How do you work out lag time on a hydrograph?

The lag time is the difference between the time of concentration and the time of peak flow. To work out the lag time on a hydrograph, you need to identify the time of concentration and the time of peak flow.

1. Identify the time of concentration. The time of concentration is the time it takes for water to flow from the most remote point in the watershed to the outlet. It can be estimated using various methods, such as the Kirpich formula or the Rational method.

2. Identify the time of peak flow. The time of peak flow is the time when the discharge of water in the river is at its maximum. It can be identified by looking at the hydrograph and finding the highest point.

3. Calculate the lag time. The lag time is the difference between the time of peak flow and the time of concentration.

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