Runoff

Comparing the runoff map with the rainfall maps shows that runoff is not proportional to precipitation, largely due to the demands of evaporation and transpiration. For example, low rainfall and high temperatures, such as occur in the north west of the state during summer, combine to reduce runoff to negligible levels.

"Nevertheless, the highest runoff is from the high rainfall areas. Each local 'high' on the runoff map is the source of several important rivers.

From almost one third of the state, the north-western section with under 500 mm of rain, there is virtually no reliable surface runoff." (Atlas of Victoria, pp.32-33)

Related Pages on Victorian Resources Online

The amount of water that runs off the land surface to streams, rivers and lakes, or that infiltrates to groundwater storages, depends on a number of local factors.

The major factors are:

rainfall intensity and duration;
losses through evaporation and transpiration;
the type of soil and rock;
the amount of water already held within the soil and rock (the moisture content);
the slope of the ground (landform);
and the vegetation type and density (land use).

Sources:

R.E. Holmes, 1982, Chapter 8:Water Resources, IN: J.S. Duncan (Ed.), Atlas of Victoria
Water Victoria: An Environmental Handbook, Dept. Water Resources, 1989
Water Victoria: The Next 100 Years, Dept. Conservation and Environment, 1991

Median Annual Runoff Map
This map shows median annual runoff, expressed as a depth of water across the catchment. This enables comparison of runoff with rainfall. It has been adapted from a map in the 'Atlas of Victoria' (1982).

"North of the 10 mm line, runoff is negligible, rainfall is generally below 400 mm and potential losses from evapotranspiration are high - over 2,000 mm a year. Areas of high runoff typically have over 1,400 mm of rainfall and potential evaporation about 1,400 mm."(Atlas of Victoria, p.32)

Flow Indicator, River Murray weir, 1992 Photo: Steve Page

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