WEAP-MODFLOW model as an integrated water management Decision Support System (DSS), a case of the Middle Awash sub-basin, Ethiopia

Optimum allocation of the limited water resources to the fast-growing demand requires new approaches. In this study, a new tool that considers a conjunctive use of surface-groundwater resources to assess both supply and demand-side perspectives is proposed. Using coupled Water Evaluation and Planning (WEAP) and three-dimensional groundwater flow (MODFLOW) models, a basis for an integrated water management Decision Support System (DSS) is configured for the Middle Awash sub-basin. The coupled models characterize water demands for domestic, industry, irrigation, and livestock and water supply from surface and groundwater sources. Using systematically structured near-future scenarios, the likely impacts of natural and anthropogenic stresses (population growth, climate change, and groundwater irrigation) on the surface and groundwater supply sources were evaluated up to the year 2030. The model results under the reference scenario indicated that the annual average industrial, livestock, domestic, and irrigation water demands are 1.3MCM, 21MCM, 41MCM, and 1.6BCM, respectively. If a 2.6% annual population growth rate is considered, the domestic water demand will be more than double (90MCM) in 2030. With the existing domestic water supply infrastructure, the population growth will cause a significant shortage of domestic water supply (70MCM), resulting in maximum domestic unmet water demands between October to December. The highest water supply deficit is recorded for irrigation water demand, with the annual average estimated at 393MCM. The maximum shortfalls of irrigation water demand occurred between January to March. Awash River is the only irrigation water supply source for large-scale irrigation schemes in the basin. However, groundwater can supplement surface water irrigation of large irrigation schemes with careful use that will not deplete the groundwater storage of the basin. Groundwater irrigation scenario tested for the Middle Awash sub-basin reduced the average annual irrigation unmet demand from 393MCM to 265MCM. The effects of climate change were analyzed under four climate scenarios: increase in Temperature, High Precipitation (HPPT), Medium Precipitation (MPPT), and Low Precipitation (LPPT). The climate change scenarios indicated more irrigation water deficiency would occur under the LPPT (443MCM) and increased Temperature (415MCM), while unmet irrigation water demand will be reduced by 366MCM and 328MCM if the MPPT and HPPT scenarios are prominent, respectively. To evaluate the combined effects of population growth, climate change, and groundwater abstraction for irrigation, META scenarios were tested. If the LPPT and increased temperature would be a likely near-future scenario, the irrigation unmet demand will increase from 393MCM to 468MCM. The annual average irrigation unmet demand would be reduced from 393MCM to 294MCM, even when the precipitation is low and the temperature is high, indicating groundwater can endure some of the stresses that might result from low precipitation and an increase in temperature. The WEAP-MODFLOW results can be used as a basis to develop a better-integrated water management decision support system in the Basin.