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The Ewaso Ng’iro North Basin faces critical water security challenges driven by competing sectoral demands, upstream-downstream allocation inequities, climate variability, and fragmented governance across five counties: Laikipia, Samburu, Isiolo, Meru, and Nyeri. This study employed the Water Evaluation and Adaptation Planning (WEAP) system to assess current and future water supply-demand dynamics under multiple scenarios, providing evidence-based recommendations for integrated water resources management that balances agricultural productivity, domestic water security, hydropower generation, and ecosystem health. This study was made possible through the valuable contributions and collaborative efforts of numerous partners and stakeholders across the Ewaso Ng’iro North Basin. We extend our sincere gratitude to the Centre for Training and Integrated Research in ASAL Development (CETRAD), Mount Kenya Integrated Development Programme (MIDP), National Drought Management Authority (NDMA), Water Resources Authority (WRA), Ministry of Water, Sanitation and Irrigation, and the Kenya Meteorological Department (KMD) for providing essential data, technical expertise, and institutional support that formed the foundation of this modeling exercise. Special appreciation goes to the County Governments of Nyeri, Meru, Isiolo, Samburu, and Laikipia for their active participation, data sharing, and commitment to collaborative water resources planning across the basin. We are particularly grateful to the Water Resources Users Associations (WRUAs), local communities, civil society organizations, private sector representatives, and all stakeholders who participated in consultative workshops, validation sessions, and scenario co-creation processes. Your deep knowledge of local water challenges, practical insights into community needs, and collaborative spirit in jointly developing the six WEAP scenarios were instrumental in ensuring that this modeling framework reflects real-world conditions and priorities. The participatory approach strengthened the relevance, credibility, and applicability of the findings, transforming this study from a technical exercise into a truly community-informed decision-support tool for sustainable water management in the basin. Some of the key findings include: Model Calibration Success: The WEAP model achieved excellent performance metrics (NSE=0.72, R²=0.85, PBIAS=7.1%, NRMSE=9.2%), validating its reliability for water resource planning and scenario analysis across the basin; Tightening Water Budget: Despite projected 7.5% rainfall increase by 2050, streamflow decreases by 10% (485 to 438 Mm³) while evapotranspiration rises 8% and human abstractions surge 40% (245 to 342 Mm³), indicating intensifying water stress; Sectoral Water Use: Domestic consumption dominates at 43% of total water use, while agricultural irrigation (commercial 38%, subsistence 16%) accounts for 54%, highlighting the critical role of efficient water management in both sectors; Upstream-Downstream Inequity: Upper catchment counties (Nyeri, Meru, Laikipia) command 99.7% surface water abstractions, creating chronic shortages in downstream pastoral counties (Samburu, Isiolo) where reliance on groundwater reaches 43%; Climate Vulnerability: Water supply fluctuates dramatically between wet (520-550 Mm³) and dry (360-390 Mm³) years—a 40% reduction—while demand remains stable at 290-350 Mm³, creating severe deficits during droughts with water stress exceeding 130%; Scenario Analysis Results: Business-as-usual and county-focused scenarios (S1, S2) increase unmet demand to 142-174 units by 2050, while integrated cooperation (S5) and ecosystem restoration (S6) scenarios reduce unmet demand by 78-88%, achieving 48-88% environmental flow requirements compared to just 22% under baseline conditions. The modeling results demonstrate that fragmented, county-siloed water management approaches will exacerbate water stress, threatening livelihoods and economic development across the basin. Conversely, integrated interventions combining agricultural efficiency improvements (irrigation technology, crop shifting), infrastructure investments (storage expansion, hydropower diversification), and cooperative governance mechanisms can reduce unmet water requirements by 15-25%, lower greenhouse gas emissions, and generate sustained agricultural productivity gains through 2050. Critical to success is the establishment of legally-binding inter-county water allocation frameworks with benefitsharing mechanisms that address the upstream-downstream disparity. Without coordinated action, downstream pastoral communities in Samburu and Isiolo—already experiencing 60% food poverty rates—will face escalating water insecurity, while upper catchment agricultural systems risk undermining their own long-term sustainability through over-abstraction