The effect of farm dams and constructed banks on hydrologic connectivity and runoff estimation in agricultural landscapes

System coupling and landscape connectivity control the flow of water and sediment through landscapes. Although coupling is well known to control long-term landscape development and shorter-term sensitivity to disturbance, the anthropogenic influences on coupling are seldom considered in hydrologic investigations. In particular, the building of small-scale water diversion (earth banks) and collection (farm dams) infrastructure on hillslopes in dryland agricultural areas may significantly alter hillslope-channel coupling. Twelve sub-catchment basins in a dryland agricultural region were investigated under their natural (ignoring infrastructure) and modified (including infrastructure) conditions to investigate the influence of water collection infrastructure on hydrologic connectivity, and whether manual modification of a Digital Elevation Model (DEM) could account for the impact of these factors in hydrologic simulation of hydrologic and geomorphic processes. Dam numbers and density have both increased over the period of available aerial photography (1965-1999), resulting in an average 39.5% reduction (range 4.3-86.7%) in the area retaining hydrologic connectivity with the basin outlet. Analysis of basins dominated by either banks or dams, and with combinations of both was performed using the Cumulative Area Distribution (CAD), Hypsometric Curve (HC), Simplified Width Function (SWF) and Instantaneous Unit Hydrograph (IUH). The geomorphic descriptors (CAD and HC) showed little change in basin structure as a result of farm dam and bank construction, but hydrologic descriptors (SWF and IUH) indicate that hillslope processes are significantly altered by farm dams and banks. Because runoff models are sensitive to catchment area, incorporating hillslope water capture and diversion infrastructure into the base data sets may offer a solution to improved parameterisation of spatial models of hydrology, particularly in dryland agricultural regions.

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