Hydrographs
A hydrograph shows two graphs 鈥 a bar chart showing rainfall and a line graph showing discharge from before, during, and after the rain storm.
A hydrograph shows you the ways in which a river is affected by a storm. This helps us to understand The volume of water in a river passing a point in a given time. Measured in cumecs (cubic metres per second). patterns of a particular The area of land that is drained by a river and its tributaries. and helps to predict flooding and plan flood prevention measures.
As you can see in the graph below, the peak rainfall is the time of highest rainfall. The peak discharge is the time when the river reaches its highest flow. There is a delay because it takes time for the water to find its way to the river. This is called lag time.
The normal (base) flow of the river starts to rise when run-off, ground and soil water reaches the river. This is shown on the hydrograph as the rising limb.
The falling limb shows that water is still reaching the river but in decreasing amounts. The run-off/discharge of the river is measured in cumecs 鈥 this stands for cubic metres per second. Moisture that falls from the air to the ground. Includes rain, snow, hail, sleet, drizzle, fog and mist. is measured in millimetres (mm).