In running some basic long-term continuous simulations I’ve noticed that SWMM output may not be very sensitive to the slope subcatchment parameter. For instance, I ran two simple continuous SWMM5 models with over 50-years of hourly rainfall data and a 1-acre catchment. One model had a slope of 0.5% and the other 50%. All other subcatchment parameters were the same. While I would have thought that the 50% sloped catchment would produce more runoff than the 0.5% catchment, the results turned out to be about the same. The runoff coefficient for 0.5% slope was 0.212 while that for 50% slope was 0.223, only a 5% increase. The peak flow rate for 0.5% slope was 0.122 cfs while that for 50% was 0.124 cfs.
My questions are:
- Do you think this slight increase in runoff volume is a realistic result or do you agree with me that it seems low?
- Are there more appropriate ways to set up a SWMM model to account for increased runoff caused by steeper slopes?
- Are there other continuous hydrologic models that are better at accounting for changes in runoff with slope (HSPF, HMS, etc)?
- Is the low sensitivity to slope due to any other parameters you can think of (time step, catchment area, etc)?