Long-term studies of land degradation in the Sneeuberg uplands, eastern Karoo, South Africa: A synthesis

© 2017 Elsevier B.V. For the past 15yr, the Sneeuberg uplands in the eastern Karoo, South Africa, have been a focus for research on land degradation by the above authors and other colleagues. Earlier work in the Karoo emphasised vegetation change whereas we concentrate on physical changes to the landscape at the small catchment scale, e.g., bare, degraded areas (badlands) and gully (donga) systems. Analysis of sedimentation in farm dams allows for reconstruction of environmental histories using 210Pb, 137Cs, geochemical and mineral magnetic properties of the sediments. Erosion rates on badlands are monitored using arrays of erosion pins. Sediment source tracing within small catchments points to the importance of hillslope sources and the relative erosional inactivity of gully systems in recent decades. Sediment supply from hillslope and colluvial sources is maintained by high rates of weathering on mudstones and sandstones. Current degradation should be viewed in the context of a c. 200yr history of overgrazing by European-style stock farming and limited areas of former cultivation in the valleys. Grazing pressures are now much reduced but the loss of soils and vegetation suggests that landscape recovery will require several decades. Additional drivers of past degradation are likely to have been periods of drought and fire (natural and managed) and a gradual increase in both rainfall intensity and the frequency of extreme rainfall events. The future of the degraded Sneeuberg landscape will depend on future farming practices. Desirable options include more sustainable livestock practices, adoption of wildlife farming and other more benign regimes involving mixes of agriculture, tourism, and wildlife protection together with landscape rehabilitation measures

Using 3D observations with high spatio-temporal resolution to calibrate and evaluate a process-focused cellular automaton model of soil erosion by water

Future global change is likely to give rise to novel combinations of the factors which enhance or inhibit soil erosion by water. Thus, there is a need for erosion models, necessarily process-focused ones, which are able to reliably represent the rates and extents of soil erosion under unprecedented circumstances. The process-focused cellular automaton erosion model RillGrow is, given initial soil surface microtopography for a plot-sized area, able to predict the emergent patterns produced by runoff and erosion. This study explores the use of structure-from-motion photogrammetry as a means to calibrate and evaluate this model by capturing detailed, time-lapsed data for soil surface height changes during erosion events. Temporally high-resolution monitoring capabilities (i.e. 3D models of elevation change at 0.1 Hz frequency) permit the evaluation of erosion models in terms of the sequence of the formation of erosional features. Here, multiple objective functions using three different spatio-temporal averaging approaches are assessed for their suitability in calibrating and evaluating the model's output. We used two sets of data from field- and laboratory-based rainfall simulation experiments lasting 90 and 30 min, respectively. By integrating 10 different calibration metrics, the outputs of 2000 and 2400 RillGrow runs for, respectively, the field and laboratory experiments were analysed. No single model run was able to adequately replicate all aspects of either the field or the laboratory experiments. The multiple objective function approaches highlight different aspects of model performance, indicating that no single objective function can capture the full complexity of erosion processes. They also highlight different strengths and weaknesses of the model. Depending on the focus of the evaluation, an ensemble of objective functions may not always be necessary. These results underscore the need for more nuanced evaluation of erosion models, e.g. by incorporating spatial-pattern comparison techniques to provide a deeper understanding of the model's capabilities. Such calibrations are an essential complement to the development of erosion models which are able to forecast the impacts of future global change. For the first time, we use data with a very high spatio-temporal resolution to calibrate a soil erosion model.</p

Multiperspective analysis of erosion tolerance

Erosion tolerance is the most multidisciplinary field of soil erosion research. Scientists have shown lack in ability to adequately analyze the huge list of variables that influence soil loss tolerance definitions. For these the perspectives of erosion made by farmers, environmentalists, society and politicians have to be considered simultaneously. Partial and biased definitions of erosion tolerance may explain not only the polemic nature of the currently suggested values but also, in part, the nonadoption of the desired levels of erosion control. To move towards a solution, considerable changes would have to occur on how this topic is investigated, especially among scientists, who would have to change methods and strategies and extend the perspective of research out of the boundaries of the physical processes and the frontiers of the academy. A more effective integration and communication with the society and farmers, to learn about their perspective of erosion and a multidisciplinary approach, integrating soil, social, economic and environmental sciences are essential for improved erosion tolerance definitions. In the opinion of the authors, soil erosion research is not moving in this direction and a better understanding of erosion tolerance is not to be expected in the near future

The concept of transport capacity in geomorphology

The notion of sediment-transport capacity has been engrained in geomorphological and related literature for over 50 years, although its earliest roots date back explicitly to Gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering. Despite cross fertilization between different process domains, there seem to have been independent inventions of the idea in aeolian geomorphology by Bagnold in the 1930s and in hillslope studies by Ellison in the 1940s. Here we review the invention and development of the idea of transport capacity in the fluvial, aeolian, coastal, hillslope, débris flow, and glacial process domains. As these various developments have occurred, different definitions have been used, which makes it both a difficult concept to test, and one that may lead to poor communications between those working in different domains of geomorphology. We argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons. First, as sediment becomes entrained in a flow, the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment. Secondly, environmental sediment transport is complicated, and the range of processes involved in most movements means that simple relationships are unlikely to hold, not least because the movement of sediment often changes the substrate, which in turn affects the flow conditions. Thirdly, the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space. Consequently, new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems