ASSESSING DRIVERS OF SALINIZATION IN THE LOWER LIMPOPO RIVER, MOZAMBIQUE

Sea level rise and reduced river discharge are well-documented drivers of freshwater salinization in estuarine rivers. Few of these studies assess the relative contribution of both of these variables concurrently, and even fewer consider the impact of coastal land subsidence on local rates of sea level rise. In the lower Limpopo River basin (LLRB), located in southern Mozambique, stakeholders have reported increasing salinization of the lower Limpopo River. Upstream movement of the Limpopo River’s saltwater-freshwater boundary poses a threat to the region’s extensive agricultural schemes, which rely on the river for irrigation. In response to these stakeholder concerns, this study utilized remotely sensed and in-situ data to quantify trends in land deformation, sea level and river height to determine the relative contribution of these variables to river salinization. Land deformation was measured during the dry season (May to October) of 2017 to 2021 using ESA’s Sentinel-1 interferometric synthetic aperture radar (InSAR) and extrapolated to mean annual rates. Mean annual eustatic sea level rise was calculated via daily tide gauge measurements collected at Durban, South Africa from 1971 to 2019. Mean annual height of the Limpopo River was calculated from a hydrometric gauge at Xai-Xai, Mozambique from 1996 to 2019. We found that land subsidence is occurring in the lower Limpopo River floodplain at an average rate of -2.98 cm/yr, based on the dry season acquisitions. River height decreased at a rate of -1.93 cm/yr and eustatic sea level rose at a rate of 0.11 cm/yr. Our findings indicate that relative sea level rise rates are greater than eustatic sea level rates and, therefore, land subsidence is likely contributing to reported increases in river salinization, along with reductions in river height.