An assessment of lake extent Changes Using four sets of satellite imagery from the Terra Look database: a case study of Lake Chad, Africa
Lake Chad is located in Central Africa. This lake is shared by four countries namely Cameroon, Nigeria, Chad and Niger. The total population of the five urban areas that make up the Lake Chad drainage basin in 1991 was about 22 million inhabitants, with an average density of 22 persons/ km2. The population of the drainage basin is growing rapidly and was estimated at about 37 million people in 2004 with a high average growth rate of 2.4-2.6%. The water resources provided by Lake Chad are important to the people in the region. Agriculture has always been the backbone of the regional economy and continues to engage about 60% of the basin’s population. However, annual average rainfall over the entire basin is rather low at 320 mm and varying between 1, 500 mm in the southern parts of the region to less than 100 mm in the northern parts of Chad. Thus, with a small quantity of water coming into the lake and a projected increased future demand, the future of Lake Chad is questionable.
The major aim of this study was to test if it is possible to estimate former changes in the spatial extent of Lake Chad using four sets of satellite imagery from the Terralook database (USGS, 2007). Satellite data (images) covering Lake Chad for four time periods÷ (Landsat MSS (1975), Landsat TM (1990), Landsat ETM+ (2000) and ASTER (2007)) were used as data source. These images were classified using GIS and remote sensing techniques to create land cover maps which were used to estimate lake extent changes of Lake Chad. This study is unique as it tests the ability of a newly available, user friendly and publicly available dataset (Terralook) to be used in conjunction with remote sensing techniques. The results showed that Lake Chad has shrunk over the past 35 years. In the interval 1975- 1990, lake area increased by 15 %. The lake area declined by 9% in the interval 1990-2000. During the period 2000-2007, the lake area declined again by about 11%. Still, the Terralook database appears not to be an adequate dataset for assessing lake extent changes over a long period under the methods used in this current study. Terralook suffers because of limitations with time of data collection and the resampling method used for data creation. As such, it is difficult for Terralook data to be used ‘off the shelf’ by those with limited or no experience in remote sensing practice.
The findings of this current study contribute relevant data to future research developing a hydrologic model for the Lake Chad basin. The results of this study could also provide a useful tool to local takeholders (e.g., Lake Chad Basin Commission, policy makers, national and international NGOs, World Bank and the African development bank) for decision making in the field of water management and agricultural policies, irrigation water distribution and drought management.
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