Tilake's explanation of dry weather flow time patterns in SWMM 5 is essentially correct. The patterns are simply user-assigned factors that multiply a node's "average" dry weather inflow. They do not necessarily have to sum to 1.0, and the "average" DWF does not have to be the actual average. Maybe we should call it a "nominal" value instead.

The reason that the total DWF inflow reported in the results for Example 3 in the Beta A release did not agree with the value computed by hand had to do with the way that Beta A was updating the simulation clock time for variable time steps, which was used in this example. If you turn off the variable time step feature (by setting the Courant Factor to 0 in the Dynamic Wave Options) the program will report a correct total DWF inflow.

This anomaly with the clock time for variable time step simulations has been corrected in Beta B, which should be posted on the EPA SWMM web site sometime next week.

Lewis Rossman
U.S Environmental Protection Agency
rossman.lewis@epa.gov