Three decades of shoreline change investigation over Lake Sibaya, South Africa, using Landsat imagery and geospatial methods.

Abstract

Lake Sibaya has experienced a significant decline in shoreline changes over the past three decades, in part, due to land use/cover (LU/LC) changes and drier climate conditions. In response, this study examined shoreline changes of Lake Sibaya between 1986 and 2020, LU/LC changes surrounding it, and climate variability using Landsat data series, the Digital Shoreline Analysis System (DSAS) geospatial modelling approaches, and climate data. DSAS provides the unique ability to compute the rate of change statistics using a suite of metrics, but in this thesis, only two variables were used (Net Shoreline Movement (NSM) and End Point Rate (EPR)) to understand shoreline dynamics and predict the future shoreline position over the next decade. The Landsat analysis showed a reduction in Lake Sibaya’s surface area from 70.7 km2 in 1986 to 49.5 km2 in 2020, while the DSAS analysis showed that the changes occurred at a NSM of −1338 m and an EPR average of ±6 m/year. Clear, conspicuous retracted changes were observed near the planted forests of Mbazwana between the southeastern/western basins and Manzengwenya in the northeastern region of the northern arm. Shoreline projections for the future showed the lake would continue to shrink from 49.5 km2 in 2020 to 39.1 km2 by 2030. Field observations and photographs validated DSAS results of declining shoreline changes in the lake and further in the vulnerable edges. Regarding the LU/LC, the results showed an expansion of forest plantations from 155.2 km2 in 1986 to 248.3 km2 in 2020, while water bodies decreased considerably, as indicated above, with a clear increase of barren land from 129.3 to 210.5 km2 over the 34 years, mostly the accumulation of sandbars in areas previously occupied by the lakes’ water. The study established a link between the lake’s surface changes and drier climate conditions during the extreme El Niño phases. During El Niño/La Niña events, the lake surface decreased/increased, with precipitation showing a significant decrease in the 2014/2016 intense drought period. During this period, higher temperatures were in the order of 32°C, and these conditions are believed to have caused further reduction in lake water levels. Overall, this research demonstrates how LU/LC change and drier climate conditions have interacted to shape Lake Sibaya's shoreline over the past 34 years and highlight the importance of integrated management approaches to sustaining this vital ecosystem. Further research is recommended to examine the development of diverse vegetation communities in dry areas of the lake as well as the intensification of illicit forestry practices in this catchment area and how these might impact the ecological function of this water resource.