LONG-TERM VARIATIONS IN REGIONAL RAINFALL IN THE SOUTH-WEST OF WESTERN-AUSTRALIA AND THE DIFFERENCE BETWEEN AVERAGE AND HIGH-INTENSITY RAINFALLS

The south-west of Western Australia is characterized by a 'Mediterranean' climate with dominantly winter rainfall. It is the only Australian region where the annual rainfall has significantly decreased during the period 1911-1990. Based on daily rainfall records from 70 stations in the area, high intensity rainfall (which is closely related to flooding, soil erosion, and gully formation) does not show a concurrent decrease. Reasons for this unconformable trend include a greater temporal variability of high intensity rainfall and an increase in the frequency of occurrence of rainfall events at high intensities during the summer. This tends to balance out a decrease during the winter months. While it is unwise to assume that there will be similar trends for average rainfall and high intensity rainfall for a given region, evidence from the instrumental records supports, to some extent, the notion that in a CO2-warmed world, high intensity rainfall could occur more frequently irrespective of local change to average rainfall.