Classical water-yield relationships were developed primarily under pressurized irrigation systems and lightertextured soils; their quantitative behavior under gravity-fed surface irrigation on shrink-swell Vertisols remains insufficiently tested. The Gezira Scheme, Sudan's largest gravity-fed irrigation system, faces persistent challenges of low application efficiency and uneven water distribution. A two-season field experiment (2017/18 and 2018/19) compared seven irrigation regimes-including full irrigation and stage-specific deficit schedules-on chickpea (Cicer arietinum L.) yield, seasonal actual crop evapotranspiration (ETc,act), and water productivity under Vertisols conditions. Full irrigation produced the highest yields (1816 and 1280 kg ha-1), whereas deficits imposed during reproductive stages reduced yield by up to 33%. In contrast, a controlled vegetative-stage deficit (W3) saved 6% of seasonal irrigation water (35 mm) without significant yield reduction and achieved the highest crop water productivity (CWP) and irrigation water productivity (IWP). Grain yield correlated strongly with ETc,act but exhibited diminishing returns at greater irrigation depths, reflecting soil hydraulic constraints and surface irrigation inefficiencies. Seasonal climatic variability-particularly elevated vapor pressure deficit (VPD) and wind speed-further reduced yields and water productivity in 2018/19. A 9-10% increase in reference evapotranspiration (ETo) was associated with an approximately 30% decline in yield. These results demonstrate that irrigation thresholds in gravity-fed Vertisols are governed by the interaction among soil hydraulic properties, irrigation dynamics, and atmospheric demand, rather than irrigation depth alone. The quantified water-yield relationships and management implications are broadly relevant to chickpea production in other semi-arid environments and provide practical guidance for climate-resilient, site-specific irrigation management in canal-based systems on heavy clay soils.