What outputs reflect water in sewer systems and Waste Water Treatment Plants (WWTPs)?
In the outputs of i-Tree Hydro, impervious flow is a component of total flow which represents water flowing on impervious surfaces (such as roads and sidewalks) and in subsurface pipe networks (such as sewer systems) going to an outlet of the project area. Users examining land use change impacts on sewers or WWTPs should look at impervious components of Hydro's outputs. This is based on the assumptions that 1) water flowing over impervious surfaces will be collected in the impervious drainage/pipe network being studied, and, if assessing changes to flow in waste water treatment plants, 2) the pipe network collecting runoff will enter a waste water treatment plant.
How does i-Tree Hydro account for impervious drainage networks like sewers?
The impervious flow component of total flow in i-Tree Hydro represents a) water flowing along roads, sidewalks, etc. overland "directly" connecting to an outlet (where water leaves the system being studied), and b) water draining into surface/subsurface pipe networks which connect to an outlet (a treatment plant and/or water body). The vast majority of a) becomes b), as most impervious infrastructure ultimately drains into a pipe network prior to reaching an outlet. There are parameters (time constants) in Hydro's processes that reflect the different rate of distribution of those two forms of impervious flow.
How does i-Tree Hydro account for WWTPs?
The water entering a WWTP will mostly, if not entirely, be represented by the impervious flow component of total flow. It is possible that parts of the impervious flow (water running along directly connected impervious surfaces/subsurface networks) will not enter a WWTP and will instead go to a different outlet. i-Tree Hydro v5 is not fully spatially distributed and therefore cannot identify how much of each flow component would go to a specific outlet in a non-watershed area.
To maximize confidence that the impervious flow output of i-Tree Hydro represents water entering a WWTP, users should model the WWTP's sewershed. By doing so, the outlet or 'pour point' of the watershed-area project will be the WWTP itself, and all surface flow components in a watershed/sewershed drain to their pour point.
Section for FAQs regarding i-Tree Hydro
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