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First-ever environmental flow assessment in Benin successfully completed

11 March 2020

How much water from the Ouémé river in Benin should be reserved to maintain important ecosystems and biodiversity in the Ouémé Delta? Benin’s first-ever environmental flow assessment, carried out by local experts with support from IUCN NL, provides the answer. The experts recently presented their findings to decision-makers, in order to allow them to plan for water allocation and aquatic infrastructure in the river basin without jeopardizing the integrity of important ecosystems.  

In the Ouémé River Basin, there are plans for dams and large water abstractions, which will alter the hydrological regime of the Ouémé River. If not planned properly, these developments will impact important ecosystem services such as fisheries, fertile soils, flood retention and water quality downstream in the Ouémé Delta.

By determining the environmental flow requirement in the Ouémé Delta, it is possible to reserve this part of the river flow for important ecosystems and their functions for nature and people. This way, it is clear how much water can be withdrawn for other uses whilst maintaining biodiversity in important ecosystems, including the goods and services we derive from them.

“Large irrigation, hydropower development and infrastructure projects are already affecting the frequency and quality of floods in the Ouémé Delta,” says Fidèle Sossa, director of the Beninese NGO AquaDeD. “Together with climate change, they constitute the main threats to ecosystem services linked to flows.”

Defining water allocation

Especially when planning for water allocation in the river basin for hydropower, irrigated agriculture and other large-scale water using activities, the results from the environmental flow assessment will define the maximum amount of water that can be allocated without compromising the functioning of important ecosystems.

It is important to include the results of the Environmental Flow Assessment in the water management plans of the Ouémé River Basin. “The environmental flow assessment is a compass and an important tool to manage the current developments in the Ouémé River Basin. We presented our findings to decision-makers, so they can take environmental flows into account in the legal and institutional framework for these developments,” says Sossa, who is part of the research team for the Environmental Flow Assessment, together with IUCN NL’s partners from NGOs CREDI and BEES and the National Water Institute of Benin and with technical assistance of IHE Delft.

Read the executive summary (in French)

Mapping environmental flows in wet and dry season

As part of the environmental flow assessment, the team created an overview of the different habitats and ecosystem services that the Ouémé Delta provides (see below map).

The dry season is characterized by low flows, resulting in the drying up of the wetlands in the Delta. Because seawater can easily intrude in this season, salinity in the river is increased. This causes aquatic species such as the manatee to migrate up the river where it resides in lakes during the low flow season.

The increased salinity in the river and Lake Nokoué also reduces the growth of water hyacinth, an invasive species that floats on the water and takes away nutrients and oxygen for other species.

When the rains start, marking the onset of the wet season, water in the river swells and causes overbank flow in to the wetland. Fish migrate into the floodplains to spawn, and the salinity in the water decreases. Water hyacinth gets flushed out to the ocean when the flows have their peak.

Recommendations to sustain ecosystem services

This dynamic between high and low flows is important to sustain all the different ecosystem services in the Ouémé Delta. The graph below shows how much of the high and low flows are needed to maintain these services. From the graph it can be derived that in the dry season, all the available water must be reserved for the environment, whereas in the wet season, this is only 52%. This means that in the wet season, the other 48% of the available water can be allocated to other uses such as agriculture, drinking water supply or industries.

Sossa explains: “February, March and April are the most critical months when 100% of the available water flowing through the Ouémé River at the town of Bonou is needed for the ecological survival of the Ouémé Delta. Water abstraction during this period must be regulated, in particular for hydro-agricultural exploitation.”

Future water management in the basin

Several management actions aimed at ensuring the sustainability of the Ouémé Delta resources are underway. These include the Dutch government funded OmiDelta Plan, the establishment of the water reserve of the lower valley of the Ouémé, but also plans at a smaller level on specific important ecosystems such as the Vallée du Sitatunga and the Grand Nokoué Reserve. The integration of environmental flows should be based on these different ongoing actions.

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