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Background

What is drought?

A good working definition of drought (see Druyan 1996b) is:

An extended period - a season, a year, or several years - of deficient rainfall relative to the statistical multi-year mean for a region.

This very roughly translates into: an extended period of below-average rainfall.

We still don't have a universal definition of drought. In Australia, drought is detected when the observed three-month total rainfall is in the lowest 10% of the long-term precipitation record. This is called 'meteorological drought' and it is determined by the 'Rainfall Deciles method' of Gibbs and Maher (1967). The Bureau of Meteorology website gives more information about climatic aspects of drought: see www.bom.gov.au/climate/drought .

Most, but not all, droughts in Australia are linked to the El Niño phase of the El Niño/Southern Oscillation phenomenon. (For more information, see www.bom.gov.au/climate/enso.)

Drought is not the same as aridity (which is the situation found in arid and semi-arid regions where annual rainfall is always expected to be small), though it has some similar effects.

At the start of a drought, the rainfall that is normally expected at that season fails to occur. The soil begins to dry out, there is no surface runoff, and no rain to recharge the groundwater. In the early stages of drought, temporary standing water bodies (such as ponds, wetlands) may dry up; and temporary flowing waters (ephemeral creeks) may shrink and turn into a series of separate pools.

As drought builds, water levels and volumes decrease in natural perennial waters. Flowing waters (rivers, streams) may flow only slowly for extended periods. Eventually the water stops flowing - a critical stage. Streams then become a series of pools and even these may disappear in severe drought. This is known as 'surface-water drought'.

It is important to recognise that in severe drought the levels and volumes of groundwaters will also fall. This is 'groundwater droughtâ', and it is poorly understood (out of sight, out of mind).

Groundwater drought diminishes the streams and wetlands that are normally kept wet by an inflow of baseflow (groundwater that flows from the ground into streams) during short dry spells.

When normal rainfall returns, meteorological drought typically breaks before surface-water drought and well before groundwater drought.

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Useful references:

Keywords:

rainfallView Frequently Asked Questions    View Bibliography
surfaceView Frequently Asked Questions    View Bibliography
groundwaterView Frequently Asked Questions    View Bibliography
wetlandsView Frequently Asked Questions    View Bibliography
flowView Frequently Asked Questions    View Bibliography
poolsView Frequently Asked Questions    View Bibliography
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