Daily evaporation rates were below average in 2000, slightly above average in 2001, and more than 10% above normal in 2002 (see Table 1). These annual rates are driven largely by solar radiation (measured as sunshine hours), although wind, temperature and humidity can also play a role. In general, we expect evaporation rates at land-based sites to increase if conditions are sunny, hot and windy; and to decrease if conditions are cooler or the air is moist. The above-average evaporation rates in 2002 coincided with dry, hot and windy conditions, and longer than normal sunshine hours.
|Table 1. Annual evaporation statistics in the ACT, 2000–02|
|Annual mean (mm)||4.3||4.7||5.1|
Monthly average evaporation rates at Canberra Airport were below the long-term mean in most months from January 2000 to April 2002 (Figures 1 and 2), and then increased rapidly through winter and spring 2002. They returned closer to normal during late summer 2002–03. The spring and summer months of all three years show the largest wind speed anomalies; conditions were closer to normal during autumn and winter, although even the winter of 2002 experienced unusually windy conditions.
Figure 1. Monthly evaporation anomaly at Canberra Airport, January 2000 to June 2003 compared with the long-term mean (1967–96). (a) Departure from the long-term mean in mm/day. (b) Standard deviation from the long-term mean for each month.
Approximately half of all months in the period January 2000-June 2003 recorded below-average evaporation rates (19 of a total of 42 months). The largest negative evaporation anomaly occurred in November 2000, when the monthly average of 4.6 mm/day was 1.9 mm/day below the long-term mean (Figure 1a). However, since the variability of evaporation in spring and summer is higher than in winter, the lowest standardised evaporation rate was recorded in July 2001 (Figure 1b), when average daily evaporation was 1.3 mm/day.
The highest monthly average evaporation rate of 9.1 mm/day was experienced in January 2002; the largest positive anomaly in evaporation was that of November 2002, when the rate of 8.6 mm/day was 2.1 mm/day above the mean (Figure 1a). Once again, the largest standardised evaporation anomaly was in winter, in this case July 2002 with 2 mm/day (Figure 1b).
As is expected, evaporation rates are anomalously low in the wettest months, such as November 2000 and February 2002, and unusually high in the hottest, driest months (e.g. November 2002).
Evaporation rates at Canberra Airport vary about the long-term mean with no clear pattern (Figure 2). There is a slight downward trend in annual average evaporation since the early 1980s, but this is not sufficiently marked to be significant.
Figure 2. Annual averages of daily evaporation at Canberra Airport from 1967 to 2002. The long-term (1967–96) average is indicated by the horizontal line through the centre of the graph (scale in mm/day).
The year with the highest evaporation rates was the El Niño year of 1982, when evaporation averaged 5.6 mm/day (0.9 mm/day above the mean). Evaporation was lowest in 1992, with an annual average of 3.6 mm/day (1.1 mm/day below the mean); 1992 was a wet, cooler than average year.
Analysis, graphs and tables by Dr Janette Lindesay, School of Resources, Environment and Society, The Australian National University.
Evaporation has been measured for an extended period of time at only one official Bureau of Meteorology station in the ACT, located at Canberra Airport. The record there begins in 1967, and there are 34 complete years of observations. Other organisations (e.g. ACTEW, CSIRO and ACT Parks and Wildlife) may make independent measurements relevant to the area; their data may be made available independently, or through the Bureau of Meteorology. Most of these records are of considerably shorter duration, but often contain invaluable information for otherwise sparsely-monitored areas.
Information on Bureau of Meteorology weather stations and climate data can be obtained from:
|Organisation||National Climate Centre|
|Phone (BH)||(03) 9669 4082|
|Fax||(03) 9669 4515|