The impact of land use land cover (LULC) change and climate variability on infiltration/runoff evolution in the Dar Es Salaam coastal floodplain

Both groundwater and surface water are essential sources of freshwater worldwide. In urban and peri-urban areas of Sub-Saharan Africa, such as Dar es Salaam city (Tanzania), the scarcity of good quality surface water has led to a groundwater overexploitation in the coastal aquifer. This is due to the growing water demand driven by demographic and economic development of the city. The rapid and uncontrolled urban expansion, together with the increase of impermeable areas, have negative impacts on the aquifer recharge, water table decrease and runoff increase, causing higher risk of flooding, erosion and water shortage. This paper presents the separate assessments of land use land cover change (LULCC) and climate variability impacts on water budget in the study area, using remote sensing datasets, which have followed the evolution of Dar Es Salaam City during the period 2002–2022. The assessments show how rapid urbanization has increased runoff and reduced aquifer recharge, leading to flooding risks and groundwater degradation. In 20 years, within the hydrogeological basin, the aquifer lost 30% of the recharge water volume, on average. Climate variability has affected the specific annual recharge and runoff volumes, especially in the last decade, due to an important rainfall increase. This paper demonstrates that combining the analysis of LULCC and climate variability in catchment areas with geomorphological ones and hydrogeological water budget method can provide a thorough understanding of uncontrolled urban spread effects on water resources balance and support evidence based decision-making spatial planning and environmental management. The study is part of the WECOAdapt project (Water management through ECOhydrology for climate change ADAPTation) that focuses on the reduction of negative impacts on floods and droughts due to unsustainable urban development, aiming to reverse/reduce the degradation of water and land resources.