Cross-Scale Structural and Functional Connectivity as a Driver of Hillslope Erosion on Disturbed Landscapes

Abstract:

Knowledge regarding the effects of cross-scale structural and process connectivity on runoff and erosion events on disturbed rangelands remains limited due to the individual plot-scale nature of most research. This study quantified surface susceptibility and runoff and erosion across point- to hillslope-scales at unburned and burned woodland-encroached shrublands using high-intensity rainfall simulation, overland flow experiments, and hydrologic modeling. High levels of runoff (31-47 mm) and erosion (154-1893 g m^-2) were measured at the patch scale (13 m²) associated with accumulation of fine-scale (0.5 m²) runoff and sediment and high velocity (0.10-0.26 m s^-1) concentrated flow through contiguous bare zones (64-85% bare ground). Burning increased continuity of runoff sources and sediment availability. For unburned and burned conditions, cumulative runoff was generally consistent across plot-scales while erosion increased with increasing plot-area up to the patch scale. The cross-scale increase in erosion is attributed to enhanced sediment detachment and transport in concentrated flow paths. At the hillslope scale, predicted runoff and erosion rates reflected the measured patch-scale runoff and erosion trends and the connectivity of hydrologic and erosion processes and sediment availability. For similar rainfall across unburned conditions, predicted hillslope runoff (19-35 mm) was slightly less than or equal to measured patch-scale runoff (27-37 mm), and predicted erosion (100-413 g m^-2) was greater than or similar to patch-scale measured erosion (124-311 g m^-2). For burned conditions, predicted hillslope runoff (36-41 mm) was greater than or equal to patch-scale runoff (28-34 mm), and predicted hillslope erosion (314-516 g m^-2) was either consistent with or decreased relative to patch-scale measures (274-884 g m^-2). The decline in simulated hillslope-scale erosion relative to measured patch-scale erosion for burned conditions is attributed to limited sediment availability at the hillslope scale. The cross-scale experiments and simulations clearly demonstrate that the magnitude of hillslope response on disturbed rangelands is governed by the connectivity in surface susceptibility, runoff and erosion processes, and sediment availability and the rate or volume of rainfall input.