Bega Valley

Rainfall Analysis

The main rainfall observing sites used in this analysis were Bega - Newtown Road (Bureau of Meteorology Station No. 069002) and Eden Caravan Park (No. 069015). Data from other nearby stations were used to calculate values for months with missing data at these two sites (see Appendix 2 for further information).

Note that in the following analyses rainfall totals for individual months or years (where the year is defined as the 12 months from July to June) are compared to the long-term average (or climatology) from 1961-1990. A 30-year period is used to calculate climatological averages to conform with World Meteorological Organisation practice; this takes into account year-to-year variations and less frequent events such as El Niño or La Niña. This particular 30-year period is a standard climatological average period used internationally.

As shown in the table below, Bega and Eden were both drier than average in three of the four years in the period 2004/05-2007/08. The driest year at Bega was 2007/08, when rainfall was 32% below the long-term average; at Eden the driest year was 2004/05 with 15.6% less than average rainfall. The wettest year in the last four years was 2006/07 at Bega (5.7% above average rainfall) and 2005/6 at Eden (12.7% above average).

Table 1. Annual Rainfall Statistics at Bega and Eden, 2004/05-2007/08
Rainfall Statistics Bega Eden
2004/5 2005/6 2006/7 2007/8 2004/5 2005/6 2006/7 2007/8
Annual Total* (mm) 733.6 753.1 924.4 595.0 739.6 986.8 849.2 751.2
Anomaly (mm) -140.8 -121.4 49.9 -279.4 -136.2 111.0 -26.6 -124.6
Anomaly (%) -16.1 -13.9 5.7 -32.0 -15.6 12.7 -3.0 -14.2
Mean (1961-1990) (mm) 874.5 875.8

*Note: annual totals were calculated for the 12 months July-June.

Monthly Rainfall, 2004-2008

A comparison of monthly rainfall totals with the long-term monthly average rainfall over this period at Bega and at Eden (Figure 1a and b) shows that there have been a considerable number of months of below average rainfall interspersed with months of extremely heavy rainfall, particularly at Eden. The wettest months occurred mostly as a result of the development of east coast low pressure systems that tended to affect Eden (on the coast) rather than Bega (further inland). The wettest months were February 2007 at Bega (334 mm) and in February 2008 at Eden (265.5 mm) (Figure 2). The prolonged period of below-average rainfall in 2006/07 was associated with an El Niño event that coincided with a negative phase of the Pacific Decadal Oscillation, which tends to amplify the impacts of El Niño Southern Oscillation events on rainfall in eastern Australia.

Figure 1. Bega (a) and Eden (b) monthly rainfall totals (blue bars) compared with the long-term monthly mean rainfall shown in yellow (all in mm), July 2004-June 2008.

a Bega (a) monthly rainfall totals (blue bars) compared with the long-term monthly mean rainfall shown in yellow (all in mm), July 2004-June 2008.

b Eden (b) monthly rainfall totals (blue bars) compared with the long-term monthly mean rainfall shown in yellow (all in mm), July 2004-June 2008.

Figure 2. Monthly rainfall at Bega (green) and Eden (blue) (mm) for the period July 2004-June 2008.
Monthly rainfall at Bega (green) and Eden (blue) (mm) for the period July 2004-June 2008.

Of the 48 months in this period, 34 (71% of the record) had above-average rainfall at Bega, and 33 (69%) at Eden (Figure 3). Conditions in late 2004 were around average, with generally dry conditions then predominating for the rest of the period; Bega was drier than Eden. Rainfall is generally similar at the two locations, although in wetter months Eden tends to have more rain than Bega; and in drier months Bega often has less rain than Eden. The two-year period 2006/07 to 2007/08 was particularly dry apart from winter 2006 and late spring/summer 2007/08. The driest individual month at Bega was October 2006 with 7.2 mm (long-term average 72.8 mm); at Eden the driest month was May 2008 when no rain was recorded (long-term average 68.4 mm). The monthly rainfall total of 334 mm in February 2007 at Bega was the sixth wettest February on record since1888, and the 265.5 mm in February 2008 at Eden was the ninth wettest February on record since 1870.

Figure 3. Bega (green) and Eden (blue) monthly rainfall totals expressed as percentage anomalies, or deviations from the long-term monthly average, July 2004-June 2008.
Bega (green) and Eden (blue) monthly rainfall totals expressed as percentage anomalies, or deviations from the long-term monthly average, July 2004-June 2008.

The results of the generally dry conditions at Bega, and the less severe rainfall deficits at Eden, during the 2004-2008 period can be seen in Figure 4. Accumulated monthly rainfall totals at Bega went into deficit (relative to the long-term average rainfall) in October 2005. This deficit increased considerably during 2006, was halted in winter 2007, and then increased again from late 2007 into 2008. At Eden accumulated rainfall totals fluctuated around the norm from 2004-2005 to September 2006, after which the rainfall deficit increased apart from some individual months of heavy rainfall. The total deficits for the four-year period were 528.3 mm at Bega and 165.6 mm at Eden. These deficits represent 60% of the long-term average annual July-June rainfall at Bega and 19% at Eden.

Figure 4. Accumulated Bega (a) and Eden (b) monthly rainfall totals (in mm), expressed as anomalies (differences between the actual amount of rainfall that accumulated from month to month during the period July 2004-June 2008, and the amount that would have accumulated if average rainfall had been received each month where the averages are based on the 1961-1990 period).

a Accumulated Bega (a) monthly rainfall totals (in mm), expressed as anomalies (differences between the actual amount of rainfall that accumulated from month to month during the period July 2004-June 2008, and the amount that would have accumulated if average rainfall had been received each month where the averages are based on the 1961-1990 period).

b Accumulated Eden (b) monthly rainfall totals (in mm), expressed as anomalies (differences between the actual amount of rainfall that accumulated from month to month during the period July 2004-June 2008, and the amount that would have accumulated if average rainfall had been received each month where the averages are based on the 1961-1990 period).

The climatological context for rainfall at Bega and Eden

The long-term perspective

As is typical of the region, Bega and Eden have experienced large fluctuations in annual (July-June) rainfall, including several extended periods of above- and below-average rainfall since records began. In general, annual rainfall was lower and less variable early in the 20th century at Eden, followed by a period of greater variability (Figures 5 and 6). The dry period evident in the last part of the records at both Eden and Bega is reflected in other parts of the record such as the early 20th century and the period 1937-45, and therefore lies within the range of natural variability.

Figure 5. Annual (July-June) rainfall at Bega from 1885 to 2008 depicted as deviations from the long-term mean. A 5-year running mean is superimposed to highlight wetter and drier periods. Deviations were calculated as the difference between the rainfall for each year and the mean for the period 1961-1990 (scale in mm).
Annual (July-June) rainfall at Bega from 1885 to 2008 depicted as deviations from the long-term mean. A 5-year running mean is superimposed to highlight wetter and drier periods. Deviations were calculated as the difference between the rainfall for each year and the mean for the period 1961-1990 (scale in mm).

Figure 6. Annual (July-June) rainfall at Eden from 1870 to 2008 depicted as deviations from the long-term mean. A 5-year running mean is superimposed to highlight wetter and drier periods. Deviations were calculated as the difference between the rainfall for each year and the mean for the period 1961-1990 (scale in mm).
Annual (July-June) rainfall at Eden from 1870 to 2008 depicted as deviations from the long-term mean. A 5-year running mean is superimposed to highlight wetter and drier periods. Deviations were calculated as the difference between the rainfall for each year and the mean for the period 1961-1990 (scale in mm).

Figure 7. Comparison of accumulated monthly totals between Bega and Eden (in mm), expressed as anomalies (differences between the actual amount of rainfall that accumulated from month to month during the period July 2004-June 2008 (a) and July 2000-June 2008 (b), and the amount that would have accumulated if average rainfall had been received each month where the averages are based on the 1961-1990 period.)

a Comparison of accumulated monthly totals between Bega and Eden (in mm), expressed as anomalies (differences between the actual amount of rainfall that accumulated from month to month during the period July 2004-June 2008 (a) and July 2000-June 2008 (b), and the amount that would have accumulated if average rainfall had been received each month where the averages are based on the 1961-1990 period.)

b Comparison of accumulated monthly totals between Bega and Eden (in mm), expressed as anomalies (differences between the actual amount of rainfall that accumulated from month to month during the period July 2004-June 2008 (a) and July 2000-June 2008 (b), and the amount that would have accumulated if average rainfall had been received each month where the averages are based on the 1961-1990 period.)

There is a distinct difference in the accumulated rainfall deficits between Bega and Eden in the 2004-2008 period (Figure 7a), particularly after October 2005. While dry conditions predominated through 2006/07, particularly at Bega, conditions were alleviated at Eden by heavy rainfall in some months. Wetter months had less effect on the accumulated deficit at Bega. During this 48-month period Eden received more rainfall than Bega in 32 months, while Bega received more rainfall than Eden in 15 months. Overall Eden received 320.7 mm of rain more than Bega over this time.

The extent of the persistent dry period over the region is evident in accumulated rainfall over the longer period 2000-2008 (Figure 7b). The accumulated deficit that had developed between 2000 and 2004 continued to increase between 2004 and 2008, with the difference between the two sites also increasing. During this period Eden received more rainfall than Bega in 67 months (70%) while Bega received more rainfall than Eden in 28 months (30%). Overall Eden received 934.1 mm of rain more than Bega during this eight-year period. The total deficits over this period were 1608.2 mm at Bega (184% of Jul-Jun annual total) and 590.2 mm at Eden (67.4% of Jul-Jun annual total).

One possible explanation for the difference in rainfall between the two locations is that there were two successive El Niño events during this period (2002/03 and 2006/07), without a compensating wet period between them. In dry years rainfall tends to be particularly impacted by local conditions. The coastal location of Eden means that it may receive rainfall from weather systems that are confined to the coast and do not penetrate inland to reach Bega. During generally wetter years more widespread rainfall may occur across the region.

  • Rainfall at Eden in the late 19th and early 20th century was generally below average (to about 1950), with the notable exception of a short wet period in the 1930s. Since then annual rainfall totals have been more variable.
  • Several wetter-than-normal years around 1950 and 1960 produced generally wetter periods then.
  • A generally drier period around 1980 was followed by slightly above-average rainfall until the early 1990s. Since 1993 conditions have been generally drier than average.

The wettest year (July-June) on record at Eden was 1948/49 (1819 mm); the driest year was 1993/94 (363 mm).

  • Rainfall at Bega has been more variable than that at Eden, although the record reflects similar extended dry periods to those at Eden (around 1940, the late 1960s, and around 1980).
  • Wetter conditions occurred around 1950, 1960 and in the mid-1979s.
  • Since 1993 rainfall has generally been below average at Bega.

The wettest year (July-June) on record at Bega was 1933/34 (1867 mm); the driest year was 1885/86 (354 mm).

About the data

Rainfall appendix

 

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