Computing structural and functional flow and sediment connectivity with a new aggregated index: A case study in a large Mediterranean catchment

Hydrological connectivity in large catchments is influenced by natural and human-induced heterogeneities and dynamic processes. In this study, a new aggregated index (AIC) based on topography, C-RUSLE factor, RUSLE2 rainfall erosivity, residual topography and soil permeability, was proposed to model structural and functional flow and sediment connectivity (FSC). It was tested in a large Mediterranean catchment (Vero River, NE Spain, 380 km²) with contrasted physiographic and climatic conditions (19 land uses and 15 types of lithology). Twelve weather stations were used and simulations were done at 5 m of pixel resolution using a LiDAR-derived DEM and the D-Infinity algorithm. Structural FSC (FSC-st) was computed with both an updated version of Borselli's index (IC) and the AIC. Values of connectivity with AIC followed a normal distribution with a wider range of values compared with the non-normal distribution obtained with Borselli's approach. The differences in the values of FSC-st between the different land uses were similar with the two indices and in agreement with the soil erosion rates reported in comparable landscapes. The spatial characteristics at sub-catchment scale were better reflected with AIC although values of FSC-st in the river and outlet were similar between both indices. Functional FSC (FSC-fn) was computed with AIC during 96 months (September 2009-August 2017) characterising the spatio-temporal dynamic at catchment scale (18% of coefficient of variation). FSC-fn was higher in September, October, June and July and lower during the period December-February. Variation of connectivity in the stream was higher than in the hillslopes. Modelling testing with river flow was satisfactory between November and March, and during the months with high discharge values and weak during the summer, suggesting different runoff and sediment responses over the year. The new AIC appeared as a suitable tool for geomorphic and hydrological studies at catchment scale.