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 (KAZA) transfrontier conservation area of southern Africa (300,000 km2). 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 spatiotemporal patterns were was associated with soil type; loam soils were most likely to hold water for longer periods in the study area. From the wettest time period to the driest, there was a ~44,000 km2 (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.
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