WEAP-MODFLOW dynamic modeling approach to evaluate surface water and groundwater supply sources of Addis Ababa city

Abstract

The integration between WEAP and MODFLOW models coupled via LinkKitchen helps to create a dynamic link between surface water and groundwater supply sources of Addis Ababa city. Possible impacts of natural and anthropogenic stresses on surface water reservoirs volume and groundwater storage have been assessed through water supply scenario analysis. Besides, contrary to other surface water hydrological models, the unique nature of WEAP adds water demand assessment by simulating Addis Ababa city near future water demand coverage under three population projection scenarios. The water demand projections of Addis Ababa city indicates 100% water demand coverage will not be achieved for high (4.6%), medium (3.8%) and low (2.8%) population growth rate projections, even with all the emerging and planned water supply projects start production up until 2025. Supply scenario projections indicate, as surface water reservoirs are highly sensitive to climate change and variability, the city groundwater supply sources will be noticeably affected by the emerging and planned groundwater supply expansion schemes. If groundwater abstraction continues to reach to zero unmet demand, more than 30-meter groundwater level decline can be registered in 2025. To foresee the combined effect of both natural and anthropogenic stresses on Addis Ababa city water supply sources, best case (considering conditions which improve Addis Ababa city water supply) and worst case (considering conditions stressing Addis Ababa city water supply) scenarios were tested. The best case scenario results zero unmet water demand in Addis Ababa city in most wet months of future projection years up to 2025, with likely decline of about 6 meter on the groundwater level. The worst case scenario to the contrary shows, Addis Ababa city water demand coverage will potentially be reduced to a maximum of 35% in 2025, with seasonal and annual variability. The dynamic link between surface water reservoirs and groundwater supply sources helps to gain insight into the potential consequences of continuously changing natural and anthropogenic conditions on Addis Ababa city water supply sources. Consequently, the significant predicted near future pressure on Addis Ababa city surface water and groundwater supply clearly indicate planning and developing alternative water supply sources outside of the boundary (Upper Awash basin) where the city is located should be immediately started in order to endure the pressure from the ever increasing demand. Otherwise, not only Addis will continue suffering unmet water demand for the years to come, but also the water supply sources will be severely impacted. Nonetheless, wherever the water supply sources, minimizing water loss, recycling and improving water use efficiency should be given at most priority.