Neglecting farmer cropping adaptation can overstate water shortages in large-scale hydrological modeling assessments.

Abstract Threats to water security are a paramount global concern, largely driven by human pressures on scarce water resources. The irrigation of croplands, which accounts for the lion’s share of human water consumption, is critical in understanding water shortage trajectories. Despite irrigation’s defining role, large-scale hydrological modeling (LHM) frameworks typically impose trajectories of land use that underlie irrigation demand, neglecting dynamic feedbacks in the form of human instigation of and subsequent adaptation to water shortage via irrigated cropping changes. We extend an LHM with adaptive farmer agents, applying the model to the Continental United States to evaluate water shortage outcomes that emerge from the interplay between hydrologic-driven water availability, reservoir management, and farmer cropping adaptation. Hypothetical comparative simulations reveal that neglecting farmer cropping adaptation regularly leads to pronounced overestimation of water shortages, with adaptation reducing U.S.-wide annual water shortage by as much as 42 percent in an experiment that mimics U.S. hydrology from 1950–2009.