Calculating Lag Time on a Hydrograph

Lag time is the interval between the time of concentration (when runoff from the farthest point of the watershed reaches the outlet) and the time of peak flow (when discharge hits its maximum). To calculate it:

1. Determine the time of concentration using methods like the Kirpich formula or Rational method.
2. Identify the time of peak flow by locating the highest point on the hydrograph.
3. Subtract: Lag time = Time of peak flow − Time of concentration.

For example, if the time of concentration is 2 hours and peak flow occurs at 4 hours, the lag time is 2 hours.