Hydrological Connectivity Does Change Over 70 Years of Abandonment and Afforestation in the Spanish Pyrenees

Runoff connectivity depends on topography, rainfall, man-made elements (terraces, trails, roads, drainage systems) and vegetation. In this study we quantified the effects of 70 years of human activities on runoff connectivity in the mountaineous Araguás afforested sub-catchment (17.2 ha; Central Spanish Pyrenees). The index of connectivity (IC) of Borselli et al. (2008) was chosen to perform this metric over 6 land use scenarios at high spatial resolution (1 x 1 m of cell size). The current scenario (year 2012) was simulated with three flow accumulation algorithms (MD, MD8 and D8). MD8 was linked with the most frequent hydrological response of the sub-catchment (rainfall intensity and stream flow during seven years) and generated the most representative pattern of connectivity, especially in the linear landscape elements (LLE). This algorithm was chosen to simulate the 5 past scenarios (1945, 1956, 1973, 1980 and 2006). In all scenarios the highest connectivity appeared related to trails and roads, as well as to streams and gullies, whereas the lowest appeared related to stonewalls in 1945 and 1956 to hillslopes in 1973, and the following afforestation. Changes in connectivity mainly depended on the changes in the vegetation factor and in a minor way in the total length, spatial location and type of LLE. Afforestation promoted lower and more stable connectivity at both local and catchment scales.