An Assessment of Small Wetland Ecohydrological Dynamics Using Sentinel-2 MSI Derived Spectral Indices in Semi-Arid Environments of South Africa

Abstract

Water presence within small wetlands serve as a determining factor influencing biodiversity productivity and wetland functionality. Small wetlands remain largely unprotected hence, they are more sensitive to frequent exposure to environmental modifications, and are less resilient to changing rainfall patterns, climate change and variability, droughts and changing land use practices. Accurate and up to date spatial and temporal information on changes in surface water and inundation extent becomes imperative for the proper management of these wetlands. Therefore, this study sought to extract and monitor wetland ecohydrological dynamics (surface water and inundation extent) using monthly Sentinel-2 MSI remotely sensed datasets. These dynamics were assessed for the period between July 2020 and June 2021 using the modified normalised difference water index (MNDWI), normalised difference moisture index (NDMI) and normalised difference phenology index (NDPI) derived from Sentinel-2 MSI data. The results showed that the rainy season (Dec 2020 – Feb 2021) had a larger-water coverage extent (10948 m2 (0.05%) to 31594 m2 (0.13%)) when compared to the dry season (July 2020: 19157 m2 (0.04%) and June 2021:14429 m2 (0.03%)). The extent of surface area declined during the dry period due to less rainfall (0.20 mm) and decreased actual evapotranspiration (9.90 mm – 10.43 mm). Further, the NDPI showed a high concentration of wetland vegetation between the months of October 2020 and April 2021. In contrary, higher moisture content was observed between December 2020 to April 2021. The increase in vegetation concentration and moisture content reflects the spatial extent of inundation extent. Wetland water extent, soil moisture, and vegetation condition were assessed with high overall accuracies, ranging between 70.83% and 97.36%. Overall, the results indicate that small wetlands are characterised with significant variations in levels of inundation and productivity throughout the year.