Always update CN method inter-event time when there is no rainfall #206
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@mathieudseguin, thank you for bringing this issue to our attention. If I am understanding you correctly, I think this is one where the documentation may have to be updated to reflect what is in the code. As long as there is some infiltration capacity left, the ponded depth + runon can be functionally considered rainfall and the time for that period should not count towards replenishing infiltration capacity. |
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No, there's something wrong with the implementation here. Once the soil moisture capacity is depleted, the routine falls into a state where the depth must be depleted by evaporation before the inter-event time can increase. The infiltration rate can only increase once a new rainfall event occurs and the variables are reset. This can lead to two subcatchments with slightly different CN but are otherwise identical to yield very different results. Here's the response from two catchments with the drying time set to 1 day and depression storage set to 5mm subject to two rainfall events 6 days apart. CN=86 CN=87 The soil capacity is exceeded during the event for the CN=87 subcatchment and since I haven't applied any evaporation, the depth is trapped and the inter-event time is never increased. |
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Thanks for the additional information and clarification. I understand your point now. I will add it to list of issues to address in future releases. |
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By the way @mathieudseguin, are you able to share the instance of the model you used to generate the results you shared? |
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Here's a copy of the model I used to generate the results. |
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The CN infiltration method implementation only increases the inter-event time if no depth is present on the catchment. In cases where the soil moisture capacity is exceeded with depth remaining on the catchment, the infiltration rate is set to the rate at the last time step, zero, and the depth is only depleted by evaporation.
The current implementation is inconsistent with the computational scheme outlined in the SWMM Hydrology Manual and can lead to cases where the variables are not reset as expected despite the time between rainfall events exceeding the input drying time.
Stormwater-Management-Model/src/solver/infil.c
Lines 966 to 978 in d4e11b1
To be consistent with the hydrology manual, the routine should be changed to something like this:
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