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Climate Variability and Change (continued)

Average evaporation rate [A Indicator 1.18]

Evaporation is the transfer of water vapour from a free water surface to the atmosphere at temperatures below the boiling point of water. Evaporation from a surface depends on meteorological parameters such as net radiation, wind speed, temperature and humidity of the air, and also on the physical characteristics of the evaporating water surface. Evapotranspiration is the collective water vapour transfer from both water surface and vegetation and is a large component of the water balance. Almost 90% of rainfall that falls on the continent is returned through Evapotranspiration to the atmosphere. The remaining portion returns to the sea and some is stored in groundwater.

Evapotranspiration is very difficult to measure. Potential evaporation, or the evaporation that would occur from a completely wet surface, used to be measured from US Class A pans, which are standard pans to measure evaporation, at certain locations in Australia.

Evaporation and evapotranspiration data have undergone the same level of quality control as applied to rainfall and temperature data. Figure 33 shows the spatial pattern of annual average potential evapotranspiration based on Class A pan (guarded or protected by wire mesh from birds etc.) provided by the BoM (2001) . Annual average potential evapotranspiration varies from under 800 mm in western Tasmania to over 3400 mm in central Australia (in the desert regions of Western Australia and Northern Territory). Evapotranspiration values are lower over the Great Dividing Range in Victoria and New South Wales. In the wet tropical regions, annual average values vary from 2200 to 2800 mm. There are significant seasonal differences in potential evapotranspiration between summer and winter seasons. Trends and year-to-year variability of evapotranspiration rates are closely associated with trends and variability in solar radiation, surface temperature, wind patterns and speed, and rainfall.

Implications

Projected increasing temperatures and reduction in rainfall over most parts of Australia under enhanced greenhouse conditions could lead to an increase in potential evapotranspiration. This has implications for future sustainable agricultural and human settlement via an increase in water demand for crops, as well as increase heat stress for human and livestock. It could lead to changes in agricultural patterns in marginal areas.

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