Periodically COMET produces online, instructor-led training for specific groups of learners. In this three-day course for water resource professionals, state/local planners, and hydrologists, participants worked with hydrologic models and climate change scenarios to assess impacts of a changing climate on surface water hydrology. Using sample watersheds, participants also learned how to evaluate climate change questions across different time and space scales. The following is Part One of a three-part lecture given by Holly Hartmann, PhD, during the course and available online in the MetEd lesson, Communicating Climate Change Scenarios with Decision Makers.