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| Created: | Aug 06, 2025 at 4:43 p.m. (UTC) | |
| Last updated: | Feb 25, 2026 at 8:43 p.m. (UTC) | |
| Citation: | See how to cite this resource | |
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Abstract
Workflow for Swift et al 2026; all code available on Google Earth Engine. This code uses Otsu thresholding on median AWEI values, calculated from Sentinel-2 MSI imagery, to generate a binary "isWater" raster where 1 = water and 0 = not water.
Paper abstract
Reliable freshwater access drives terrestrial wildlife movements and habitat use globally. However, the small, rain-fed seasonal pools critical for dryland wildlife persistence are vulnerable to the rising temperatures and unstable precipitation regimes under climate change. In southern Africa, drying surface water may impact area-sensitive and water-dependent mammals by inhibiting seasonal migrations and increasing resource competition and human-wildlife conflict.
We present a framework and empirical analysis of fine-scale surface water mapping in the 520,000km2 Kavango Zambezi Transfrontier Conservation Area (KAZA), the world's largest terrestrial conservation area. From 2019-2025, we implemented Otsu thresholding on median Automated Water Extraction Index imagery from 10m Sentinel-2 MSI, leveraging high wet season contrast between vegetation and water as a dry season positive mask. We created >35 quasi-monthly KAZA-wide Ephemeral Surface Water (ESW) rasters (mean classification accuracy 87%), and found wet-season precipitation drivers of fill levels did not extend into the dry season. Then, using GPS data from 27 African savanna elephants (Loxodonta africana), which typically visit water every 48 hours, we compared elephant water visitation rates based on ESW to existing 30m Global Surface Water (GSW) maps. Models using ESW estimated 99% of elephant data came within a 48-hour window, compared to 42% for GSW, suggesting that ESW is a better proxy for actual water use.
Subject Keywords
Coverage
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Content
Related Resources
| Title | Owners | Sharing Status | My Permission |
|---|---|---|---|
| KAZA Surface Water Maps | Maggie Swift | Public & Shareable | Open Access |
Credits
Funding Agencies
This resource was created using funding from the following sources:
| Agency Name | Award Title | Award Number |
|---|---|---|
| WWF-US | None | None |
Contributors
People or Organizations that contributed technically, materially, financially, or provided general support for the creation of the resource's content but are not considered authors.
| Name | Organization | Address | Phone | Author Identifiers |
|---|---|---|---|---|
| Graham McCulloch | Ecoexist | |||
| Robin Naidoo | WWF-US | |||
| Piet Beytell | Namibia MEFT | |||
| Anna Songhurst | Ecoexist |
How to Cite
This resource is shared under the Creative Commons Attribution CC BY.
http://creativecommons.org/licenses/by/4.0/
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