The Australian Bureau of Meteorology currently monitors rainfall deficiencies over periods based on the timing of agricultural growing seasons or periods of peak percentage area of the country experiencing drought. Drought is defined in terms of rainfall deciles, with areas of the country in the lowest decile over a certain period regarded as being rainfall deficient, i.e., in meteorological drought. The use of the rainfall in the lowest decile range had been shown previously to correspond with incidences of drought occurrence (Gibbs and Maher 1967). More generally, a decrease in rainfall, or meteorological drought, is often a good first approximation of both agricultural and hydrological drought.
Recently the Bureau of Rural Sciences examined the utility of seasonal indices for monitoring and assessing drought (White 2006). The report concluded that all rainfall indices, when used in isolation, are inadequate for reliably assessing agricultural drought. Temperature and evapotranspiration were found to be important fields when determining the severity of drought in relation to agriculture and it is speculated that under global warming scenarios they may become more important. Furthermore, under global warming the nature of droughts themselves may be changing with a change in the intensity, duration and frequency of rainfall events.
Cai and Cowan (2008) examined the evidence of rising temperatures on inflows. They found that over the 2001-2007 period, inflows to Australia's longest river system, the Murray-Darling, reached historical lows. Furthermore, they find that increases in temperatures due to the enhanced greenhouse effect are linked to reductions in climatological inflows. Nicholls (2004) also raised the possibility that the enhanced greenhouse effect is increasing the severity of Australian droughts by raising temperatures and thus increasing evaporation. It is becoming obvious that measuring drought purely in terms of rainfall deciles may not be enough to service future needs of the user community.
The severe drought conditions Australia has been experiencing in recent years and the interaction of drought with climate change has triggered a broad ranging review of how drought is defined and the role of government. A recent study by CSIRO and the Bureau of Meteorology (Hennessy et al. 2008) found that the frequency and severity of droughts are likely to increase with the study finding that the existing trigger for government assistance, based on a one in 20-25 year event, is not appropriate under a changing climate. This is a complex issue as climate change is expected to trigger both a change in the frequency of drought (for example as the amplitude and frequency of El Niño events change) as well as a more general shift in the climate system to greater aridity through much of southern and eastern Australia.
The Bureau of Meteorology is thus undertaking a review of its current drought service with the aim to extend this to include some consideration of temperature. Consideration also must be made of the need to separate increasing aridity as a result of climate change from cycles of drought associated with climate variability.
Cai W.J. and T. D. Cowan (2008). Evidence of impacts from rising temperatures on inflows to the Murray-Darling Basin, Geophys. Res. Lett., 35, L07701, doi:10.1029/2008GL033390.
Gibbs, W.J. and Maher, J.V. (1967). Rainfall deciles as drought indicators. Bureau of Meteorology, Bulletin 48, Melbourne, 73 pp.
Hennessy K et al. (2008). An assessment of the impact of climate change on the nature and frequency of exceptional climate events. A consultancy report by CSIRO and the Australian Bureau of Meteorology for the Australian Bureau of Rural Sciences, 33pp. www.bom.gov.au/climate/droughtec/.
Nicholls, N. (204). The changing nature of Australian droughts. Climatic Change, 63, 323-336.