I am new to the site and hoping to get a few questions answered about the newest version of SWMM. I am currently a student working on a design project and we have choosen SWMM to model stormwater mangement in a cluster resdential development. I started using HEC HMS and then switched to SWMM a little later and I used the curve number method to generate my runoff depth for pre development and post development conditions. For some reason the pre development conditions are extremely off, HEC HMS gave me about 0.62 inches of runoff and SWMM gave me about 1.5 inches of runoff (there is no impervious area pre development). The post development was much closer (only about 0.2 inches off).
I've checked the curve numbers to make sure they are right and other parameters (area, width, rainfall, etc.) are the same.
Has anyone experienced this problem before? Any insight or suggestions would be greatly appreciated!