The Integrated Decision Support Group (IDS) at Colorado State University has developed a set ofmonthly reference evapotranspiration (Err) maps for the western slope of Colorado. The reference evapotranspiration maps were developed using geographic information systems (GIS) to manipulate spatial data. After the monthly alfalfa-based ETr maps were complete, a Graphical User Interface (GUI) was developed to allow the user to interactively view and query the maps. In accordance with current developments in the irrigation science community the Penman-Monteith was used to calculate ETr and alfalfa was selected as the reference crop for this study; Data parameters required as input for the Penman-Monteith equation are maximum and minimum temperature, relative humidity (or dew point temperature), solar radiation, and wind-speed. During the course of this project, several data sources have been identified. Where possible, data have been subjected to a quality check.