Monitoring Recent Fluctuations of the Southern Pool of Lake Chad Using Multiple Remote Sensing Data: Implications for Water Balance Analysis

Wenbin Zhu,Jiabao Yan,Shaofeng Jia

doi:10.3390/rs9101032

Abstract

The drought episodes in the second half of the 20th century have profoundly modified the state of Lake Chad and investigation of its variations is necessary under the new circumstances. Multiple remote sensing observations were used in this paper to study its variation in the recent 25 years. Unlike previous studies, only the southern pool of Lake Chad (SPLC) was selected as our study area, because it is the only permanent open water area after the serious lake recession in 1973–1975. Four satellite altimetry products were used for water level retrieval and 904 Landsat TM/ETM+ images were used for lake surface area extraction. Based on the water level (L) and surface area (A) retrieved (with coinciding dates), linear regression method was used to retrieve the SPLC’s L-A curve, which was then integrated to estimate water volume variations ( Δ V ). The results show that the SPLC has been in a relatively stable phase, with a slight increasing trend from 1992 to 2016. On annual average scale, the increase rate of water level, surface area and water volume is 0.5 cm year−1, 0.14 km2 year−1 and 0.007 km3 year−1, respectively. As for the intra-annual variations of the SPLC, the seasonal variation amplitude of water level, lake area and water volume is 1.38 m, 38.08 km2 and 2.00 km3, respectively. The scatterplots between precipitation and Δ V indicate that there is a time lag of about one to two months in the response of water volume variations to precipitation, which makes it possible for us to predict Δ V . The water balance of the SPLC is significantly different from that of the entire Lake Chad. While evaporation accounts for 96% of the lake’s total water losses, only 16% of the SPLC’s losses are consumed by evaporation, with the other 84% offset by outflow.

Highlights

Lake Chad, one of Africa’s largest freshwater lakes, lies in an endoreic basin on the southern margin of the Sahara Desert (Figure 1)
Water surface area of the southern pool of Lake Chad (SPLC) was extracted from 904 Landsat TM/ETM+ images
Because of the lack of in situ observations, the accuracy of the altimetry products was evaluated by correlation analysis, and only water levels correlating significantly with the extracted lake surface area were adopted

Summary

Introduction

Lake Chad, one of Africa’s largest freshwater lakes, lies in an endoreic basin on the southern margin of the Sahara Desert (Figure 1). Like most lakes located in a hydrologically closed drainage system, the fluctuation of Lake Chad is directly related to river inflow, which varies according to the annual rainfall over the basin [1,2]. The climate of the Lake Chad basin can be classified as tropical hyper-arid, with four distinct climate zones of different rainfall levels. Moving from the north to the south of the basin, the climatic zones are the Saharan climate, the Sahelo-Saharan climate, the Sahelo-Sudanian climate and the Sudano-Guinean climate, respectively. Approximately 90% of Lake Chad’s water comes from the Chari/Logone River system in the south of the basin, with the remaining 10% coming from local precipitation and from the El Beid and Komadougou Yobe Rivers [4,5,6,7]

Objectives

Methods

Results

Discussion

Conclusion