Connectivity and degradation in semi-arid systems: patterns, thresholds and feedback effects

Hydrologic, geomorphic and vegetation processes in drylands are tightly coupled through feedback mechanisms. These feedbacks have implications for the equilibrium and resilience of the emerging vegetation and landform patterns, and are particularly important for potential degradation effects under climate and anthropogenic pressures. Here we analyze semi-arid systems with patchy vegetation at the hillslope scale. As increased surface runoff connectivity has been linked to degradation, we focus on evolving “connectivity patterns” resulting from these feedback effects. We analyze the connectivity patterns using a landform evolution model (LEM) that includes coupled dynamic vegetation, hydrology, and soil depth evolution modules. Surface connectivity resulting from different (initial) topographies and soil depths, as well as varying soil erodibilities and vegetation characteristics are analyzed and contrasted. Model results are compared with vegetation and connectivity patterns obtained from high-resolution satellite images from both well preserved and degraded semi-arid sites in the Northern Territory and New South Wales (Australia). The analysis from satellite images reveals a major role of surface connectivity on the spatial organization of patchy vegetation, suggesting that transitions on the distribution of vegetation leading to degradation are related to sharp variations on the landscape surface connectivity. More general model results further suggest that these sharp variations are related to thresholds on slope and soil erodibility, regulated by feedback effects dependent on vegetation response times and soil depths. These results have important implications for restoration efforts in degraded semi-arid areas.