Projecting water availability and quality for reuse under scarcity in the Bahr El-Baqar catchment in Egypt using the SIWARE model.

Agricultural drainage water reuse is a vital strategy for mitigating freshwater scarcity in Egypt. This study applies the SIWARE (Simulation of Water Management in the Arab Republic of Egypt) model to project water quantity and quality under scarcity at two strategic nodes in the Bahr El-Baqar catchment. The first node is the Bahr El-Baqar Feeder (BBF), which supplies the world's largest wastewater treatment plant. The second node is the Bilad El-Ayad Pump Station (BAP), which supports direct local irrigation. The model was parameterized using extensive data from four Egyptian national authorities. It was rigorously calibrated and validated for 2020-2021. The model achieved strong performance (R² up to 0.92, Nash-Sutcliffe efficiency up to 0.91). Freshwater allocation reductions to the Eastern Delta were simulated at 5% increments from 0% to 50%. Results reveal starkly contrasting node-specific responses. At BBF, baseline total dissolved solids (TDS ~ 2200 ppm) already exceeds Egypt's 2000 ppm regulatory limit. Closing this gap would require a theoretical increase in dilution, but such an increase is infeasible under Egypt's fixed 55.5 billion cubic meters (BCM) annual Nile allocation. Under a 50% reduction, TDS at BBF exceeds 3000 ppm (a 36% increase) with a 47% discharge decline. This poses operational risks including accelerated membrane fouling, increased energy consumption, and shortened maintenance cycles. In contrast, BAP shows greater resilience. TDS increases by only 16.4% under the same scenario, remaining below 2000 ppm. However, discharge declines by 45%. Site-specific exponential relationships (R² ≈ 0.98 for curve fitting, 95% confidence intervals ± 4-6%) predict discharge and TDS for any allocation reduction. This enables rapid assessment without full model re-runs. A deployable portfolio of interventions is presented. These include membrane-based pretreatment (up to 82% TDS reduction), temporal abstraction management, upstream salinity source control (75.67% TDS reduction), salt-tolerant crop varieties, and operational blending optimization. The contrasting responses underscore the need for node-specific management. This study provides a quantitative framework and actionable interventions. It supports adaptive water governance and infrastructure planning under increasing scarcity. The work contributes directly to Sustainable Development Goal (SDG) 6 (clean water and sanitation) and SDG 2 (zero hunger).