Abstract
In this study, an algorithm to transform LANDSAT 8 images of Lake Guiers into chlorophyll-a concentration maps was developed. The objective was to see the feasibility of this method as a mapping tool for monitoring the trophic state of the Lake. The available data are 15 in-situ measurements of the CHL-a content of Lake Guiers’ water using a Micro FLU device and images of the lake acquired in 2013 and 2016 by the LANSAT 8 OLI sensor. A series of correlation tests (simple linear functions, multi-variables, or containing two- order terms) were performed between the spectral sensor signal of June 19, 2016 LANSAT 8 image (“corrected” and converted into reflectance) and the CHL-a content of Lake waters measured on the same day of the satellite record. This was, to find the best regression equation that can be inferred to develop an algorithm to transform LANDSAT 8 image into CHL-a concentration maps. The best equation obtained by correlating the ground truths and the reflectance ratios of the visible and near infrared bands (B2, B3, B4 and B5), is as follows: where B2 = Blue band, B3 = Green band, B4 = Red band, B5 = NIR band. The obtained R2 is 0.703 (e.g., a correlation coefficientof 0.84), revealed that the measured concentrations are well reproduced by the regression model. Applying this model to the 2013 and 2016 image data revealed the monthly variations of the lake CHL-a contents and their seasonal evolution and interannual variations. The obtained results are quite consistent with those from similar studies and provide valuable information regarding Lake Guiers’ vulnerability to algal development. The calculation and analysis of the variability of the CHL-a contents over the study period made it possible to evaluate the trophic state of the Lake, which shows a general tendency for eutrophication process, leading to a progressive degradation of the quality of the water resource.
Highlights
The collective effects of climate change and environmental impacts associated with the development activities in the Lake Guiers’ basin have impacted adversely on the quality of this important resource for drink water supply in Senegal
Remote sensing has the advantage of providing regular data coverage over a large scale area, and many studies have demonstrated its ability to monitor surface water quality parameters like chlorophyll-a or turbidity, suspended sediment loads and coloured dissolved organic matter ([6]-[11])
The purpose of this study is to demonstrate the usability of LANDSAT-8 images for monitoring the CHL-a content of Lake Guiers waters
Summary
The collective effects of climate change and environmental impacts associated with the development activities in the Lake Guiers’ basin (i.e. intensive agricultural and industrial activities, changes in land-use practices, waste-disposal practices of domestic and industrial waters) have impacted adversely on the quality of this important resource for drink water supply in Senegal. The problems that have emerged are essentially a continuous quality deterioration of the environment of the lake, with the risk to failing to meet its beneficial uses ([1] [2]) To deal with this situation, the state authorities proposed the establishment of Lake Guiers Office (Office du Lac de Guiers) in 2009; a state agency whose mission is the protection and management of this natural resource. Under these conditions, monitoring of the lake water quality has become a major challenge. Remote sensing has the advantage of providing regular data coverage over a large scale area, and many studies have demonstrated its ability to monitor surface water quality parameters like chlorophyll-a or turbidity, suspended sediment loads and coloured dissolved organic matter ([6]-[11])
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