Remote Sensing to Power Senegal’s Water Use

High-resolution satellite imagery helps determine the spatial extent of aquatic vegetation and chlorophyll concentrations in Lac de Guiers. Photo credit: Richard Rapier

Lac de Guiers is a complex ecosystem south of the vast rice paddies along the Senegal River Delta in the Louga and Saint-Louis regions of Senegal.  Not only does the lake and its surrounding land comprise an important bird area, fisheries, and a biosphere reserve, but it also produces rice, sweet potatoes, and sugar. Additionally, it supplies 70 percent of the water consumed in Dakar and its suburb where nearly 4 million people live, meaning upstream management of Dakar’s critical drinking water supplies is paramount. It would be difficult to overstate the importance of managing this water ecosystem in achieving Sustainable Development Goal 6 (SDG 6) and multiple interdependent SDGs in Senegal.  This is why the government sought the assistance of USAID’s Water for Africa through Leadership and Institutional Support, or WALIS. 

WALIS is working with Senegal’s national Directorate of Water Resources Management and Planning to support the government’s management of the freshwater ecosystem and measure the change of the Lac de Guiers over time. The activity is designed to take the first substantive steps to systematically monitor the spatial extent of aquatic vegetation and its seasonal growth and to measure efforts to reduce its spread. This effort will also integrate SDG Indicator 6.6.1 monitoring methodologies into current policy, procedures, guidance, and practices. WALIS’s support to the Government of Senegal will better equip and train its water managers and technicians to improve water resources management decisions. 

The use of state-of-the-art methods to measure seasonal changes in the aquatic vegetation of the Lac de Guiers and adjacent wetland areas by implementing partner Centre de Suivi Ecologique is key to WALIS’s support. Under this model, different types of medium and high-resolution satellite imagery are cycled through an algorithm to determine the spatial extent of aquatic vegetation and chlorophyll concentrations in the area studies. High resolution imagery and the algorithm are then used to pinpoint aquatic vegetation, most notably cattails and reeds. 

The proliferation of emergent vegetation like cattails in the Lac de Guiers and surrounding wetlands has limited water access for the surrounding communities, reduced the potential for agriculture, and may, according to some research, increase occurrences of diseases like bilharzia, also known as schistosomiasis. This increase is attributed to people collecting the vegetation for building material or wading in the water to fish. Sentinel-1 (cloud-penetrating radar) and Sentinel-2 (13 spectral band optical) satellite imagery of the European Copernicus program will also be used to estimate the extent of aquatic vegetation, especially during the rainy season when cloud cover increases. This 10–20 meter resolution imagery also captures change over time. The Sentinel-1 revisits every place in the world on a 12-day cycle, while the Sentinel-2 revisits all land surfaces and coastal areas every five days. This imagery will be used even beyond the Lac de Guiers area in eight principle water-related ecosystems affected by emergent vegetation in Senegal’s planning regions. These ecosystems cover an area over 1,000 square miles. 

The processing and analysis of the satellite imagery in combination with the other work supported by WALIS, including ambient water quality field monitoring, will lead to the formulation of a policy and strategy to better manage surface water in the areas studied. These results will also inform a “polluters-pay principle” policy and strategy to help counteract increasing pollution from anthropogenic sources.  These improvements to Senegal’s water-related ecosystem monitoring over time are a clear example of how demand-driven, country-led initiatives can lead to success. 

By Richard Rapier, WALIS Chief of Party, and Theophane Boutrolle, WALIS WASH Specialist