Sponge Functions for Semi-Arid Cities: Assessment and Recommendations for Water Resilience – The Case of Johannesburg (2000–2020)

Urban water resilience is crucial for semi-arid cities that are facing the challenges of climate change and rapid urbanisation. This thesis presents a Composite Sponge Index (CSI) which assesses Johannesburg's capacity to intercept rainfall, slow runoff and store water between 2000 and 2020. It examines temporal trends in sponge functioning and provides evidence-based recommendations for water management. The research employs a sequential explanatory mixed-methods design, integrating quantitative geospatial analysis with qualitative stakeholder validation and expert consultation. The CSI framework comprises three weighted variables representing core sponge functions: the Vegetation Interception Index (VII) is derived from MODIS NDVI and evapotranspiration data (30%); the Surface Flow Regulation (SFR) is derived from DEM slope analysis and land cover Manning coefficients (30%); and the Observed Surface Water Volume (OSWV) is derived from Global Surface Water satellite data (40%). This reflects the importance of storage functions in semi-arid environments. Satellite data are processed through Google Earth Engine and QGIS across five temporal snapshots (2000, 2005, 2010, 2015 and 2020). Semi-structured interviews with water sector stakeholders, as well as consultation with sponge functions specialists, contextualise the quantitative findings within local governance frameworks. The analysis examines the temporal dynamics of all three sponge functions, revealing the spatial and temporal variability of Johannesburg's water retention capacity. The results demonstrate the influence of urban development patterns, climate variability and land cover changes on the city's hydrological responses over the twenty-year period. The integrated assessment identifies critical trends in vegetation interception capacity, flow regulation effectiveness and surface water storage evolution. The findings inform practical recommendations for improving urban water resilience through solutions that are adapted to semi-arid regions. This framework establishes a replicable methodology for assessing sponge functions in regions where water is scarce, using freely available satellite data.