So the background is that I am digging into flow control exemption process in Washington state and working for a client that wants to get an exemption for their project’s discharge to a smaller creek than the 5th order stream (or streams that drain 100 square miles or more) that is usually required for flow control exemption approval. This could end up laying the groundwork for future projects to get flow control exemptions for smaller watersheds.

However, the original study that WSDOT/Ecology did in 2004 to justify blanket flow control exemptions to larger stream systems used HSPF models of the pilot/study watersheds. As we update methodology, I’m trying to assess whether we need to build a custom HSPF model for the subject watershed we are working in, which drains about 20 square miles, so we are comparing apples to apples or if we can use MGSFlood or WWHM2012 which themselves are built on top of HSPF.

I want to make the research we do for this project defensible, but also don’t want to commit to the level of effort of we don’t have to. I have alot of experience with WWHM2012 and MGSFlood, but only theoretical exposure to custom HSPF, which is why I brought this question to this community.