What does the lag time showon a hydrograph?

The lag time in a hydrograph represents the time it takes for the peak flow to arrive at the outlet of the watershed or draining system after the occurrence of the rainfall event. It is the difference between the time of peak rainfall intensity and the time of peak discharge. The lag time is indicative of the watershed's storage capacity, infiltration rates, vegetation cover, slope, and drainage network characteristics.

Here's how the lag time is shown on a hydrograph:

1. Time Interval on the X-Axis: The x-axis of a hydrograph typically represents time, usually in hours or days.

2. Flow Discharge on the Y-Axis: The y-axis represents the flow discharge or water discharge, which is the volume of water flowing past a specific point in the watershed per unit time.

3. Rainfall Event: The hydrograph may include an overlay or annotation indicating the timing and intensity of the rainfall event.

4. Rising Limb of the Hydrograph: The rising limb of the hydrograph represents the increase in flow discharge from the beginning of the rainfall event to the point of peak discharge.

5. Lag Time: The lag time is measured from the start of the rainfall event (or the point where the flow discharge begins to rise significantly) to the peak flow. It is typically indicated as a vertical line or shaded area on the hydrograph.

By examining the lag time on a hydrograph, hydrologists and water resource managers can gain insights into the watershed's response characteristics and develop appropriate strategies for managing water flow, flood control, and water conservation measures.