A hydrograph shows how a river responds to a period of rainfall.
The lag time can be short or long depending on different factors. For example, if there is no vegetation in an area, the water runs off into the river quicker, therefore it would have a short lag time. Alternatively, if there is plenty of vegetation in the area, the lag time would be longer as the plants would intercept the rainfall. A shorrt lag time means water is reaching the river quickly, so there is a greater chance of a flood.
Factors influencing lag time include: