Erosional development of small scale drainage networks

Abstract

Drainage networks are usually determined for large scale river systems. Small scale drainage networks for upland eroding areas have rarely been studied. In this scale, drainage networks of surface runoff substantially affect soil erosion. The objective of this study was i) to explore the similarities between drainage networks of runoff on eroding surfaces and those of river systems and ii) to determine the interrelationships between drainage network development and soil erosion. In flume experiments sequences of simulated rainstorms and overland flow were subjected to soils of initially different surface configurations. Before and after each rainstorm and overland flow test, digital elevation maps (DEMs) of the soil surface were generated using a laser scanner with 3 mm grid spacing. Drainage networks were determined from the DEMs and characterized with Horton's ratios, fractal characteristics, and with single stream properties like gradient, sinuosity, and orientation. Horton's ratios indicated convergence and organization for all determined networks. When expressed with Horton's ratios and fractal characteristics, drainage networks of runoff on eroding surfaces were similar to those of river systems. Initially different network configurations yielded different erosion values but resulted in similar network characteristics at the end of the rainstorm and overland flow experiments. Raindrop detachment, clod destruction, and microrelief changes were identified as important mechanisms of network configuration and stream property changes during the rainstorm and erosion events. The network changes led to network structures that resulted in continuously decreasing soil erosion values. The results support the idea of optimization in drainage network development.