacted the headwaters of river basins that generate most of the water resources, reducing streamflow This study analyses the evolution of hydrological and climatic drought in headwater catchments of Spain and it explores the extent to which vegetation can reinforce trends in hydrological drought severity in comparison to the evolution of cFlimatic drought severity. We have used the Standardized Precipitation Evapotranspiration Index and Standardized Streamflow Index to examine hydrological and climatic drought, respectively, and examined the changes in the frequency, magnitude, and duration of climatic and hydrologic droughts over non-perturbed headwater catchments of Spain from 1961 to 2013. We quantified vegetation changes over time in the analysed catchments and we compared the changes in the climatic and hydrological droughts with changes in vegetation coverage using Pearson’s r correlations and linear regression model. The results show that the trends toward higher frequency, duration and severity of hydrological droughts are more marked than the observed trends in climatic droughts, which can be associated to the dominant increase in vegetation coverage and activity in the study domain. Finally, it is concluded that the spatial differences observed between the trends of climatic and hydrological droughts show some relationship with the patterns of forest succession observed in recent decades. Our results stress the relevance of land transformation processes on trends in water resources availability, particularly during periods of climatic droughts in which competence between vegetation water consumption and streamflow production is much more relevant.