Improved Water Retention From Soil Carbon Accrual Delivers Only Marginal Agrohydrological Benefits Under Most Conditions

Abstract Despite growing interest in soil carbon sequestration as a climate mitigation strategy, its practical hydrologic benefits in agricultural systems remain uncertain. While soil organic carbon (SOC) is known to increase available water capacity (AWC), it is unclear whether SOC induced AWC gain produces meaningful and perceptible changes in field‐scale water budgets and irrigation demand. To address this question, we applied a carbon‐sensitive soil water balance model to quantify the impact of SOC accrual achievable under U.S. Department of Agriculture‐Natural Resources Conservation Service conservation practice standards (CPS) on total available water (TAW), crop evapotranspiration (ET a ), deep percolation, surface runoff, and irrigation demands. Hydrologic responses to upper‐bound SOC accrual were evaluated across soil textures, aridity classes, and water management regimes for 42 growing seasons (1981–2022). Decadal SOC accruals were as high as 25% under humid, irrigated conditions with multiple CPS implementation. Even under most optimistic SOC accrual, TAW only modestly improved with median gains of <2%, and maximum improvement of ∼6% in fine‐textured soils. Median changes in irrigated fluxes were small (<14 mm), although irrigation water savings of 31–84 mm occurred under rare, soil‐specific conditions. Overall, the marginal and uncertain water budget response suggests limited hydrologic incentives for CPS adoption in most years and soils, constrained by insufficient moisture inputs to capitalize on increased AWC.