Depression Storage vs Retention Basins

Hello,

I am modeling a basin in RUNOFF and EXTRAN. The basin is approx. 200ac. and drains to a storage node. Within the basin are 4 small subdivisions that are approx. 20ac. each, and each with its own onsite retention facility. The remainder of the watershed consists of pasture and woods.

Due to the lack of data to define the conveyance system from the subdivisions, I have decided not to sub-divide the basin. My approach is to model the basin by converting the volumes in the retention facilities into a basin wide pervious depression storage.

Here are my questions to the list;

1. Is this a valid approach to modeling the basin?
2. Would this approach significantly effect the peak runoff volume and rate?

Thanks in advance for your comments.
Clay Campbell
cac@bdi-ae.com