SUSTAINABLE WATER MANAGEMENT IN THE DAKAR AGRICULTURAL BOTTOM-LANDS.

In Sub-Saharan region of Africa, the rainfall amount decrease has dramatic consequences on the hydric resources. The today annual precipitation does not permit any “efficient” recharge of groundwaters. That is the case in the Dakar agricultural bottom-lands which are depressions located in the sand dunes. The main characteristic of these lands is that the top-table quaternary sand groundwater (Nappe des Sables Quaternaires or NSQ) reaches or overflows the soil surface. Thereby, agricultural activities are based on irrigation using generally traditional farming practices; production supplying Dakar city in fruits and vegetables. Because of recent climatic crisis, the groundwater depletion threatens the agricultural activity on the long time. Concerning strategies used to solve the surface water problems, some management practices existing in the African cities are matter of debate, more precisely in Sahalian zones. For example in the case of Dakar, rainwater management techniques are diverting waters toward the sea or evaporation basins. In a rainfall scarcity context, and with the possibility of storing drained waters in the sand dunes and reusing them for irrigation, this strategy leads to a loss of resource. This study aims to promote an innovating approach and technical measurements for increasin g rainwater infiltrating volume feeding the NSQ groundwater, in order to reduce losses and enhance water availability. These measurements would allow to mitigate the current climate crisis and urbanization related constraints. This strategy is supported by the following hydrodynamic indicators 1/ the groundwater static level annual balance and 2/ the rainwater infiltrating rate variabilities. These parameters are calculated according to three pluviometric models -average, low and excess rainfall- representing the 80 last years rainfall variability.