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. 2023 Nov 8;13(1):19424.
doi: 10.1038/s41598-023-46319-1.

Detecting desertification in the ancient oases of southern Morocco

Louise Rayne  1 , Filippo Brandolini  2 , Jen Lavris Makovics  2 , Emily Hayes-Rich  3 , Jackson Levy #, Hope Irvine  2 , Lima Assi  2 , Youssef Bokbot  4
Affiliations

Detecting desertification in the ancient oases of southern Morocco

Louise Rayne et al. Sci Rep. .

Abstract

Understanding what led to desertification in the long-term is crucial for adaptation to climate change and pressures on resources in North Africa, but existing maps do not accurately show the extent of degraded land or the traditional water systems which underpinned cultivation. These products rely on recent vegetation trends and hindcasted statistical data. Desertification which occurred prior to the later twentieth century is poorly represented, if at all. However, large areas of abandoned fields are distinctive in satellite imagery as brightly reflectant and smooth surfaces. We present a new and open-source machine-learning workflow for detecting desertification using satellite data. We used Google Earth Engine and the random forest algorithm to classify five landcover categories including a class representing desertified fields. The input datasets comprised training polygons, a 12-band Sentinel-2 composite and derived tasselled cap components, and a Sentinel-1 VV-polarisation composite. We test our approach for a case study of Skoura oasis in southern Morocco with a resulting accuracy of 74-76% for the desertification class. We used image interpretation and archaeological survey to map the traditional irrigation systems which supply the oasis.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
(a) Long profile of a typical khettera. (b) Satellite image shows khettera shafts and outlet: KH9 1217-100011-F023c, 12th May 1982. (c) Satellite image of the same feature: Google Earth Pro map data: 2023 CNES/Airbus, Maxar Technologies https://www.google.com/intl/en_uk/earth/versions/#earth-pro. Image prepared using ArcGIS ArcMap 10.6.1 https://www.esri.com/en-us/home and GIMP 2.10.34 https://www.gimp.org/.
Figure 2
Figure 2
Skoura oasis and khettara irrigation network. Channels recorded as actively flowing or dry at the time of the field visit (Nov-Dec 2022) are labelled accordingly (Fig S1 shows visited khettara and data S1 is the shapefile). Others are labelled according to their appearance on satellite imagery. Areas of desertified soils are light coloured and bright in appearance. Map prepared using ArcGIS ArcMap 10.6.1 https://www.esri.com/en-us/home. Service layer source: ESRI, MAXAR, EARTHSTAR Geographics.
Figure 3
Figure 3
Google Earth Engine Desertification Detection workflow https://developers.google.com/earth-engine.
Figure 4
Figure 4
Example of landcover types represented in each input into the data stack. The VV band of Sentinel-1 is used. The Sentinel-2 image is a false colour composite (near-infrared, red, green) and the Tasselled Cap is a composite of the brightness, greenness and wetness components derived from Sentinel-2. The Sentinel data was accessed via Google Earth Engine https://developers.google.com/earth-engine : Copernicus Sentinel data 2021 and contains modified Copernicus Sentinel data 2021. High resolution images were obtained via Google Earth Pro map data: Google, ©2023 CNES/Airbus, Maxar Technologies https://www.google.com/intl/en_uk/earth/versions/#earth-pro. Images prepared using InkScape 1.2 https://inkscape.org/.
Figure 5
Figure 5
Land cover materials visited and photographed by Lavris Makovics and Rayne November–December 2022. (a) Desertified fields with a typical smooth and bright appearance. (b) Dense palm cultivation. (c) Desertified fields with some surviving palms. (d) Natural material. (e) Typical 3-part oasis with palms, fruit trees and crops. (f) Mudbrick and pisé kasbah. Image prepared using GIMP 2.10.34 https://www.gimp.org/.
Figure 6
Figure 6
Classification result with proportion of sample points correctly classified. The Sentinel data was accessed via Google Earth Engine https://developers.google.com/earth-engine: contains modified Copernicus Sentinel data 2021. Basemap is KH9 1217-100011-F023c, 12th May 1982. Classification generated using Google Earth Engine and map prepared using ArcGIS ArcMap 10.6.1 https://www.esri.com/en-us/home. Service layer source: ESRI, EARTHSTAR Geographics.
Figure 7
Figure 7
Khettara Groups and chronology of khettara construction in Skoura oasis based on analysis of historic KH9 satellite imagery. Note desertification around Groups 10 and 20. Basemap is KH9 1217-100011-F023c, 12th May 1982. Classification generated using Google Earth Engine and map prepared using ArcGIS ArcMap 10.6.1. https://www.esri.com/en-us/home. Service layer source: ESRI, EARTHSTAR Geographics.
Figure 8
Figure 8
(a) Line of khettara shafts on the ground. (b) Line of shaft revitalised with concrete. (c) Water flowing from outlet lined with concrete. (d) Imagery showing stages of revitalisation of khettera. Photographs taken by Makovics and Rayne November–December 2022. Google Earth Pro map data: 2016 CNES/Maxar Technologies, 2017 MAXAR Technologies, 2018 MAXAR Technologies https://www.google.com/intl/en_uk/earth/versions/#earth-pro. Image prepared using GIMP 2.10.34 https://www.gimp.org/.
See this image and copyright information in PMC

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