My problem is: Why is the difference of external outflow volume so significant, 7.733 and 14.791? Shouldn't the two values be close, if not exactly the same? Did I set up something wrong in Option 1 which includes gutter flow? Pleseae see the attachments of two options for reference. Thanks a lot.
Option 1 allows flows to travel along the curb and gutter system (which possibly has different properties from the pipe network and in some case increases TOC) prior to it entering the pipe network below. Thus, longer TOC for the time span would reduce outflow volume. Option 2 allows all flows to enter directly into the system without having to travel through a curb and gutter system first. This results in a single TOC which would be based on the parameters of your pipe network. If the two time spans are identical in both options it is possible that option 1 show less volume than option 2.