Freshwater in Average Year

Runoff is closely linked to rainfall and soil moisture since rainfall onto saturated soil leads to high runoff. Similarly rainfall onto an impervious surface (such as paved roads) also leads to runoff and urbanisation is often seen as increasing runoff.

Runoff is important to consider because it is linked to where water is flowing. Rainfall or melting snow/ice flows downwards either over the land surface or into the soil. The timing of rainfall or snowmelt is also important because a heavy rain shower occurring over a short period of time causes high runoff which can lead to flooding.

Also it is important for habitat development because changes in soil moisture means a particular region will not be able to sustain its habitat. By comparing a given month over a number of years extreme events can be detected, if the frequency of extremes events increases (such as floods or droughts) then changes in habitat are to be expected.