The Eastern Lesotho Highlands (ELH) are often known as the water tower of southern Africa, as they provide water to South Africa, via the Lesotho Highlands Water Scheme, as well as to local subsistence farms. This project will use water isotopes to track moisture sources across Lesotho, and assess the implications for future water security within the wider region. The ELH receives moisture from the Indian Ocean in summer, as well as through the regional Tropical Temperate Trough and local convection. Atlantic-derived snowfall in winter come from mid-latitude cyclones. Despite a broad understanding of the different moisture sources, there is spatial heterogeneity in their importance across Lesotho, which is poorly understood. Oxygen- and hydrogen-isotope values in meteoric and surface waters are sensitive tracers of moisture source and will be used in this project to investigate hydroclimatic processes across the region. The student will collect surface water samples from Lesotho in two (summer and winter) sampling campaigns to assess spatial and seasonal variability in moisture sources and their implications for water resources. Isotope analyses will be undertaken in UCL. The student will also synthesise existing water isotope data from various disparate sources from across Southern Africa. The primary and secondary data will be further investigated using back trajectory modelling. The student will be fully trained in field techniques, instrumentation and data processing. The project will benefit from the close existing relationship between UCL and University of the Witwatersrand and preliminary fieldwork undertaken in Lesotho by members of the supervisory team.