Modelling Soil-Regolith Thickness In Complex Weathered Landscapes Of The Central Mt Lofty Ranges, South Australia

We describe a preliminary model to predict soil-regolith thickness in a 128,000 ha study area in the central Mt Lofty Ranges in South Australia. The term soil-regolith includes the A, B, and C horizons of a soil profile to the lower boundary of the highly weathered bedrock zone. The thickness of the soil-regolith has a major control on water holding capacity for plant growth and movement of water through the landscape, and as such, it is important in hydro-ecological modelling. Thickness estimates have direct application in groundwater salinity studies and in evaluating land suitability, e.g., for forestry and agriculture. Geology and landscape evolution within the area is complex, reflecting the variable nature of bedrock materials, and the partial preservation of deeply weathered profiles as a consequence of weathering processes dating back to the Cenozoic or possibly older. These characteristics, together with strong climatic gradients across the area, make it an ideal location to understand the environmental and landscape evolution controls on weathering depth. It also covers the spectrum of environmental and landscape age characteristics found across many parts of southern Australia. We use a digital soil mapping piecewise linear decision tree approach to develop the model to predict soil-regolith thickness. This model is based on relationships established between 677 soil-regolith thickness measurements and 18 environmental covariates (e.g., rainfall, slope, gamma-ray spectrometry). Results establish an R-2 of 0.47 on a validation data. These results are preliminary and further work is being done to better understand and refine the model, and to test other modelling approaches.