What does lag time mean in geography?

In geography, lag time refers to the length of time the water will take to find its way to the river after a period of heavy rainfall.

Below are other terms related to "lag time" (see hydrograph on the side):

http://www.bbc.co.uk/bitesize/higher/geography/physical/hydrosphere/revision/1/

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Hydrographs are graphs which show river discharge over a given period of time and show the response of a drainage basin and its river to a period of rainfall.

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.

The lag time is the time difference between the peak precipitation and and the peak discharge. A long lag time indicates that it’s taking a long time for precipitation to enter the river. Conversely, a short lag time indicates that the precipitation is entering the river fairly quickly. See more at Geographyas.info.

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