Farm dam siltation, sediment management and sediment source tracing in the Zeerust-Swartruggens Area, North West Province
Abstract
The North-West province in South Africa is a semi-arid landscape vulnerable to soil erosion and sediment related problems. There is high dependence on small farm dams for the provision of water for cattle and irrigation. However, loss of small farm storage capacity due to sediment accumulation is a growing threat to water provision which cannot be ignored. There is widespread experience from pioneering work of Foster and co-workers in the Karoo region of South Africa on the use of environmental magnetism and environmental radionuclides to reconstruct important events in the history of catchments and to understand the drivers and trends in soil erosion and sediment dynamics. Such work needs to be extended to other parts of the country where similar problems exist to better assess the techniques and help better understand and address the dual problems of soil erosion and sedimentation.
The first part of the study entailed an inventory and mapping of small farm dams in the farming areas of Zeerust and Swartruggens in the North West province. It included the survey of sediment-related problems and sediment management at farm level. In the second part two specific sub-catchments were selected to test the application of environmental magnetism and environmental radionuclides for sediment tracing.
Forty four small farm dams were mapped and investigated. The newest was less than 5 years old and the oldest over 100 years old. Most of the dams were built with mud and stone along main stream courses to capture runoff and provide water for livestock and irrigation. Thirty four percent of the dams are heavily silted. However, sixty four percent of them are still functional in terms of holding water for a full season. The loss of dam’s storage impacted negatively on the farming operations and most of the farmers (81 %) were aware of this. However, most of them were not addressing the source of the problem of dam siltation as many chose to manage the sediment through cleaning the sediment from the dams every 4 to 6 years. Thus dam siltation in the study area is a big challenge with huge potential financial and livelihood implications for the farmers.
On the use of environmental magnetism for sediment source tracing, the statistical analysis showed that only magnetic susceptibility (MS) was able to distinguish between topsoil and
subsoil and between burnt topsoil and unburnt topsoil in one of the two sub-catchments investigated. None of the remanant magnetic parameters managed to distinguish the potential sediment sources. The analysis of the environmental radionuclide concentrations in sediment and potential sediment source materials confirmed the potential of unsupported 210Pb as an alternative to 137Cs in soil erosion and sedimentation studies in the catchments. The successful magnetic and radionuclide parameters were used in a multivariate sediment mixing model to estimate the proportion of sediment coming from the identified sources in Dam 1. The result showed that 100% of the sampled sediment was mobilized from top soil. Although unexpected, this result may be a reflection of the complexity of sediment mobilization processes influenced by various factors including the nature and spatial distribution of the rainfall. The result may also reflect the sediment sampling which considered only recently deposited sediment, which may have come from a specific source.