Mathematical modelling has become an indispensable tool for engineers, scientists, planners, decision makers and many other professionals for making predictions of future scenarios as well as real impending events. As the modelling approach and the model to be used are problem specific, no single model or approach can be used to solve all problems, and there are constraints in each situation. Modellers therefore need to have a choice when confronted with constraints such as lack of sufficient data, resources, expertise and time. Environmental and Hydrological Systems Modelling provides the tools for students and professionals by presenting different approaches to modelling the water environment over a range of spatial and temporal scales. Their applications are shown with a series of case studies, taken mainly from the Asia-Pacific Region. Coverage includes: linear systems conceptual models data driven models process-based models risk-management models model parameter estimation model calibration, validation and testing.