Wood Rodgers developed an interactive design precipitation manual for Yolo County using ArcGIS. The design manual is used to calculate precipitation values based on a particular return period and storm duration. It is also used to determine the design value of the coefficient of variation that will be included in the run-off model. Mean Annual Precipitation data was obtained from Parameter-elevation Regressions on Independent Slopes Model (PRISM) climate mapping system. This custom tool was developed within ArcGIS using Visual Basic for Applications, ArcObjects and Microsoft Excel.