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Currently when we model a combined sewer subcatchment, the contributing area is 100% of the total area. For a Sanitary sewer network, we would reduce the contributing area to reduce the runoff. However this also has the effect of reducing the pervious area, and therefore the amount of water infiltrating to the Ground Infiltration Soil Store to the point that there is not enough volume available to properly calibrate the ground infiltration.
Our workaround has been to artificially increase the Pervious Runoff Surface by factors of 25-1000%, but obviously this has the consequence of that value not being relatable to the actual land use.
I would like an option to have the contributing area to apply only to certain Runoff Surfaces, so we can maintain reasonable runoff surface land use values, but still limit runoff and have the groundwater being recharged from the rainfall.
I've had it suggested to alter the Fixed Runoff Coefficient in the Runoff Surface for impervious surfaces and leave the contributing area at 100%, however that would require a whole mess of new Runoff Surfaces for each calibration region and new Land Use profiles to tie them all together.
