Building the case for a flood forecasting and early warning system in Senegal using satellite climate and environmental data
Senegal, a country of 15.2 million inhabitants on the Atlantic coast, is vulnerable to four natural hazards: drought, locust invasion, flooding, and sea level rise. Although there is significant variation in rainfall within the country—Northern Senegal is arid and drought prone, while the wooded south receives abundant rainfall—flooding occurs in all 14 regions of the country. The most affected are communities in urban areas of Dakar, Saint Louis, and Kaolack. For example, in September 2020, heavy rain resulted in flooded streets, neighborhoods, bridges and the displacement of more than 3,000 people in the suburbs of Dakar and the department of Thiès. The floods also isolated some localities and due to limited accessibility, some communities could not be reached by the emergency services.
In some areas, flooding occurs after rivers overflow (especially Gambia and Senegal rivers) due to heavy rains, while in others, particularly in peri-urban areas of Dakar, floods are the direct result of heavy rainfall combined with lack of appropriate drainage infrastructure. Traditionally, flood risk management in Senegal has focused on infrastructure, such as the construction of dams and flood walls or post-event reconstruction and compensation activities. However, a broader approach of planning and establishing regulations and early warning systems can significantly reduce flood losses. Evidence-based, multidisciplinary planning, particularly targeting vulnerable populations, is crucial to save lives and property from climate disasters. According to Gora Mbengue, the head of the Monitoring and Evaluation Division at the Department of Planning and Environmental Watch (DPVE) Senegal, there is “…a problem of land use planning and sanitation. It would be necessary to agree on the fact that many people have gone to live in areas without proper infrastructure. There should be a platform containing data relevant to inform the authorities about the risks of flooding.”
Between October and December 2020, experts from Togo, Guinea, and Senegal met via Zoom for capacity-strengthening activities organized by the Global Partnership for Sustainable Development Data ( Global Partnership) and led by the United Nations Environment Program and the Danish Hydraulic Institute (UNEP-DHI) on the UNEP-DHI Flood and Drought Portal, an Earth observation- (EO-)based tool. The Flood and Drought Portal is a series of open-source technical applications that help stakeholders to perform baseline assessments using readily available satellite data, impact assessments through data analysis, planning options, and dissemination of environmental data to affected groups or individuals, allowing authorities to be better prepared to act early, anticipate the risks of extreme weather events, and empower citizens to avoid flood prone areas.
Senegal joined the training to increase its capacity to use satellite imagery to produce reliable data for environmental protection and climate change adaptation policies. The participants came from the Directorate of Water Resources Management and Planning, the National Agency for Statistics and Demography, the Ecological Monitoring Center, the Agricultural and Rural Prospective Initiative (IPAR), and the Department of Planning and Environmental Watch (DPVE) of the Ministry of the Environment and Sustainable Development of Senegal. Following the three-session training, participants identified a use case for their newly acquired skills in their respective countries. The Senegal team, including Mbengue, decided to tackle the issue of flood management.
Cheikh Faye, statistical engineer at IPAR, noted that in Senegal, “local actors and humanitarian organizations often have to work with traditional meteorological sensors (ground instruments capable of detecting temperature, relative humidity, atmospheric, wind direction, speed and radiation, for example) and climate data that is dated, poor quality, or nonexistent, due to the limited ability to access and work with EO data in near-real time.” In addition to data, indicators are needed to allow the combination or interpretation of data as a basis for making sound decisions or policies and to help compile complex values into a simple interpretation of a particular situation, for example, conditions and types and severity of drought or flooding. These indices are linked and a combined analysis of the relevant scenarios measured enables one to effectively define the flood risks for a given area.
The Senegal team assessed a range of flood indicators available on the portal, including the Standard Precipitation Index, the Normalized Antecedent Precipitation Index, Soil Health Index, Flood Variability Index, and Flood Risk Index. The team worked on these indicators and evaluated the Flood and Drought Portal’s algorithms with reference to the country's geospatial data and different eco-geographic zones. The objective of this assessment was to validate the availability and accessibility of near-real-time EO flooding data with ground-level data, with the hope of justifying the use of EO data in decision-making processes for predicting and addressing environmental risks.
Through the Flood and Drought Portal, the Senegal team discovered that several indicators can be used to improve existing knowledge on flood-prone areas and effectively identify areas at risk of flooding to inform early warning systems, as well as urban planning. From the team’s point of view, the potential of the information drawn from the Portal presents a case for the development of a spatial mapping database and a decision support tool for all stakeholders in Senegal to access the data, visualizations, and reports on flood risks in the country.
For example, viewing the Soil Moisture Index in Senegal through the Portal shows that soil moisture decreased from south to north and from west to east across the country, reflecting the national records of floods from 2007 to 2020, where the north has a longer dry season and little to no flooding with annual rainfall of up to 380mm, while flood incidences increase as they move south and east, where the rainfall can reach up to 1,500 mm per year. Compared to ground measurements, the remote sensing technique as demonstrated by the Flood and Drought Portal can capture soil moisture information across the country in a single snapshot, which takes less time and labor. The data, which also complemented traditional rainfall data over the same period, can help predict flooding based on predicted rainfall relative to soil moisture levels.
The Senegal team observed some limits with the portal due to a lack of data disaggregated at regional and departmental levels. With a global tool, it wasn’t feasible to generate small-area statistics for all administrative areas across the country in the Flood and Drought Portal, especially since Senegal wasn't a country that the UNEP-DHI team had initially worked with in developing the tool and delivering training activities. The data and indicators in the Portal are available at resolutions based on satellite sensors (e.g., the soil water index is at a 0.1-degree spatial resolution). However, the Portal’s map features offer the possibility to disaggregate all indicators at the regional level that can then guide supplementary analysis at sub-regional levels.
According to Mbengue, the training enabled the team to acquire new knowledge on climate indicators. The open-source nature of the Portal, which allows data exports to different platforms with lower-level metrics for further analysis, is also a very useful feature, further supporting the need for the country to invest in such a platform that can inform authorities as well as the public about flood risks, as this does not yet exist in the country. “We have learned to explore and use data on deforestation, drought, and flooding in Senegal, which is not often collected at the national level. New indicators were discovered in the platform with concepts and methodology thanks to the existing documentation,” he said. "This has allowed us to better understand the concepts and methods that are used to measure or appreciate the extent of deforestation, flooding, or drought and to strengthen analyses on these indicators. In addition, we were able to obtain a new source of data on these various national issues.”
Mbengue added that the team had acquired a baseline to easily assess the effectiveness of environmental projects that will be implemented in the future. This saved the team time and resources to collect this data, which is easily accessible on the portal. “The idea was to collect data to improve the foundations of knowledge,” said Mbengue. “It turned out that we could not focus on the small administrative areas of the city, but the aim was to improve the knowledge base, and we have now acquired the ability to analyze new indicators of flooding in flood-prone areas in order to anticipate and warn about the risks of flooding.”
The DVPE AND IPAR plan to present their findings and recommendations to the Civil Aviation and Meteorology Agency, the Ministry of Water and Sanitation, and the Ministry of Regional Planning, as well as other technical and financial partners, in a bid to build partnerships for the development of the flood observatory system, which can be based on the data provided by the Flood and Drought Portal in conjunction with more granular field data and provide relevant analysis based on a range of indicators and indices.
Once the information obtained has been well refined and analyzed, the Senegal team will be able to make evidence-based recommendations to the authorities for the implementation of the country’s Ten-Year Flood Management Program (2012-2022). On the training, Faye observed: “The training allowed us to acquire new knowledge. Even though some platforms could not inform us of some more granular indicators that are relevant for the country, this was useful due to the rapid availability of this information which can easily be transferred and used in other software.”
The Global Partnership is organizing a knowledge sharing event to support the dissemination of lessons and recommendations from this training through facilitation of the three countries involved in the training and peer countries and stakeholders by the end of December 2021
This project was managed by Francois Kamano, Francophone Africa Program Manager, and the case study was written by Muthoni Mugo, MEL Program Officer.