Watershed models simulate natural processes of the flow of water, chemicals, nutrients, and microbial organisms within watersheds, as well as quantify the impact of human activities on these processes. Simulation of these processes plays a fundamental role in addressing a range of water resources, environmental, and social problems. This book introduces some of modern modelling software technologies used for watershed delineation as well as their modelling. The management of watershed resources to produce more than one product or amenity reflects the multiple-use concept. In almost any discussion of natural resource management, multiple uses are cited as a guiding principle. We believe that this still is the case and that the concepts of ecosystem management and environmental protection all fit within an overall multiple-use concept. However, although there has been little difficulty in gaining a general acceptance of the multiple-use concept, implementing multiple-use programs is not always easy.