Assessing the effects of site preparation treatments on erosion processes and sediment yield on a commercial Eucalyptus plantation: case study at Two Streams, KwaZulu-Natal.

Abstract

Soil erosion monitoring and modelling is critical in the face of climate change, as erosion is detrimental to environmental and human health. It reduces soil productivity through degradation, compromises water quality through nutrient loading freshwater sources, and decreases reservoir capacity through sedimentation. This is a global challenge which is being amplified by increased levels of soil erosion on cultivated lands (e.g. commercial forestry), the combined effects of which hinder the success of several United Nations Sustainable Development Goals. However, commercial forestry is essential for human survival, providing a host of resources for human consumption, and expanding in its global coverage each year; although, this expanding need for commercial forestry creates a paradox, as it has the potential to damage environmental health and biodiversity (systems which humans rely on for survival), particularly through soil erosion and sedimentation of freshwater systems. Afforestation of plantations has been widely considered a land-use activity which reduces soil erosion; although, this is dependent on the management of the commercial plantations, where certain commercial forestry management techniques exacerbate soil erosion, such as the well-used site preparation technique of burning. Therefore, an investigation into the effect of commercial forestry site preparation techniques such as burning (at different severities) and mulching on soil erosion and the modelling thereof is required, as only a paucity of research has reported on this. Soil erosion measurements were conducted on a newly planted Eucalyptus dunnii stand, which consisted of three different site preparation techniques, namely a hot burn, a cold burn and a mulch treatment, in the Two Streams catchment, Kwa-Zulu Natal, South Africa. Micro-runoff and runoff plots were used to respectively measure splash and rill erosion of sediment, nitrogen, phosphorous, dissolved organic carbon, particulate organic carbon loss and runoff on each treatment at different slopes. In addition, soil erosion and runoff of this catchment and treatments were modelled using the ArcSWAT model, and the observations were used to validate the simulated outputs. The mulch treatment had the most consistent reduction in runoff and erosion, while the burn treatments of different severities generated greater respective runoff and erosion quantities through different erosional processes (splash vs rill). The ArcSWAT model over-simulated runoff on the hot burn and mulch treatment, while under-simulating on the cold burn treatment; however, the model consistently over-simulated sediment and nutrient loss on all treatments, indicating the model’s inability to simulate soil erosion on the defined land-use treatments. The reduced runoff and erosion produced by the mulch treatment is attributed to the protection that the mulch provides to the soil from splash erosion and the resistance posed to overland flow reducing rill erosion. The burn treatments generating more erosion and runoff through different erosional processes was attributed to the differing nature of debris produced by each burn severity. This research will contribute towards the data sets necessary to refine the land-use management tools of the ArcSWAT model to better model soil erosion on different land-use treatments. Furthermore, this research demonstrates the erosion processes that differing site preparation treatments are susceptible to, and what this means for future research and protecting soil and downstream water quality in the face of climate change.