Traditional Analysis Approaches
Traditional approaches for analyzing antecedent moisture effects rely on physically based models derived from first principles, such as the principles of energy, momentum and continuity, which rely on measurements of many parameters for input and simulation. These include programs such as the Storm Water Management Model, Mouse RDII, or other rainfall/runoff simulation programs. These models are frequently calibrated to a specific antecedent moisture condition observed during a single storm. Fitting data from several storms that occurred during various antecedent moisture conditions requires modifying the model parameters and recalibrating the model. At the end of this process, the modeler is left with several models, each of which can fit a specific storm that occurring during a specific antecedent moisture condition, but none of which are capable of simultaneously fitting all of the data. This is the challenge of using event-based models with traditional approaches: it requires the user to select a particular antecedent moisture condition for design simulations.
Some modeling approaches, such as the Hydrologic Simulation Program - Fortran (HSPF) or the Stanford Watershed Model developed by Crawford and Linsley (1966) attempt to address antecedent moisture conditions through a complex physically based representation of the transport paths of water on the surface and in the subsurface. These tools have their place in researching and studying the various complexities associated with hydrologic transport processes. However, the great number of parameters in these models, the difficulty of measuring the many parameters, and the sensitivity of the model output to slight variations in the parameters makes using these models to simulate antecedent moisture in sewer systems challenging. The principles of parsimony and Occam's razor provide evidence of these challenges from a systems perspective.
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