Runoff

Only a small proportion of Australia’s rainfall becomes runoff; this is due to the high rate of evapotranspiration, temporal and spatial variability in rainfall intensity and frequency and generally flat topography across most of the continent. Only northern Australia and parts of Tasmania, where annual rainfall is much higher, have high runoff volumes. Deep drainage (or groundwater recharge) has a similarly uneven distribution due to these biophysical characteristics.

From the national perspective the Water 2010 model created by Bureau of Rural Sciences, provides estimates of the runoff, deep drainage (recharge), rainfall and evapotranspiration for each catchment. For AWR 2005 the results from this model have been used for many of the water balances as no other information on runoff was available. It should be noted though that the results from the model have a reliability category of either B (error of ±25%) or C (error of ±50%) for most areas.

A summary of the total available water, defined here as the available runoff and deep drainage, is given below for each state and territory. Based on the Water 2010 model the surface water runoff and groundwater infiltration totalled about 292,000 GL in 2004-05. Some users of these estimates have expressed concern over the datasets used in tropical areas where it is believed to over-estimate runoff. However, in the temperate south-eastern Australia and south-western Australia, comparisons with runoff values from other methods show good correlation.

Total runoff and recharge from the Water 2010 model by jurisdiction for 2004-05 (Source: Bureau of Rural Sciences)

  State/Territory
  ACT NSW NT QLD SA TAS VIC WA Australia
Runoff (GL) 149 30,266 47,151 93,018 1,285 32,084 14,266 24,560 242,779
Deep drainage (recharge) (GL) 29 7,133 8,382 13,599 2,925 1,228 2,695 13,182 49,174
Total Runoff and Recharge (GL) 178 37,399 55,533 106,618 4,210 33,312 16,961 37,742 291,953

For 2004–05, continental runoff was estimated to be approximately 242,799 gigalitres. Runoff was greatest in the Gulf of Carpentaria drainage division (62,060 gigalitres), the Timor Sea drainage division (50,240 gigalitres), and the North-East Coast drainage division (40,210 gigalitres). More than 65 per cent of runoff in Australia comes from far northern Australia and coastal Queensland; only 6 per cent of Australia’s runoff comes from the Murray-Darling Basin, which is where more than 50 per cent of Australia’s water consumption occurs. This demonstrates the imbalance between runoff and water use across the northern and southern areas of Australia.

Australia’s annual average distribution of runoff from each drainage division in 2004-05 (Source: Bureau of Rural Sciences)

Australia’s runoff volumes in 2004-05 from each drainage division (Source: Bureau of Rural Sciences)

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A comparison of runoff results for 2004–05 with estimates of average runoff shows modelled runoff for 2004–05 being below average in most drainage divisions. Two drainage divisions, Western Plateau and Bulloo-Bancannia, were predicted to have generated less than 25 per cent of mean annual runoff in 2004–05. Runoff in the northern drainage divisions, including the Timor Sea, Gulf of Carpentaria, North East, and Lake Eyre drainage divisions, was estimated to be between 51 per cent and 70 per cent of mean annual runoff. Runoff from Tasmania was estimated to be two thirds of the annual average. The South Australian Gulf, Murray-Darling Basin and South East drainage divisions were slightly below average, with 2004–05 runoff at between 71 per cent and 90 per cent of the long-term average. Only in the South-West Coast was the volume of runoff in 2004–05 close to the long-term average.

Australia’s runoff in 2004-05 compared to the mean annual runoff from each drainage division (Source: Bureau of Rural Sciences)

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The impact of the drought since 1997 is seen in the reduced inflows to major water storages across many areas of Australia, in particular the eastern states and the south-west of Western Australia. Examination of the graph below that shows the annual inflow into dams supplying the Perth urban area, highlights the significant reduction in storage inflows in Perth since the 1970s. This step change in climate and runoff over this time is significant. Rainfall in the Perth region has decreased by 14 per cent and runoff by 48 per cent between 1975 and 1996.

Recent planning in Victoria for the Central Region Sustainable Water Strategy similarly acknowledges that a similar step change may have occurred since 1997 in that region. Inflow to Lal Lal reservoir, which supplies Ballarat, is depicted for the past 75 years. This also highlights a step-wise shift in reservoir inflows in 1997.

The inflows into the Canberra storages have declined steadily since the mid 1990s, however these low inflows in recent years are not the lowest on record as similarly low inflows have occurred a few times since 1880. The problem with the recent drought and low flows is that over the last 100 years, development of the resource has occurred during wetter periods and so many users are reliant on higher rainfall and higher runoffs than currently being experienced.

Annual inflow (from May to April) into dams providing water to the Integrated Water Supply Scheme in Western Australia (Source: WA Water Corporation)

Annual inflow (from May to April) into dams providing water to the Integrated Water Supply Scheme in Western Australia (Source: WA Water Corporation)

Inflows to Lal Lal Reservoir (Victoria) over the past 75 years (Source: Dept of Sustainability and Environment, Victoria)

Inflows to Lal Lal Reservoir (Victoria) over the past 75 years (Source: Dept of Sustainability and Environment, Victoria)

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

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Last Updated 22/06/2007