Environmental flow calculation

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To calculate the environmental flow of a river, I have monthly rainfall data and calculated monthly flow through this equation:  Q (mm/month) = Q(m^3/s) *1000*24*number of days in month*3600/ Area (m^2)

Which method can I use to calculate environmental flow?

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8 Answers

  1. Hi,

    if you mean by environmental flow is the minimum flow that should exist within one section of the river, it involves many ecological parameters. An example is by using sustainable index of living habitats of certain sensitive organism (fish, riparian animals, etc.) which needs minimum amount of river flow in order to sustain.

    However, if what you mean is baseflow of a river, there are various studies on baseflow separation method which you can search over the internet.

    Hope this helps.

     

    Best regards,

    Muhammad Alfalah Fauzi

  3. Hi Mehdi,  while others have given very good advice, I think you are asking for the base flow calculation and not the total or storm flows.  This is usually determined from the rainfall/runoff for the driest months of the year, and then you may also wish to determine this for the average year, or alternatively for e.g. a 10 year drought year.  Now in many cases the rainfall in the driest months of the year may be zero or close to zero, so using the rainfall-runoff calculations does not give you an acceptable result.  Instead you will need to estimate the contribution to the stream flow from underground sources (springs and seeps) which are geologically controlled as well as linked to the recharge of the groundwater.  A simpler way is to measure the streamflow in the critical period (if it has not already been abstracted for human endeavours upstream).

    I am pleased though that you are following this path - it is often neglected and we ignorantly disturb our river eco-systems thrrough ignoring this factor.  Best wishes

  4. Apart from the environmental flow calculation based on run-off, there is also a socio-economic factor that needs to be considered like aquatic life, transport, entertainment, municipal needs (waster and wastewater) etc... these can be determined using various secondary methods.... 

  5. Mehdi,

    Istvan Szamosvari is right. In the USA we have a site that gives us the 100 years storm event for 24 hours duration. You can start with this site, where you put the site address (Latitude and Longitude of a Point (getlatlong.net)), to get the Latitude and longitude of your site. In California, we would go to this site (PF Map: Contiguous US (noaa.gov)) to enter above Latitude and longitude to get the date for storm event and interval we want. 

    The formula we use is Q = CIA; where “Q” is the runoff flow in Gallons per Minute (gpm), “C” is the runoff coefficient based on land uses, “I” is the rainfall intensity in Inch per Hour taken at the time of concentration, and “A” is the surface area in ft2. We use I=1 if the area is covered by concrete.

    I hope this helps. 

  6. you can also use SCS-CN (Soil Conservation Service Curve Number) .

    You can use monthly rainfall P (mm)

    Q = (P-0.2S)2/(P + 0.8S)   P³ 0.2S                                  

    CN = 1000/(10 + S)         

    Q = direct runoff , and P = total rainfall

    SCS-CN depends on the ​different land-​uses of the ​catchment area. SCS-CN is tabulated  and easy to find (i.e. https://www.hec.usace.army.mil/confluence/hmsdocs/hmstrm/cn-tables

    To find direct runoff in m3

    1mm corresponds to 10m3 per ha, so if you have your area, you should just multiply Q by A and adapt yours units

     

     

  7. Unfortunately it is not correct. Only a certain part of the rainfall is flowing into surface waters, that is characterized by runoff coefficient (a). The value of runoff coefficient is 0,90 for built-in areas (like cities, parking lots), and is around 0.05 in case of plains with natural land cover. Rather sue the following:

    Q=α×I×t×A

    where

    Q: is the amount of flow

    a: is the runoff coefficient

    I: is the intensity of the rain

    t: is the duration of the rain

    A: is the catchment area

    If you have only monthly amount of rain, You can use it instead of I×t part of the equation.

     

    The runoff coefficient is to be calculated based on the different land-uses of the catchment area.

  8. Yikes, I'm not a hydrologist - that is not in my area of expertise! Sorry.  I'm an environmental chemist, have dabbled in hydrology - but don't know enough to be helpful.

    1 Comment

    1. William, yes not rainfall that falls on the land gets into the river. Most is lost back to evaporation again. Depending on how moist your soil is the soils will also soak up a lot. You only get runoff once the soil is saturated and then it starts to flow. Or if you get a high amount of rain in one event a lot will flow, hence the reason we get floods. Predicting stream flow from rainfall is really tricky. If you have a stream flow gauging station best to use that as then tells you how much water is flowing down the river + or - depending on how good the stream flow rating curve is. But what do I know about surface water hydrology, I'm a hydrogeologist. At least you can see what's happening on the surface. Subsurface now that's an art, full of mystery and intrigue.