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RAINFALL AND WATER AVAILABILITY
Based on long-term averages, the MDB receives 530,618 GL of rainfall annually, of which 94% is evaporated or transpired (table 1.9). Almost 2% of rainfall enters the soil and groundwater as deep drainage. In the MDB, approximately 23,609 GL or 4% of rainfall appears as run-off. Run-off is "the part of precipitation in a given area and period of time that appears as streamflow" (NWC 2007:87).
Proportionally more evapotranspiration (94% of rainfall) occurs in the MDB than for the whole of Australia (89%). This results in less rainfall being transformed into run-off in the MDB (4% of rainfall) compared with the whole of Australia (9%). This means that rainfall is less likely to become available for use from river basins in the MDB.
Regional distribution of rainfall
The spatial distribution of rainfall in the MDB is important as an indicator for vegetation growth - a key driver for agricultural production in Australia. In 2005-06, the highest levels of rainfall occurred in the south eastern and eastern areas of the MDB, declining towards the western and north western boundary as shown in map 1.10.
The distribution of rainfall across the river basins within the MDB is extremely variable. Based on long-term averages, annual rainfall (expressed in volume terms) is highest in the Condamine-Culgoa (85,755 GL), Murrumbidgee (48,691 GL) and Lachlan (46,120 GL) river basins (table 1.11). Rainfall is lowest in the Lake George (686 GL), Kiewa (2,374 GL) and Campaspe (2,658 GL) river basins.
Rainfall expressed volumetrically is influenced by the size of each river basin. Generally, larger river basins have higher rainfall volumes. Therefore, in area-adjusted (GL/km2) terms, river basins with the highest concentration of rainfall are the Kiewa (1.24 GL/km2), Upper Murray (1.18 GL/km2) and Ovens (1.06 GL/km2) river basins.
At the river basin level, more run-off occurs in the Upper Murray (4,472 GL), Murrumbidgee (3,831 GL) and Goulburn (2,686 GL) river basins compared with others. Run-off also exceeds 1,000 GL in the Ovens, Macquarie-Bogan, Lachlan, Namoi, Condamine-Culgoa and Border Rivers basins. Some parts of the MDB have negligible run-off, for example, the Paroo (1 GL), Benanee (3 GL) and Darling (6 GL) river basins (table 1.11).
The inter-relationship between rainfall, temperature, topography and geology affect the pattern of run-off in the Australia and the MDB (map 1.12). This is significant because it influences where water becomes available for use by society and the environment. Based on long-term averages, annual run-off levels are highest in the north eastern Victoria and south eastern New South Wales river basin sub-catchments, and lowest in the western and north western sub-catchments (map 1.13).
Rainfall anomalies 2000-01 to 2005-06
Rainfall anomalies measure the deviation from the long-term average (1960-1990) rainfall for given locations (BoM 2008). The rainfall anomalies across Australia are described in the following section and illustrated in maps 1.14 and 1.15, for the period 2000-01 to 2006-07.
In 2000-01, average levels of rain fell in the majority of the MDB. Northern and central Australia experienced more rainfall than normal.
The 2001-02 year was drier than average in the majority of the MDB, but not as severe as 2002-03. This pattern was similar to the trend over most of the rest of Australia, except in central Australia which was wetter than average.
The 2002-03 year was extremely dry throughout the MDB, particularly in the eastern and south eastern areas of the Basin where rainfall is usually highest (see map 1.10). The resulting reduction in catchment run-off severely affected water storage levels in large dams (see Chapter 3, graph 3.18). The reduced rainfall experienced in the MDB was reflected over most of eastern Australia in 2002-03. The northern part of the Northern Territory was wetter than usual, but far north Queensland was much drier.
The 2003-04 year was drier than average in the MDB, but not as dry as the previous two years. Northern and central Australia received more rainfall than normal.
The 2004-05 year was drier than average in the MDB, and geographically exhibited a similar rainfall anomaly pattern to 2003-04. Many areas of north eastern, north western, northern and central Australia were drier than normal.
The 2005-06 year was drier than average in the MDB, especially in the northern part of the Basin. North eastern, north western, northern, and central Australia experienced more rainfall than normal.
The 2006-07 year was extremely dry throughout the MDB, particularly in the eastern, northern and south eastern areas of the Basin where rainfall is usually highest (see map 1.10).