Distributed Parameter Hydrologic Models

Basin discretization is the first process to be done when a rainfall–runoff model is set up. However, there is no clear procedure to perform this task in urban basins. Neither it is fully obvious to what extent these models can provide reliable simulations over a wide range of spatial scales . Many hydrologic models are available, varying in nature, complexity, and purpose .
The SWMM was developed by the US Environmental Protection Agency (EPA), and it is one of the
most successful models produced by the EPA for the water environment . SWMM is a dynamic
rainfall–runoff simulation model, used for single-event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas . SWMM is widely used worldwide, and it
was the model used in this study. SWMM5  was used as the main modeling tool since it was developed especially for urban hydrology, even when SWMM has been applied successfully to non urban basins .
The runoff component of SWMM operates on a collection of sub catchment areas that receive precipitation and generate runoff and pollutant loads. Urbonas  pointed out that for distributed
rainfall–runoff models such as SWMM, research and studies are needed to develop guidance on how
to discretize a study catchment properly, at least from the end user’s perspective. Urbonas  specifies
that an issue not solved yet is how to reduce an irregular-shaped catchment with an array of street and
development patterns into regular shapes called for in models such as SWMM to get consistent and
accurate results.
Selecting a higher resolution in distributed hydrologic modeling implies a subsequent set of activities:
data acquisition, defining the model parameter values, building the model, simulation, calibration,
and maintenance; all the cost of these tasks are increased when selecting a higher resolution. On the
other hand, selecting a lower resolution greatly reduces the workload, but there is the risk of losing the advantage of the distributed modeling approach, leading to poor results due to lack of consideration of important spatial features. Clearly, there is a trade off between both approaches.

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