This article measures irrigated agricultural outcomes under the prior appropriation doctrine by developing a model of farmers’ land allocations in response to water supply change. The modelling approach considers the institutional factors of water rights and permits the inclusive determination of water and land allocations. We utilize farm-level data of irrigated agriculture in Water District #1 in Idaho to examine the predictions from our theoretical model. The water rights priority date is consolidated at the farm level and used to differentiate farmers’ responses. We test a set of hypotheses that relate to water supply and crop revenue. Our results suggest that the water rights priority has a profound impact on agricultural outcomes, indicating strong institutional effects and weak influence of market-based approaches. Farmers respond to both long-term and seasonal water supply change and variation, and they respond to seasonal water supply forecasts in varied ways depending on their water rights portfolios, thus signalling a disproportionate distribution of potential impacts of water supply change.