EPAs for water

EPA, in this case, stands for Environmental Protection Agency (of the United States).
We use a couple of models developed there, especially in my class of Hydraulic Constructions.
These products are:

• Epanet   (see also this post)
• SWMM (see also this post)

and they are available as open source.

There is  an organization of people gathering who are interested in working on them.  You can join them at:

https://github.com/OpenWaterAnalytics

http://wateranalytics.org/

I do not endorse them as the best models ever. But they are there, opens source, and have a community. I see them as a starting point for injecting new ideas and extensions.