Watershed Models for Storm Water Management: Comparing Hydrologic and Hydraulic Procedures

Many different watershed models are available today that have various complexities, strengths, and weaknesses. Selecting the most appropriate model to achieve the most accurate and efficient solution possible is a challenging task. Despite the fact that all models are mathematical engines, their basic mathematical foundation is often overlooked due to high demands on model applications such as the ease of the graphical user interfaces and the incorporation of geographic information systems. This often leads to modelers' and managers' making decisions based on model marketers' potentially non-objective pitch. The objective of this investigation is to provide a mathematical-based review of available watershed models used for storm water management and provide insightful facts for the end users, particularly water resource managers, so that an informed decision can be made when selecting a model. Fourteen watershed models simulating storm event runoff were reviewed and evaluated resulting in the development of six summary tables which compile, rank, and compare the models. The dominant hydrologic and hydraulic procedures used in the models are compiled, including rainfall excess or infiltration (most rainfall loss), subbasin or overland (land surface) runoff routing, channel flow routing, and subsurface flow simulation. The rankings are based on the formulations and their relative complexities and accuracies for routing overland runoff and channel flows which provide some base for comparing the models because these procedures are backbones of any watershed simulation model, and ultimate performance of a model depends largely on accuracies and speed of these procedures. The compilations and the rankings would be helpful to managers and modelers in understanding and comparing the models, selecting the most suitable model for a project or an application, and using it to its full potential. The comparison approach and the tools (tables) presented would be useful to conduct objective comparisons of other watershed models as well.