Integrated Hydrological Modelling of Climate Change Impacts in Semi-Arid Urban Watershed of Niamey, Niger

Abstract

The Niger River is the sole permanent surface water used for agriculture and drinking water supply for the area of Niamey, Niger, West Africa. Given that the water distribution network does not cover the entire populated area, and because of recurrent droughts, the River cannot cover the total water demand in the area. Groundwater is pumped through open wells and boreholes to provide water to more than 35% of 1.3 million of people of the city. Groundwater demand for drinking and agriculture purpose is increasing as a result of rapid population growth and urbanization. Simultaneously, episodes of extreme low flows have become more frequent due to increasing demand, sedimentation of the River bed, and increased variability in streamflow upstream of Niamey. The minimum environmental flow for Niamey, set to 55 m3/s over 10 days, is therefore often not available. In this context, this study investigates ground water surface interactions and the whole hydrological system response under climate change. An equivalent porous medium approach was used to define a hydrogeological conceptual model to understand the hydrodynamic of the fractured aquifer system, and quantify the integrated interaction between this system and surface water resources as well as the climate change impacts. Combined used of hydrochemicals and isotopes have shown that the major source of both groundwater and surface water is provided by silicate weathering. The isotopes signals of water are exempt from strong evaporation influence, implying that groundwater recharge process is strongly dominated by rapid and localized infiltration. A large scale, high resolution fully-integrated hydrologic model was built and calibrated using HydroGeoSphere with a sequential approach of increasing levels of temporal resolution: 1) steady state average conditions; 2) dynamic equilibrium with repeating monthly normal forcing data; and 3) fully transient conditions. Simulations results show that exchange flux between groundwater and surface water are important processes in the basin. The basin average water balance highlights the importance of plant transpiration (58 % of total rainfall) over surface evaporation (8%), with groundwater recharge of up to 5% of total rainfall. Overland flow and infiltration account for 11% and 16 % of the total annual rainfall respectively, and groundwater discharge to the river is 2% of the total rainfall. Historical (1980-2005) and projected (2020-2050) climate scenario derived from the outputs of three regional climate models (RCM), under the RCP 4.5 scenario, were statistically downscaled using the multiscale quantile mapping bias correction method. The durations of the Minimum Environmental Flow (MEF) conditions, required to supply drinking and agriculture water were found to be very sensitive to changes in runoff resulting from climate changes. MEF occurrences and durations are likely to be greater for the first decade (2020-2030) of the mid-century, and then they will be reduced for the last two decades (2030-2050) of the mid-century period. All the three RCMs consistently project a rise in groundwater table of more than 10 meters in topographically high zones where the groundwater table is deep and an increase of 2 meters in the shallow groundwater table.