Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The presence of ephemeral ponds and perennial lakes in the Sudano-Sahelian region of West Africa is strongly variable in space and time. Yet, they have important ecological functions and societies are reliant on their surface waters for their lives and livelihoods. It is essential to monitor and understand the dynamics of these lakes to assess past, present, and future water resource changes. In this paper, we present an innovative approach to unravel the sediment and water balance of Lac Wégnia, a small ungauged lake in Mali near the capital of Bamako. The approach uses optical remote sensing data to identify the shoreline positions over a period of 22 years (2000–2021) and then attributes water surface heights (WSHs) to each observation using the lake bathymetry. We then present a novel methodology to identify and quantitatively analyze deposition and erosion patterns at lakeshores and in lake beds. The method therefore represents a significant advancement over previous attempts to remotely monitor lakes in the West African drylands, since it considers not only changes in water depth to explain recent declining trends in lake areas, but also changes in the storage capacity. At Lac Wégnia, we recognize silting at the tributaries to the lake, but overall, erosion processes are dominant and threaten the persistence of the lake because of progressive erosion through the natural levee at the lake outlet. This factor contributes <span class="inline-formula">66 <i>%</i>±18 <i>%</i></span> to the decreasing WSH trend, while <span class="inline-formula">34 <i>%</i>±18</span> % of the dry-season lake level changes are explained by increasing evaporation from the lake and by possibly falling groundwater tables. Due to the decreasing reservoir capacity of the lake, WSHs are declining even in the wet season in spite of positive rainfall patterns.

Highlights

  • In arid and semi-arid areas of West Africa, small reservoirs and natural lakes improve the reliability of water supplies for 15 livestock and humans and allow the diversification of agricultural activities at the locale scale (Fowe et al, 2015)

  • Precipitation and relative humidity reveal a strong seasonality with a peak around August

  • This work has demonstrated the utility of the waterline method for extracting the water levels of Lac Wégnia, a Malian lake at the boundary between the Sahelian and the Sudanian eco-climate

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Summary

Introduction

In arid and semi-arid areas of West Africa, small reservoirs and natural lakes improve the reliability of water supplies for 15 livestock and humans and allow the diversification of agricultural activities at the locale scale (Fowe et al, 2015). Water resources in Sub-Saharan West Africa are under increasing pressure due to climatic changes, population growth and land degradation (Leblanc et al, 2008; Favreau et al, 2009; Oyebande and Odunuga, 2013). A phenomenon referred to as the "the 20 Sahelian paradox" led to an increase in surface water despite a general precipitation decline during the last decades of the 20th century. The phenomenon has been first reported for small watersheds in Burkina Faso by Albergel (1987), and later for several other watersheds in the West African Sahel (Descroix et al, 2009; Amogu et al, 2010).

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