The influence of surface roughness on runoff generation and soil erosion in semi-arid Environments

Abstract

Geomorphology is essentially the study of rough surfaces; perfectly smooth, natural surfaces are extremely rare on Earth. There is no single property that can defined as 'surface roughness'; it can be characterised in a variety of ways. Therefore the selection of any roughness measure should be made in consideration of its proposed use. Recent developments in laser scanning technology have removed many of the constraints of analysing surface roughness at the hillslope scale and in natural environments. Such advances have increased the availability of topographic data so that a re-examination of common methods of characterising surface roughness is in order. This investigation compares the utility of numerous roughness measures for representing particular surface features relevant to less-measurable hydrological processes operating on soil surfaces in two contrasting semi-arid catchments. Roughness oriented in the down-slope direction is related to depressional storage, whereas that element of roughness identified from cross-slope transects represents the convergence of flow-paths as a result of microtopography. Measures that identified a specific feature of rough surfaces perform well; however, the large range of soil surface features and processes exhibited at the hillslope scale limits the ability of general roughness measures (such as the standard deviation of elevations) to represent specific hydrological processes. The division of a semi-arid hillslope into 'Morphological Runoff Zones' provides a suitable background knowledge of the nature of the soil surfaces through which subsequent fine-scale roughness measures can be considered. Spatially variable processes acting on the soil surface and the nature of the soil surface are interdependent. In semi-arid environments where Hortonian overland flow is dominant, the microtopography of natural soil surfaces is partly determined by the balance between the erosive force of raindrop impact and shear stress caused by overland flow concentrations. Microtopography combines with larger-scale topographic roughness to determine the upslope area of any point of a hillslope. The distribution of areas of water retention and detention with respect to flow concentrations and flow depths also influences the downslope transport of runoff and sediment. Depression storage preferentially occurs in flow concentrations. These are the first areas of the surface where the depression storage capacity will be satisfied. Knowledge of the spatial configuration of roughness elements and connectivity of responsive areas to the main channel network is important for understanding runoff generation. Measures of surface roughness can also be used to identify units of particular hydrological response and determine their distribution across a hillslope. Placed in the context of large-scale topographic variation of the hillslopes, this reveals suitable criteria for generalising from one hillslope to another.