Estimating flow concentration and sediment redistribution in shrub-dominated rangeland communities

Abstract:

In arid and semi-arid rangelands where vegetation is typically sparse, a synergistic relationship is assumed to exist between spatial distribution of plants and hydrologic and erosion processes. It is believed that areas underneath plant canopy have been conditioned to act as sediment, nutrients and water sinks whereas interspaces evolve into flow concentration pathways acting as source areas. This vegetation – hydrology feedback mechanism is part of a broader Vegetation Driven Spatial Heterogeneity (VDSH) concept that has been traditionally studied from a theoretical stand point but with little support from experimental data. In shrub-dominated rangeland communities, this VDSH concept implies that spatial distribution of vegetation can be used to model: (1) the level of hillslope dissection into concentrated flow and sheet flow areas and (2) the magnitude of sediment entrapment (deposition) expected from a given vegetation spatial arrangement. In this study, we developed a methodology to test these two hypotheses and derive practical equations for modeling purposes. From a series of rainfall / runoff experiments on naturally vegetated shrubland erosion plots, flow concentration and erosion / deposition processes were quantified using a combination of sediment yield data and high resolution microtopographic changes detected with advance 3D reconstruction technologies (lidar and photogrammetry). This study is expected to produce for the first time (1) predictive equations for spatial frequency of flow concentration pathways in shrub-dominated rangeland communities and (2) equations for sediment delivery ratio, a measure of the portion of eroded sediment that reaches the hillslope outlet as a function of vegetation spatial arrangement.