Tracking a blue wave of ephemeral water across arid southern Africa

Surface water in arid regions is essential to many organisms including large mammals of conservation concern. For many regions little is known about the extent, ecology and hydrology of ephemeral waters, because they are challenging to map given their ephemeral nature and small sizes. Our goal was to advance surface water knowledge by mapping and monitoring ephemeral water from the wet to dry seasons across the Kavango-Zambezi Transfrontier Conservation Area of southern Africa (300 000 km(2)). We mapped individual waterholes for six time points each year from mid-2017 to mid-2020, and described their presence, extent, duration, variability, and recurrence. We further analyzed a wide range of physical and landscape aspects of waterhole locations, including soils, geology, and topography, to climate and soil moisture. We identified 2.1 million previously unmapped ephemeral waterholes (85%-89% accuracy) that seasonally extend across 23.5% of the study area. We confirmed a distinct 'blue wave' with ephemeral water across the region peaking at the end of the rainy season. We observed a wide range of waterhole types and sizes, with large variances in seasonal and interannual hydrology. We found that ephemeral surface water was associated with loam soils in the study area. From the wettest time period to the driest, there was a similar to 44 000 km(2) (62%) decrease in ephemeral water extent across the region-these dramatic seasonal fluctuations have implications for wildlife movement. A warmer and drier climate, expected human population growth, and associated agricultural expansion and development may threaten these sensitive and highly variable water resources and the wildlife that depend on them.