Technical Abstract: Water rights, policy and emergent water markets in the semi-arid regions of the western USA, and worldwide, call for improved estimates of agricultural water budgets. Agricultural water is seen as a major potential water supply with high commodity value as municipalities seek water security under growing populations and potential climate change (including likely changes in temporal variability). Before selling water, historical “return flows” to streams and groundwater must be subtracted from the amount of irrigation water traded, and even rain-fed “return flows” must be considered part of regional water resources. Combined land-use and climate change may substantially alter return flows, adding complexity to already uncertain estimates of return flows. An agricultural systems model, RZWQM2, has been used to simulate deep drainage (pre-groundwater recharge) and surface runoff (potential return flow to streams) under historical conditions in Colorado, USA. Conditions range from dryland to limited and fully irrigated croplands. Here, we look at the potential to apply RZWQM2 under projected climate scenarios to evaluate potential changes in return flows. Interactions between projected climate change and adaptive land use and water management (including limited irrigation) can be simulated. Selected examples will be shown, as research tools are developed for broader application. This work will be placed in the general context of USDA-ARS Agricultural Systems Research together with UNESCO’s Groundwater Resources Assessments under the Pressures of Humanity and Climate Change (GRAPHIC).