ABSTRACT:

Code for case study from Swift et al 2026. DOES NOT include elephant data.

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.

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.

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.

ABSTRACT:

Validation data created using Planet (~3.4m) and Google Earth data, for each of seven validation regions in the Kavango-Zambezi Transfrontier Conservation Area. Validation data are given for one very dry season (July of 2019) and one very wet season (March of 2021).

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.

ABSTRACT:

Rasters of maximum water fill (MWF) for all hydrosheds and SIZ. MOSAIC file is one large mosaicked raster, while the MWF_HYDROSHEDS zip file contains individual MWF for each hydroshed, that was mosaicked with the maximum extent of all SIZ to create MWF_MOSAIC. Pixel values are 0 (never water) and 1 (at least once 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.

ABSTRACT:

Ephemeral surface water recurrence (ESWr) data files for the dry season (May - October), the wet season (November - April), and all times (2019-2025). Recurrence is given pixel-wise as a proportion of all time periods in which that pixel was filled (0.0 to 1.0).

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.