# Are there ways stormwater attenuation can be quantified for comparison between different systems or conditions?

Various sources agree land cover changes impact the attenuation of stormwater. E.g. forest areas generally slow the rainfall-runoff response and moderate it, reducing peaks in hydrographs, as compared to urban areas.

Is there a way stormwater attenuation can be quantified, by field measurements (eg. observed streamflow) or model outputs? If so, what are some examples of how this can be done? The reason to quantify attenuation would be to compare differences in attenuation between different basins or between different conditions within a basin.

• If you don't get an answer here, try SE Engineering. This topic is dealt with by engineers who need to consider surface drainage & the design of water retention systems, particularly in urban environments. – Fred Sep 1 '17 at 12:31
• Good point @Fred. I approach this question as an earth scientist but it is of much more immediate importance to urban stormwater engineers. I'll give it more time here and will try to get it moved if no good answers roll in after a week or so. – cr0 Sep 1 '17 at 13:10
• I have never come across a single number that represents "attenuation" for a given basin. This is because the basin's ability to attenuate does depend on basin characteristics as you note, but also on the storm characteristics (duration/intensity/size/movement). Imagine a long skinny basin and a storm traveling along its length. If the storm moves from headwaters to outlet, the peak runoff will be much higher than a storm moving from outlet to headwaters, even if all other characteristics are the same – Adam Sep 7 '17 at 15:26

## 1 Answer

A typical ratio calculated to quantify this type of response, especially at larger scales (such as for land use changes) is the runoff ratio:

$$R = Q/ P$$

where $R$ is the runoff ratio [mm/mm or dimensionless], $Q$ is the annual flow volume [mm], and $P$ is the annual precipitation [mm]. The runoff ratio is usually calculated for a given data period (multiple years of data) or on an annual basis. It is tricky to calculate for individual events since the antecedent conditions play a large role, but on an annual basis this would be a decent metric to compare different basins or the same basin in time. This is assuming you have a number of years of data to calculate the ratio from.

• Might be worthwhile to distinguish between total runoff (which this equation addresses) and attenuation (which it does not). – Adam Sep 6 '17 at 15:11
• This ratio implicitly represents attenuation for a given basin, the larger the ratio the smaller the attenuation and vice-versa. This ratio is of course impacted by other factors, but gives a comparable value to use. – Rob Sep 7 '17 at 2:40
• Do we have different definitions for attenuation? I use the word to refer to the reduction in peak flow for individual events - How quickly water leaves the basin, not how much water leaves the basin. google.com/… – Adam Sep 7 '17 at 15:37
• I suppose I was just considering overall volume retention in terms of how much water generally reaches the streams, not in the timing. That being said, I think the timing and volume are generally related at the larger scale, those with faster responses will also have a larger percentage of the overall volume moving to the stream. – Rob Sep 8 '17 at 3:56
• I haven't accepted this for the reason @Adam pointed out. I am interested in a metric to measure the delaying of runoff / buffering of peak flow at the resolution of individual events. While I see how you could use the rainfall-runoff ratio to compare general attenuation of basins, I am looking for a finer-scale way to highlight the attenuation Adam and I are referring to. Thanks anyway! – cr0 Apr 13 '18 at 23:36