Machine learning digital soil mapping to inform gully erosion mitigation measures in the Eastern Cape, South Africa

Abstract

Soil erosion is probably the most common form of land degradation, leading to on and off site detrimental effects, such as an increase in river sediment load which have been shown to drastically decrease dam storage capacity. The South African Government has announced plans to build two large dams within an area with a very high erosion rate, thus necessitating soil erosion mitigation efforts. These efforts should consider the mechanism of gully formation, as it determines which conservation measures could be used. With overland flow gullies the kinetic energy of flowing water needs to be curbed, while gullies form by piping in dispersive soils where free water could accumulate. This paper describes how a soil association map for the quarternary catchments T35DE was created with digital soil mapping methods, in order to indicate areas where piping could occur. Soil were described and classified at 600 locations determined with the conditioned Latin hypercube sampling method, within a 500 m buffer around the available road network. This soil database was then used with the multinomial logistic regression algorithm to create an initial soil association map with seven soil associations, based on their soil erosion properties. Additionally, a soil depth map was created using the cubist algorithm. The soil association map was then created again, but with the soil depth map added as an additional covariate. Including the soil depth map as a covariate improved the accuracy (68% vs 64%) and level of detail of the final soil association map, even though the soil depth map was a poor reflection of reality. The final soil association map was compared to the best current available spatial soil dataset, and found to contain 123 times more detail. Superimposing the gully extent of the soil association map confirmed that the planosols are the most susceptible to gully erosion and mitigation efforts should avoid increasing free water in the subsoil on the planosol area, and the as yet ungullied planosol should be prioritized as it could become a hotspot for gully formation if it degrades.