Drybean is one of the most important crops planted in Wyoming. Since 1950s Wyoming has almost doubled its drybean yield. Weather conditions in the state, characterized by freeze days as early as mid September and freeze days as late as mid-late May, are the most important factors affecting drybean production. The main objective of this study is to examine the implications of the interannual weather variability, on water use and yield of drybean grown in the State of Wyoming (northwestern USA). Specific objectives are to estimate water use and water productivity of drybean and to determine optimum planting dates of new varieties. A dynamic crop simulation model, previously calibrated with statewide variety trials data, observed daily weather, observed soil characteristics, and extension crop management recommendations were used in this study. Growth and yield and water use of four drybean varieties, representing Black, Great Northern, Navy, and Pinto market classes were simulated for planting dates as early as May 1 and as late as June 25 for a period of 30 years, from 1980 to 2009. Our preliminary results showed that the model satisfactorily simulated yield. The study of the simulated planting dates allowed estimating the probability of yield reduction due to the negative effects of the interannual weather variability such as low precipitation and early and late frost occurrence.