Increasing irrigation water requirements across croplands in the Contiguous United States throughout the 21st century

Irrigation is a fundamental component of agricultural production, providing crops with the water needed for growth. In the United States, approximately 42 % of total freshwater withdrawals are devoted to irrigation. Although changing climate conditions are expected to exacerbate pressures on water availability for agricultural production in the Contiguous United States (CONUS), national-level estimates of future irrigation water requirements (IWR) for major field crops remain limited, creating a gap in the information needed to support sustainable agriculture practices. In this study, we estimated crop-specific IWR across the CONUS throughout the 21st century, using climate projections from five general circulation models (GCMs) within the Coupled Model Intercomparison Project Phase 6 (CMIP6). Temporal trends and spatial patterns of IWR were analyzed under two Shared Socioeconomic Pathways: a moderate warming scenario (SSP245) and a severe warming scenario (SSP585). Our results show a continuous increase in IWR across CONUS over the 21st century. Under the SSP245 scenario, the annual average IWR depth would reach 404.4 mm yr−1 in the late 21st century (from 2080 to 2099), reflecting a 10.3 % increase relative to the period of 2000–2019. Under the SSP585 scenario, IWR depth would rise more rapidly, reaching 462.7 mm yr−1 in the late 21st century, an increase of 26.4 % relative to the early 21st century. Each 1°C increase in annual temperature is associated with a 4.2–5.0 % (15.1–18.1 mm yr−1) increase in IWR in the CONUS croplands. The largest monthly increase is projected for July. Among the ten major crop types considered in this study, rice has the highest average IWR per unit cropland area, followed by alfalfa and cotton. Regionally, the Southern Great Plains (105.0 and 120.1 billion m3 yr−1 under both scenarios) and the Midwest (103.3 and 139.6 billion m3 yr−1 under both scenarios) record the highest total IWR amount in the late 21st century, whereas the Southwest (739.6 and 790.6 mm yr−1 under both scenarios) has the highest IWR depth per unit cropland area in the late 21st century. These results underscore the growing challenges irrigation faces under changing climate conditions and highlight the urgent need for effective water use strategies that ensure sustainable agricultural production and food security.