Impacts of land system change− driven urban heat islands on surface water dynamics and moisture conditions in Gauteng Province, South Africa

Surface water is a scarce resource in Gauteng, South Africa. This study analysed the impacts of urban heat islands on surface water dynamics and moisture conditions from 1984 to 2024 using vegetation and water indices in the Google Earth Engine. The results indicate that Gauteng has experienced a slight increase in land surface temperature (LST), ranging from 62°C to 63.2°C, and low LST decreased from 24.8°C to 10.32°C. The mean high LST was approximately 63.04°C, whereas the low LST value was 11.31°C. A higher vegetation cover, ranging from 0.66 in 1984 to 0.86 in 2014, and a lower vegetation cover, ranging from -0.22 in 1984 to -0.62 in 2024, were observed. The long-term mean LST in areas with vegetation cover was 46.72°C, whereas in other land cover types, such as built-up areas, the mean LST was 48.28°C. The Spearman correlation matrix exhibited a negative relationship between the Modified Normalised Difference Water Index and LST ( ρ = –0.40). Similarly, NDVI and Water Ratio Index showed a negative correlation ( ρ = –0.69). Additionally, we found a negative correlation between LST and NDVI ( ρ = − 0.29). Moreover, heatwaves in Gauteng have increased gradually, with an average frequency of approximately one from 1984 to 2009; however, from 2014 to 2024, the average frequency increased to four. Although the average UHII for Gauteng in 1984 was –2.18 °C, it increased to 0.25°C by 2024. Water scarcity was evidenced by a decreasing trend in the water ratio index (WRI), whereas no trend was observed in the modified normalised difference water index. This research is significant for formulating research-based mitigation measures, including the expansion of green infrastructure and the protection of surface water resources.