Climate change caused by increasing atmospheric concentrations of greenhouse gases will have variable impacts on agricultural production in the United States. As agriculture is limited by water supply in many areas, potential changes in the water cycle due to climate change will drive the adaptation of agriculture. Here we report on simulations of agriculture and water resources response to scenarios of climate change derived from the HadCM2 general circulation model. We examine how the changing supply of water may affect agricultural production and irrigation, one of the more likely ways in which farmers will try to adapt. Baseline climate data from national records for 1961?1990 and the HadCM2 scenario runs for two ten year periods centered on 2030 and 2095 were used to drive the EPIC agricultural and HUMUS hydrological simulation models. Both models were run at ambient CO2 concentrations of 365 and 560 ppm to represent the lack and presence, respectively, of a ?fertilization? effect that is known to influence rates of plant photosynthesis and evapotranspiration (ET). Temperatures increase across the country under the scenarios by up to 4° for 2030 and 7°C for 2095. Precipitation also increases over most of the country and the higher temperatures cause a greater proportion of precipitation to fall as rain. The simulation models project increases in the water supply by 2030 and larger increases in 2095 over most of the country with the exception of the Gulf Coast, where water supply is decreased. The higher temperatures also cause rates of ET to rise and alter seasonal patterns of streamflow in mountainous regions. Regional changes in water supply are variable, with some regions experiencing declines and others increases. With more available water, dryland agricultural production of corn, soybeans and winter wheat improves over most of the country. A notable exception is soybeans in southern regions where higher temperatures cause a decline in production. A proxy indicator was developed using irrigation demand simulated by EPIC and water availability simulated by HUMUS to provide a sense of where in the country, and when, water would be available to satisfy change in irrigation demand for corn and alfalfa production as these are influenced by the HadCM2 scenarios and CO2-fertilization. Conditions are projected to be more favorable for irrigated agriculture over much of the country in 2030 and 2095. A notable exception is the Great Plains, a major agricultural region, where irrigation demands may not be met by available water supply under climate change.