John Ruprecht

Dryland salinity is driven by excess water in the landscape resulting from clearing of perennial vegetation. The decrease in winter rainfall and virtual absence of ‘wet winters’ has caused a slowing in the process of salinisation, and in some cases groundwater levels have fallen. Deeper bores are continuing to rise while shallow (< 8m) groundwaters have flattened. In the north-eastern
Wheatbelt, the occurrence of summer storms has created an erratic response.

Stream salinities in many streams have continued to increase because of reduced fresh runoff which, in wet years, dilutes the saline groundwater discharge. However, in catchments where groundwater levels no longer intersect the valleys because of falling levels, runoff has freshened.

South-west WA can experience extreme rainfall in winter with frontal systems and in summer with tropical storms.

In the mid-1970s in south-west WA there was a shift to consistently drier conditions (see IOCI Climate Note 5/05) for annual and seasonal rainfall. Research sponsored by IOCI suggests that both natural climate variability and the enhanced greenhouse effect have played a role. However, it is much more difficult to determine if there has been a shift in extreme rainfall. At the relatively frequent level of extreme events, such as less than a one-in-10 year annual maximum rainfall event (20- 60mm recorded at various locations), observed statistics give us some insight, but for extremes at the rare or disaster level, there is very little understanding at this point in time.

The data for many rainfall stations in the south-west indicate that larger storms do not appear as often now in winter, but the magnitude of annual winter maxima is not significantly changed. There are also observations that summer rainfall events (one-in-10 year) are typically random and have not altered significantly over the last 100 years.

Continuing research by IOCI is building our understanding of factors driving these changes, but projections of change in rare events are perhaps only able to be investigated by modelling studies at the very edge of our current capability in climate science.

The flooding problems of the Moore River in the Bidaminna area are complex and have been developing over more than three decades. In July 1995 these flooding problems were exacerbated by a large flood which caused major bank and floodplain erosion, siltation, economic loss and major inconvenience to landowners on the Moore River floodplain.

The aims of this study were to investigate the flooding issues and to develop flood management and implementation strategies for the Bidaminna area of the Moore River.