Technical Results
Lag Time

In addition to increasing the volume and rate of runoff, the addition of impervious surfaces to a watershed decreases the amount of time that it takes to see a response to a precipitation event at the watershed outlet. With LID, the goal is to preserve the predevelopment hydrologic function of a watershed, which includes the lag time. There are several different methods to quantify lag time.

Low impact development (LID) had a significantly greater centroid lag-to-peak, centroid lag, lag-to-peak and peak lag-to-peak times than traditional development. An example of one storm can be seen in Figure 1. Runoff lag times for the LID watershed were significantly greater than for the traditional watershed for small (< 25.4 mm) but not large (> 25.4 mm) storms; short duration (< 4hr) but not long duration (> 4 hr) storms; and low antecedent moisture condition (AMC) (< 25.4mm) storms but not high AMC (> 25.4 mm) storms.

More detail on lag times and runoff thresholds can be found in the following reference:
Hood, M., Clausen, J.C., and Warner, G. 2007. Comparison of stormwater lag times for low impact and traditional residential development. Journal of the American Water Resources Association, volume 43(4).

Figure 1. Stormwater response for a September 15, 2003, 11.2 mm rainfall.


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