Why did total flow go down when impervious cover went up? (Troubleshooting unexpected results)

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rcoville
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Why did total flow go down when impervious cover went up? (Troubleshooting unexpected results)

Post by rcoville » Mon Mar 04, 2019 3:44 pm

There's a few things to look for when counter-intuitive results come up. This FAQ shares a response to a user who asked why total flow was decreasing in the scenario that increased impervious cover, along with other information and explanation that has helped clarify unexpected results.

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  • How has total directly connected impervious cover changed, considering (Impervious Cover % + Impervious Cover beneath Tree Cover %) * Directly Connected Impervious Cover %? Sometimes even if Impervious Cover % has decreased, because of changes to Land Cover beneath Tree Canopy and Directly Connected Impervious Area (DCIA), the total DCIA of a scenario can actually still increase.
In this example, we have...
Base Case: (54%+7.2%) = 61.2%, 61.2%*0.191 = 11.69% total DCIA
Alt Case: (51%+15%) = 66%, 66%*0.178 = 11.75% total DCIA

From that you can see that, despite bare Impervious Cover % decreasing from 54% to 51%, total DCIA has actually increased from 11.69% to 11.75%.

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  • How has the composition of total flow changed with respect to baseflow (subsurface flow) vs. surface runoff (pervious & impervious runoff)? To consider this, I look at each scenario's total runoff (pervious runoff + impervious runoff) and how much that makes up total flow (total runoff/total flow).
In this example, we have...
Base Case Total Flow: 35,805,112.7 m3. Total Runoff: 30,314,279.6 m3 + 5,045,210.0 m3 = 35,359,489.6 m3. Ratio of Total Runoff / Total Flow = 0.988, very high and mostly from pervious runoff.
Alt Case Total Flow: 35,891,623.0 m3. Total Runoff: 30,523,680.6 m3 + 4,981,936.6 m3 = 35,505,617.2 m3. Ratio of Total Runoff / Total Flow = 0.989, very high and mostly from pervious runoff.

With a better look at what components make up total flow, we get a more intuitive view of results: in the Alt Case the Total Runoff is slightly higher and Total Runoff makes up a larger component of Total Flow.

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Baseflow can be an un-intuitive and difficult to estimate flow component. Hydro estimates baseflow using the TOPMODEL framework (as described in Wang et al. 2008). Baseflow can be very sensitive to hydrologic parameters defined in Hydro on Step 3, such as Annual Average Flow of Project Area. Often it is more intuitive for users to focus on changes in surface runoff and Total Runoff, and we plan for future updates to Hydro's tables and charts of results to direct attention more toward Total Runoff.

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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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