Closing the loop between water supply and demand in the Nile River Basin under global change

Arid and semi-arid regions such as the Middle East and North Africa are increasingly suffering from water scarcity, exacerbated by climate change and population growth. This trend calls for new strategies for managing water demand and supply to face global changes in social-economic development, water system expansions, and cross-border differences.In this work, we explore the potential to mitigate the existing conflicts over the Nile River Basin, interconnecting water demand and supply using novel technological solutions, such as desalination and aquaponics, combined with traditional uses (i.e., groundwater extraction and water reuse). We analyse the complex dynamics and tradeoffs between energy production and irrigation water supply in Ethiopia, Sudan, and Egypt. We propose innovative portfolios of interventions that combine the coordinated operation of large water dams (i.e., the Grand Ethiopian Renaissance, Merowe, and High Aswan) and the main irrigation diversions with smart water demand management options. Desalination involves the process of removing salt and other minerals from seawater, making it suitable for irrigation and other domestic uses. Aquaponics involves the cultivation of fish and plants in a symbiotic environment, with the waste produced by the fish providing nutrients for the plants and the plants purifying the water for the fish. This technology can be an efficient and sustainable way to produce food with very low water consumption.Our approach is used to study current and future tradeoffs, generating solutions that are efficient and resilient to future hydroclimatic and demographic scenarios. We first quantified the impacts of dynamically downscaled and bias-adjusted climate projections for three Representative Concentrated Pathways (i.e., RCP2.6, RCP4.5 and RCP8.5) on the runoffs of the main tributaries of the Nile. We also considered stochastic projections of water demand based on Shared Socioeconomic Pathways (SSPs), and a strategic model that reallocates crops according to future climatic and demographic scenarios, according to a balanced diet and agricultural intensification strategy to generate a positive impact on food self-sufficiency.Our results show that the Nile River Basin features both strong tradeoffs and synergies across riparian countries, with the irrigation supply in Sudan playing a major role in allocating water between competing sectors. The results show a decrease of up to 20% of the Nile River's runoff and a doubling of the Egyptian municipal demand in the most severe scenario that leads to exacerbating tensions between the three countries. Notably, the potential reduction of the Egyptian water demand through different combinations of aquaponics, desalination, reuse, and groundwater pumping in the Nile Delta, along with a substantial decrease in Sudan irrigation demand through crop reallocation, can contribute to mitigating existing and future conflicts. Further technological improvements are needed for attaining large water demand reductions via soilless agriculture and desalination, which today cannot completely substitute reuse and groundwater contributions, whose high exploitation can induce relevant environmental risks.