The first reported analysis of evaporation trends in the southern hemisphere has identified significant changes in the cycle of water through the Australian landscape over more than 30 years
A report of the study, by Cooperative Research Centre for Greenhouse Accounting scientists Dr Michael Roderick and Professor Graham Farquhar, has been published by the International Journal of Climatology.
The report details analysis of records of both rainfall and pan evaporation across Australia from 1970 to 2002. (Pan evaporation refers to the measurement of evaporation from specially built pans, a world-wide practice to determine potential evaporation from the landscape.)
The researchers found that averaged across 61 sites analysed in Australia, pan evaporation has been decreasing by about 4 mm a year.
Most scientists have long expected that as global average temperatures rise because of the enhanced greenhouse effect, pan evaporation would also increase. Dr Roderick and Professor Farquhar say this expectation was based on the assumption that as air temperatures increased everything else remained constant. However, vapour pressure had increased with global temperatures, so relative humidity had remained about the same. As a result, the increase in average temperatures had little impact on potential evaporation.
Averaged across the 61 sites, there was no significant change in rainfall over the period. However, the researchers did note large variability in rainfall. The Bureau of Meteorology has separately identified a weak increase in mean annual rainfall across Australia over the 20th century, dominated by large year-to-year variations and despite decreases in some regions. It sees rainfall trends from 1950 to 2002 – decreasing rainfall across much of eastern Australia and the south-west of Western Australia, and increasing falls in the north-west and central Australia – as demonstrating the impact of inter-decadal variation in rainfall in Australia.
The results of the new Australian study mirror earlier findings of reduced potential evaporation in the northern hemisphere.
Dr Roderick and Professor Farquhar said that the general trend for water-limited sites (such as much of Australia) was that a decline in pan evaporation at constant rainfall led to increased plant growth.
They concluded that, given the unchanged relative humidity, the observed reduction in pan evaporation could be caused only by a decline in sunlight reaching the earth’s surface (because of more cloud and, possibly, pollution) or reductions in wind-speed. “In the northern hemisphere, decreased sunlight has proved to be an important component of the decrease in pan evaporation. However, declines in wind speed may also play a role.”
Dr Roderick and Professor Farquhar said the findings highlighted the need for the impacts of climate change on evaporation and rainfall to be reassessed. They are now embarking on further analysis of the causes of the reductions in pan evaporation.