All Forums

metric conversion in SWMM5

Hello Bob

I'm trying to convert to metric a WQ example file you posted in Feb 2009 (RUNOFF43_SW5.INP). The problem seems to be in the factor C1 of the EXP function used to describe the Wash-off process.  I get a relatively close match (in the magnitudes & patterns) of the washed off constituents at the outfall, only
if  I use  ONE PERCENT  of  the metric conversion factor : [ 1 / (2.471 * 0.4536) = 1/1.121 = 0.892 ]  which converts pounds per acre to kilograms per hectare.

Why this  conversion factor  of  (~ 0.892 )* C1  for the EXP functions should be divided by 100 to approx. reproduce the original file (in the US customary units) results?  Or perhaps I'm missing something obvious?

Thanks & Regards,

You need to be a member of SWMM 5 or SWMM or EPASWMM - (not formally associated with EPA) to add comments!

Join SWMM 5 or SWMM or EPASWMM - (not formally associated with EPA)

Email me when people reply –


  • Great many thanks for your reply, Bob. 

    The EXPO example equation itself is easily converted to the Metric by replacing  C1=2.0  with C1= .907185 and keeping C2=1.4  intact, but this produces a wildy diverging results, far too off  from the original.  The main point here is that  if the example equation is obtained from a curve fitting operation to  a bunch of observed data then the same data would be described equally well by a  simple metric conversion of the the base data itself or of the fitted equation.  So perhaps SWMM5  is not trying to be quite  Metric in here. 

    Apart from its being an interesting example (#43) for me as a novice in the WQ modelling, what I actually need is to believe that  SWMM5 does work almost flawlessly in Metric when dealing with the pollution Accumulation & Wash-off  in urban settings .   Now  could you possibly  point me to a couple of  old/recent  applications of the SWMM5 (or v4) using the equations  in Metric , in your convenience please.   Many thanks all the same.


  • Unfortunately, it is not an easy conversion to understand.  The internal washoff loads are in pounds/sec and the internal flows are in cfs and the loads are converted to Kilograms in the metric output.  I need to look further into this conversion for you.  I suspect that the exponent has to be converted based on your output and input flow units.
  • Hi Mostafa,


    Maybe this will help you.  SWMM 5 internally use the old English units or what we call American Units now.  


    // --- convert units of washoff coeff.    

    if ( func == EXPON_WASHOFF  ) x[0] /= 3600.0;    

    if ( func == RATING_WASHOFF ) x[0] *= pow(UCF(FLOW), x[1]);    

    if ( func == EMC_WASHOFF    ) x[0] *= LperFT3;

This reply was deleted.