Unexpected results: increasing impervious flow led to decreased total flow

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Unexpected results: increasing impervious flow led to decreased total flow

Post by rcoville » Fri May 18, 2018 12:13 pm

In certain parameter sets, increased impervious cover can result in decreased total flow. This phenomenon is counter-intuitive at first, but a key detail here is to consider the flow components separately from total flow. A good way to visualize the responses of each flow component between scenarios is using the chart & table (in Hydro v5) Outputs>Water Flow>Alternative Case - Base Case.

In cases of this we have examined, total flow is decreasing but surface runoff (pervious runoff + impervious runoff) is increasing. The increase in surface runoff is outweighted by how much baseflow decreases, causing an overall decrease in total streamflow. While the decreased total streamflow is at first counter-intuitive considering the increased impervious cover, the results make more sense considering how - as expected - increasing impervious cover generally reduces baseflow and increases surface runoff.

It's difficult to pin down exactly why in a particular scenario baseflow decreases more than overland flow increases, given the cascade of effects on base flow and surface runoff from the addition of impervious cover replacing other land cover types. However, this phenomenon of increasing impervious cover reducing total flow only occurs with certain land cover and hydrological parameter combinations and not in all projects, and even in the projects where it does occur, it only does so within a small range of impervious cover values. For example, going from 5% to 20% impervious cover may result in decreased total flow, but in the same project going from 5% to 40% impervious cover will result in increased total flow as the decrease in baseflow is outweighted by the increase in surface runoff.
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