Abstract

There is a need for rapid cost effective methods to obtain spatially distributed data of soil depth. Soil depth determines the subsurface topography, a major control on the distribution of flowpaths in landscapes. An EM38 survey was conducted on a 12 ha site at Bloemfontein, South Africa. A significant linear relationship between soil depth and EC_a were obtained with multiple linear regression (Soil depth = 149 - 29 CV_0.5 + 34 CV₁). It was found that the equation can reasonably accurately (RMdAE = 20%, REF = 0.49) estimate soil depth from EC_a readings. This made it possible to estimate 15,000 soil depths across the study area, which contributed to the successful characterization of subsurface topography. Consequently the following conclusions could be made. There was a close correlation between surface and subsurface topography. Overland flow seems to be high causing erosion on higher elevations and deposition of sediments and accumulation of water in lower lying areas. From flow accumulation maps, sites possibly controlling the hydrology of the study area were identified. The methodology developed should contribute towards characterising hydrological research sites.

Key words: EM38, soil depth, soil water, hydrological response, thresholds, water flow.