Multi-year Satellite Observations Reveal Permanent Seasonal and Ephemeral Surface-water Regions in Arid and Semi-arid Areas Within the East African Region

Surface water in semi-arid and arid regions has been adversely affected by climate change, compounded by their inherent environmental conditions. One such area is East Africa, which consists of small reservoirs, pans, and non-perennial rivers that provide critical water resources for people, livestock, and ecosystems [1]. However, these water resources are poorly monitored and highly variable in space and time. Recent global water products overlook small and ephemeral water bodies due to the spatial resolution of the satellite data used and the target scale [2]. This monitoring gap is particularly consequential because pastoral adaptation in the region is tightly shaped by the timing and distribution of water; socio-ecological studies emphasise that tracking environmental variability is essential for understanding how mobility patterns respond to climate change [3]. Remote sensing techniques offer a way to address this gap by identifying changes in water systems, including the emergence and disappearance of temporary water bodies. The objective of this study is to develop a framework that facilitates understanding of the spatial and temporal patterns of these water bodies, providing a foundation for subsequent analyses of pastoral nomadic movements and resource-use dynamics. Here, we present a multi-year monitoring framework that integrates Sentinel-1 SAR and Sentinel-2 optical imagery to map surface-water dynamics.  We develop Sentinel-1/2 stacks for a monthly compositing window, derive spectral water indices, perform spectral analysis, and include topographic variables. We use a basic classifier trained on multi-year reference data and produce a 10m water mask. Further, time-series metrics for water occurrence, duration, the number of wet spells, and transition frequency are produced. The preliminary results indicate a trend in changes to the temporal surface water and the extent of permanent water bodies. The results reveal strong contrasts between permanent lakes and reservoirs, seasonal floodplains, and highly ephemeral channels and pans. A good understanding of the pattern and location of such water bodies contributes to informed support for livelihoods in the region and to sustainable water resource management.  

 

Keywords: Arid And Semi-arid; Water Resources; Remote Sensing; Pastoral Mobility; Sar; Optical Imagery; East Africa, Sentinel-1/2

 

Acknowledgements: The Authors would like thank the following trusts, charities, organisations and individuals for their generosity in supporting this project: The Lord Faringdon Charitable Trust, The Schroder Foundation, The Cazenove Charitable Trust, The Ernest Cook Trust, Sir Henry Keswick, Ian Bond, P. F. Charitable Trust, Prospect Investment Management Limited, The Adrian Swire Charitable Trust, John Swire Charitable Trust, The Samuel Storey Family Charitable Trust, The Tanlaw Foundation, and The Wyfold Charitable Trust.

 

References

[1] Sigopi M, Shoko C, Dube T. Advancements in remote sensing technologies for accurate monitoring and management of surface water resources in Africa: an overview, limitations, and future directions. Geocarto Int 2024;39:2347935.

[2] Miura Y, Shamsudduha M, Suppasri A, Sano D. A Global Multi-Sensor Dataset of Surface Water Indices from Landsat-8 and Sentinel-2 Satellite Measurements. Sci Data 2025;12:1253.

[3] Cho, M.A., Mutanga, O. and Mabhaudhi, T. Adaptation to climate change in pastoral communities: a systematic review through a social-ecological lens. International Journal of Climate Change Strategies and Management, 2025 17(1), pp.246-267.