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Hydrographs for infiltration and actual evaporation in SWMM

Posted by Christina on October 24, 2012 at 10:54am

Hi,

I just started using SWMM and I've got a question:

I did some simulations and as an output I would like to get separate hydrographs for infiltration and actual evaporation. Is that possible? So far I was just able to create an hydrograph for potential evaporation and one for the losses (infiltration+evaporation) in general.

I would be glad, if someone could help me. :)

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Replies

  • Christina November 28, 2012 at 6:29am

    Hi, thank you for the quick answer. You're right there was a mistake with the units. I also found a way to get the evaporation hydrograph.

    Just one more question: What equation or method does SWMM use to compute the evaporation rate from the temperatures in the climate file?

    • Robert Dickinson > Christina November 28, 2012 at 6:34am
      Computed from Temperatures

      Hargreaves' method will be used to compute daily evaporation rates from the daily air temperature record contained in the external climate file specified on the Temperature page of the dialog.

      I just copied this from the SWMM 5 help file which is an excellent source of information from Lew Rossman of the USEPA.
      • Christina > Robert Dickinson November 28, 2012 at 6:50am

        Sorry, I didnt's see that in the user manual. Thank you very much!

  • Christina November 27, 2012 at 1:18pm

    Hi, 

    so I got another problem concerning the actual evaporation. This time I'm trying to model a green roof. So I created a bio-retention cell with a drainage layer that is directly connected to an outfall. 

    When I run the simulation,I get the following Subcatchment Runoff Summary:

    Total Precip: 1971.8 mm

    Total Evap:    1374.95 mm

    Total Infil:      364.01 mm

    Total Runoff: 364.01 mm

    Runoff Coeff: 0.185 

    So what confuses me is, that the evaporation is so big. If I calculate the sum of the potential evaporation over the whole simulation time span I only get around 608 mm. How can the actual evaporation be higher than the potential evaporation?  

    Total Infil and Total Runoff is always the same figure. Does that mean everything that infiltrates into the bio-retention cell automatically becomes runoff? Is there no evaporation from the water that is stored in the bio-retention cell? 

    My second question: Does anyone know a way to calculate the hydrograph of the actual evaporation for such a bio-retention cell? 

    I would be really thankful for your help! :) 

    • Robert Dickinson > Christina November 27, 2012 at 8:41pm

      Hi, What are you using for the daily or monthly evaporation rate.  The total of your infiltration, runoff and evaporation is about the precip so the fact that the infil equals the runoff is a coincidence.   I would check the units of your evaporation, are you using a monthly rate for a day?

  • Christina November 22, 2012 at 2:08am

    Hey, thanks for the reply! It worked! Sorry for getting back so late. :) 

  • Robert Dickinson October 24, 2012 at 11:17am

    That is a little tricky as the two values are added together as you note,  I can only suggest that you run the model twice, once with and once without evaporation and plot the losses for your subcatchments.  The difference between the two plots should give you a good estimate of Infiltration.

    Tip That Worked!

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