Assessing the effects of wetland conversion to crop farming on wetland loss, water quality and plant foliar nutrients in the lower Umfolozi floodplain system
Date
2021
Authors
Dlamini, Mandla
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Abstract
Wetland ecosystems are regarded as very beneficial in the world. They provide key benefits to humans and the environment including attenuation of floods, filtering polluted water and are habitats for many flora and fauna. Despite their importance, wetland ecosystems are subjected to multiple stressors, which include but not limited to increases in food demand, growing populations, climate change and variability. From a resource management perspective, wetland agriculture (crop production) presents a fascinating conundrum. Crop production alters wetland ecosystems significantly and at an unprecedented rate, especially in developing countries but at the same time, wetlands are highly productive. Whilst wetland agriculture is recognised globally, in South Africa, there is a lack of information about spatiotemporal changes of wetland extents and water quality status. Furthermore, the water reticulation role by wetlands and crop production can negatively affect plant nutrients composition. In that regard, this study aimed to investigate the effects of crop production on the floodplain system. Small-scale cultivation is one of the most detrimental causes of wetlands loss. First, the study sought to detect and map small-scale farms in wetlands areas using WordView-2 in the lower uMfolozi floodplain system (UFS). Second, the study investigated the changes in land use and land cover (LULC), how they contribute to wetland extent loss and test how best the geographic object-based image analysis (GEOBIA) approach and multitemporal Landsat data could be used to map the spatiotemporal dynamics of land use in the study area. Third, to assess the quality of water and soil from the effects of surface water discharged from irrigated agriculture. Fourth, to determine how accurate Sentinel-2 and the random forests algorithm were at predicting nitrogen, potassium, calcium, magnesium, phosphorus, sulphur, zinc, boron, and copper in wetland vegetation, as well as evaluate seasonal nutrient variations. To achieve the study objectives, a combination of research methods were used including standard analytical, geographical information system (GIS) and remote sensing techniques for triangulation purposes. Remote sensing has a high potential as a source of spatiotemporal information urgently required in wetland agriculture. Standard analytical procedures have the potential to afford the generation of information that is precise and accurate for such systems. The findings revealed the success of WorldView-2 in detecting small-scale farms in diverse and heterogeneous environments. The findings further revealed a major conversion of wetland to agriculture, which was attributed to small-scale farms. GEOBIA can extract time-series variations in different LULC at high levels of precision, far beyond what conventional pixel-based supervised classification algorithms can achieve. Results showed that the sodium adsorption ratio, electrical conductivity, sodium, chloride, pH, and nitrate content were all above the South African agricultural water quality guidelines' allowable limits. Last, the results revealed that for the dry winter and rainy summer seasons, the average R2 was 0.66 and 0.76, respectively. During the summer, however, copper, sulphur, and magnesium were poorly correlated (R2, ≤ 0.5). Overall, the results of this study are important because they indicate that the human agent is one of the key drivers of a persistent decline in wetland habitats. Key findings from the research imply that there is an urgent need to devise and introduce objectively informed initiatives to enhance the sustainability of the UFS and others. The findings argue the generation of sound data on land-use change and concerted efforts to regularly track the characteristics of water, soils and plants in these habitats. Therefore, researchers are urged to take advantage of the above-mentioned instruments and techniques for assessing the status quo of wetlands, especially floodplains
Description
A thesis submitted to the Faculty of Science, at the University of the Witwatersrand, in fulfilment of the requirements of the degree of Doctor of Philosophy (PhD) in Geography and Environmental Studies, 2021