Natural revegetation of abandoned cultivated land in the Spanish Pyrenees and its effect on streamflow

Abstract

Abandonment of cultivated land has been a common phenomenon in the central Spanish Pyrenees. Secondary succession causes the abandoned fields to gradually transform into forested areas and thereby impacts the hydrology. River discharges decrease, raising concerns about water availability. This study examines the change of vegetation and its subsequent influence on stream flow in the Upper Aragón basin by using a combination of satellite remote sensing and a process-based hydrologic modelling technique. A pixel based linear regression analysis was run on twelve Landsat satellite images, between the years 1984 and 2013, to determine the spatial and temporal distribution of vegetation change. With the results, the effect on stream flow was modelled using the PyCatch hydrological model in a small catchment within the Upper Aragón basin. Finally, also the effect of climate change on streamflow was modelled to determine its importance relative to the effect of succession. Analysis of the satellite imagery showed that an increase of vegetation between the years 1984 and 2013 has happened in most of the Upper Aragón basin, with most of the changes occurring beneath 1600 meter on slopes with a dominantly northeasterly aspect. The change resulted in a streamflow decrease of 87% between 1950 and 2040 in the modelled catchment, with absolutely the highest changes occurring in autumn. Under only climate change the streamflow decreased by 28% between 2005 and 2040, while vegetation changes caused a decrease of 60% between those years. The results suggest that secondary succession can influence the hydrology of an area considerably, even though the found decrease of discharge overestimates measured decreases. The impact may in some cases even be considerably larger than the effect climate change has on the hydrology of an area.