Mapping spatiotemporal variability of wet season surface water processes in the Cuvelai-Etosha Basin, Namibia using stable isotopes.

The natural tracers δ¹⁸O and δ²H are essential for tracing hydrological processes by identifying water sources, tracking evaporation loss and floodwater dynamics to enhance water management and flood mitigation strategies. This study employed this approach in the ephemeral, endorheic Cuvelai-Etosha Basin (CEB), spanning northern Namibia and southern Angola, to determine its viability in capturing spatial and temporal hydrological patterns, their timing and interactions during a medium flood condition (2017), and contrasted with a drought year (2014). During the 2017 wet season 219 grab surface water samples were collected from ephemeral waterbodies in four sampling campaigns (February, March, April and May) in addition to a single campaign in May 2014 (63 samples). Samples were analysed for stable isotopes (δ²H and δ¹⁸O) at the University of Namibia laboratory using an off-axis integrated cavity output spectroscope (Los Gatos, DLT-100). Results for 2017 revealed a compositional range of -13.51 to 12.44 ‰ for δ¹⁸O and from -100.1 to 50.9 ‰ for δ²H. The 2017 samples plot along a low sloping line (δ²H = 5.19 δ¹⁸O - 13.91) indicating the dominance of the evaporation effect. Amount, seasonality, and latitude effects were also identified in the isotopic composition of the iishana water. The surface water loss to evaporation is in the range of 24-42 % from March to April and 39-69 % from March to May. The d-excess decreased from April and remains relatively low in May, which supports observations of evaporation losses. The overall large water losses from the system via evaporation reduces the water availability substantially, and the impact is more pronounced in the western part of the basin than in the eastern. Since evaporative loss begins early, even during the rainy season, proactive technical solutions such as floodwater harvesting need to be planned accordingly to mitigate losses and optimizing water use.