This simulation tool aims to support the design and management of the water structure made of contour bunds (DCN stands for Diguettes en Courbes de Niveau) in the rice-based lowland systems.

This tool is designed to simulate water flows within a type of water control facility developed for rice based lowlands. The hydraulic structure considered is contour bunds (DCN stands for Diguettes en Courbes de Niveau) largely promoted since last decades in Sudanese West Africa. It aims to spread the flood and reduce water peak damage in rainy season, and to retain water in plots at the end of cropping season. But the water control remains very partial, there is no large water storage. Originally designed to combat drought, this structure faces the problem of higher intra and inter annual variability of water regime in lowlands with climate change. Hence raises the issue of how contour bunds can mitigate hydrik risks for rice, dry spell but also increasing floods and waterlogging. This simulation tool aims to support the design and management of such water structure

Left: The Kankaniba site lowland contour bunds structure stylised for hydraulic model implementing conservation of plot areas and bunds length). Right: Conceptual scheme of water flows

Several issues are tackled with the tool “SIM DCN”. First for the design, it can support the assessment of the suitability of a specific lowland site for a contour bund structure devoted to rice and its dimensioning: height of bunds, number and size of spillway gates. Second is the management of the gates, calendar of closure versus openings in relation with the rice cropping calendar. A main risk nowadays faced by lowland farmers is the damage caused by a prolonged flooding of plots at early stage of rice cycle (1 to 40 days after sowing). Hence the tool can simulate the effect of a given flood and assess the capacity of bund gates to evacuate the flows within a time frame compatible with rice tolerance. A lack of coordination of gates management can also lead to damage, when high flows are concentrated on a few gates opened. The tool can be used in training to sensitize farmers to this management.

The tool is based on a water balance model with reservoir in cascade developed on Excel spreadsheet. The lowland scheme is represented by a set of plots and bunds crossed by a water flow. Input variables are water inflow, loss by soil/crop evapotranspiration and infiltration; parameters are the geometry of plots and bunds; output variables are water discharge, volume and height of water in each plot; decision variable is the opening or closure of spillway gates. The time step is hourly in order to describe consistent hydraulic processes linking water height and flow in spillway or over bunds. This allow a simulation for an event of a few days. The model has been applied for the Kankaniba pilot site for lowland hub Burkina Faso. It shows the suitability of the present contour bund structure to manage floods, but their limited effects to retain surface water, especially due to problems of inadequate junctions of bunds to ground soil on borders of the scheme.

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