DAFWA analysed groundwater trends for 1500 surveillance bores to assess salinity risk across the region. The analysis built on previously reports and compares three periods: 1991–2000, 2000–07 and 2007–12.
Between the periods 1991–2000 and 2000–07, the proportion of bores with rising trends fell from 60% to 40% and the proportion of bores with falling trends increased from 6% to 29%. The changes in trend were most pronounced in the north.
For the 2007–12 period, groundwater trends were reported in hydrozones, which are regions of similar hydrogeological, climate, landscape and farming system attributes. Hydrozones with variable groundwater trends covered half of the land area. Hydrozones with predominantly rising or stable groundwater trends covered 21% of the region each. Hydrozones with mainly falling trends covered 6%, and there is no data on which to base trends for 2% of the region (Figure 1).
Dryland salinity is a hydrologically-driven land degradation hazard in the south-west agricultural region of Western Australia.
Shallow-rooted annual crops and pastures transpire significantly less water than the perennial native vegetation they replaced, leading to an increase in recharge, rising groundwater levels and the development of shallow watertables. Rising groundwater mobilises soluble salts naturally stored in the regolith. These salts can be concentrated by evapotranspiration in the root zone of vegetation.
In addition to the clearing of native vegetation for agriculture, rainfall factors determine groundwater trends. Rainfall has been below the long-term average over most of the region since the mid-1970s. The change was most noticeable between 2001 and 2007, especially in the northern part of the region. There are, however, areas in the far east and in the eastern south coast where rainfall has consistently been above the long-term average.
The codes for the names of the numbered hydrozones are provided in the table below.
Groundwater levels have continued to rise in and adjacent to areas of salinity hazard in lower landscape positions over much of the region, despite a general reduction in the proportion of bores with rising trends.
DAFWA used a risk matrix to assess salinity risk for the region. The matrix combines likelihood and consequence to determine a salinity risk rating. Inputs to the risk assessment, additional to the groundwater trends and climate analyses, were the areas of salinity hazard and current extent, as determined by the Land Monitor project. The risk assessed was the expansion of dryland salinity and its consequence on agricultural land beyond its current extent.
The risk assessment calculated that 82% of the region has a moderate salinity risk, 10% has a high risk and only 8% has a low risk (Figure 2).
Over most of the region, the impact of rainfall on groundwater trends is still less than the impact of clearing. Climate variability, therefore, appears to be a secondary, rather than the driving factor in the risk of dryland salinity in the south-west agricultural region.
|3||East Binnu Sandplain|
|7||Northern Zone of Ancient Drainage|
|8||Northern Zone of Rejuvenated Drainage|
|10||South-eastern Zone of Ancient Drainage|
|11||South-western Zone of Ancient Drainage|
|12||Southern Zone of Rejuvenated Drainage|
|13||Eastern Darling Range|
|14||Western Darling Range|
|18||Scott Coastal Plain|
|26||Salmon Gums Mallee|