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

<p>$$R = Q/ P$$</p>

<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.</p>

CC BY-SA 3.0
2017-09-06T15:11:15.553Z
Might be worthwhile to distinguish between total runoff (which this equation addresses) and attenuation (which it does not).
Adam
CC BY-SA 3.0
2017-09-07T02:40:20.007Z
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
CC BY-SA 3.0
2017-09-07T15:37:09.740Z
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.  https://www.google.com/search?safe=active&biw=1067&bih=1635&tbm=bks&q=attenuation+hydrology+routing&oq=attenuation+hydrology+routing&gs_l=psy-ab.3...20556.22199.0.22366.8.7.0.0.0.0.346.346.3-1.1.0....0...1.1.64.psy-ab..7.0.0.lppuouZB6Wc
Adam
CC BY-SA 3.0
2017-09-08T03:56:24.853Z
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
CC BY-SA 3.0
2017-09-08T14:58:07.390Z
Especially in urban setting, there are small basins designed to return all runoff eventually, but with constrictions that create ponding and attenuation ("detention areas").  Similar features exist in natural settings. High Runoff & high attenuation.
 Conversely, basins with steep channels over heavily fractured rock/sand can have  low attenuation and lose significant amounts of their runoff to groundwater.
Adam
CC BY-SA 3.0
2018-04-13T23:36:24.847Z
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
CC BY-SA 3.0
2018-04-14T23:45:41.680Z
No problem, feel free to post back if you find or develop a better metric, I would be interested to follow-up if you find something better. Cheers
Rob
CC BY-SA 3.0
2017-09-01T19:03:28.893Z
2017-09-01T19:03:28.893Z
Rob
<p>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. </p>

<p><strong>Is there a way stormwater attenuation can be quantified, by field measurements (eg. observed streamflow) or model outputs?</strong> 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.</p>

CC BY-SA 3.0
2017-09-01T12:31:39.183Z
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
CC BY-SA 3.0
2017-09-01T13:10:30.733Z
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
CC BY-SA 3.0
2017-09-07T15:26:35.403Z
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
<hydrology><measurements><storms><runoff>
Are there ways stormwater attenuation can be quantified for comparison between different systems or conditions?
cr0
# Are there ways stormwater attenuation can be quantified for comparison between different systems or conditions?

(Asked by: username_0 on 2017-08-31)

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.

**Tags:** hydrology, measurements, storms, runoff

> **Comments:**
> **username_1**: 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.  
> **username_0**: Good point @username_1. 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.  
> **username_2**: 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  

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## 1 answers

### Answer by username_3 on 2017-09-01. Score: 1

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.

> **Comments:**
> **username_2**: Might be worthwhile to distinguish between total runoff (which this equation addresses) and attenuation (which it does not).  
> **username_3**: 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.  
> **username_2**: 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.  https://www.google.com/search?safe=active&biw=1067&bih=1635&tbm=bks&q=attenuation+hydrology+routing&oq=attenuation+hydrology+routing&gs_l=psy-ab.3...20556.22199.0.22366.8.7.0.0.0.0.346.346.3-1.1.0....0...1.1.64.psy-ab..7.0.0.lppuouZB6Wc  
> **username_3**: 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.  
> **username_2**: Especially in urban setting, there are small basins designed to return all runoff eventually, but with constrictions that create ponding and attenuation ("detention areas").  Similar features exist in natural settings. High Runoff & high attenuation.  Conversely, basins with steep channels over heavily fractured rock/sand can have  low attenuation and lose significant amounts of their runoff to groundwater.  
> **username_0**: I haven't accepted this for the reason @username_2 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 username_2 and I are referring to. Thanks anyway!  
> **username_3**: No problem, feel free to post back if you find or develop a better metric, I would be interested to follow-up if you find something better. Cheers  

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