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We describe a model to predict regolith thickness in a 128,000ha study area in the central Mt Lofty Ranges in South Australia. The term regolith encompasses soil (A and B horizons) and highly weathered bedrock (C horizon). The thickness of the regolith has a major control on water holding capacity for plant growth and movement of water through the landscape, and as such is important in hydropedological modelling and in evaluating land suitability, e.g. for forestry and agriculture. Thickness estimates also have direct application in mineral exploration and seismic risk assessment. Geology and landscape evolution within the study 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 to the Cenozoic, or possibly older. These characteristics, together with strong climatic gradients, make the study area an ideal location to understand environmental and landscape evolution controls on weathering depth. The area also features weathered landscape analogues to many parts of southern Australia. We use a digital soil mapping rules based approach to develop a model to predict regolith thickness. This model uses statistically-based relationships established between 714 regolith thickness measurements and 29 geographic environmental covariates including geology, geochemistry, terrain and climate themes, and with a ground resolution of 10m. Accuracy testing based on a 75:25% training:test data split on the resulting map established a correlation R² of 0.64. This result is encouraging and is a significant advance over regolith depth mapping by traditionally-based regolith-landscape mapping methods. Finally, it leads the way towards a nationally consistent regolith thickness map for landscape scale environmental simulation modelling and decision support.
