Groundwater recharge

Recharge is the inflow of water to the groundwater system from the surface. Infiltration of precipitation and its movement to the watertable is one form of natural recharge. Modelled deep drainage (water moving to below the root zone) shows a similar spatial pattern to runoff across the country, but with smaller magnitudes than runoff (up to about 250 mm each year for recharge as compared with up to about 5000 mm each year for average annual runoff). Spatially, deep drainage is highly variable and dependent on soil type, vegetation/land use, and topography, amongst other factors.

Deep drainage estimates are normally much higher than actual recharge rates to aquifers as they include water that infiltrates the soil and may only temporarily be stored before being removed either through evaporation or flow to streams from perched clay layers.

While modelled runoff coefficients (modelled average annual runoff as a percentage of average annual rainfall) for Australian drainage divisions range from 0.9% on the Western Plateau to 49.7% in Tasmania, modelled drainage coefficients (modelled average annual deep drainage as a % of average annual rainfall) only range between 1.5% and 1.8% for all drainage divisions except the South Australian Gulf and the South-West Coast, which are slightly higher at 2.5% and 3.2% respectively.

This means that while modelled runoff and deep drainage follow a broadly similar pattern at high resolution (1km x 1km grid), at a national scale there are significant differences in the distribution of total drainage between drainage divisions compared with runoff distribution. The map below also highlights the higher than expected values in northern and south-eastern Australia, including Tasmania.

The deep drainage model, involves assignment of a drainage rate constant on the basis of soil texture, with modification to account for cultivation. The estimates thus depend upon rainfall, soil type (sand or clay) and land use (cultivation), which is a gross simplification since drainage clearly depends on many other factors including vegetation type and management, topographic position and so on. Actual drainage will vary more among soil types than indicated by a simple clay/non-clay classification.

Antecedent soil moisture must also be considered when using deep drainage figures as it does not take into account the “wetting up” of the soil profile required prior to recharge occurring. This can reduce the volume of deep drainage that actually reaches the watertable and other aquifers considerably.

These drainage estimates should therefore be treated with appropriate caution. At a regional scale, however, they provide some insight to the relative amount of water making its way into the water table, and potentially to shallow and deep, confined and unconfined aquifers.

Australia’s mean annual deep drainage (recharge to groundwater) in each drainage division in 2004-05 (Source: Bureau of Rural Sciences)

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Modelled deep drainage for 2004–05 was below average in most drainage divisions. The map below shows that the northern and eastern drainage divisions—including the Timor Sea, Gulf of Carpentaria, North East, South East, Murray-Darling Basin and Tasmania drainage divisions—were estimated to have had between 71 per cent and 90 per cent of long-term average annual deep drainage. Three drainage divisions, Western Plateau, Lake Eyre and Bulloo-Bancannia, were estimated to have had deep drainage amounts of between 51 per cent and 70 per cent of the long-term annual average. Only the South West Coast drainage division was estimated to have had average amounts of deep drainage during 2004–05.

Australia’s deep drainage (recharge to groundwater) in 2004–05 compared to the mean annual deep drainage in each drainage division (Source: Bureau of Rural Sciences)

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More detailed discussion of groundwater recharge is provided in the Water Availability Level 2 Report – National Perspective, available from the Publications page.

 

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This website was last updated in 2007. It is no longer being maintained but remains here as an archive for information.20/06/2007