A hydrograph shows us the difference between the peak rainfall and peak discharge. This difference is called lag time . The greater the lag time, the less chance there is of a flood; while a short lag time shows that water has reached the river channel quickly.
A rise in discharge on the graph is called the rising limb ; the falling limb shows a decrease in discharge.
The flood hydrograph below shows:
rainfall peaking at six hours
discharge peaking at 11 hours
lag time - the time between peak rainfall and peak discharge - is five hours.