Hydrographs

You are expected to be able to construct, describe and explain hydrographs. A hydrograph shows two graphs - rainfall (in bars) and discharge (in a line).

Hydrograph

The peak rainfall is the time of highest rainfall. The peak discharge (the time when the river reaches its highest flow) is later because it takes time for the water to find its way to the river (lag time) . The normal (base) flow of the river starts to rise (rising limb) when run-off, ground and soil water reaches the river. Rock type, vegetation, slope and situation (ie is this an urban river?) affect the steepness of this 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 - this stands for cubic metres per second. Precipitation is measured in mm - this stands for millimetres.