Groundwater trends in agricultural areas of Western Australia

Page last updated: Wednesday, 16 January 2019 - 2:03pm

Please note: This content may be out of date and is currently under review.

The Department of Agriculture and Food, Western Australia (DAFWA) completed a major analysis of groundwater trends in the agricultural areas of south-west Western Australia in 2014.

Detailed records for 1500 surveillance bores showed that in the period 2007–12, groundwater levels continued to rise in and adjacent to areas of salinity hazard in lower landscape positions over much of the region.

Summary

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 trends from 2007 to 2012 show that watertables were rising in a few areas, notably north of Geraldton, around Esperance and a few others. They were mostly stable in other parts of the South-West of Western Australia
Figure 1 Dominant groundwater level trends (2007–12) within hydrozones in the south-west agricultural region

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).

Salinity risk in the South-West of Western Australia is closely aligned with rising groundwater with greatest risk north of Geraldton, around Esperance and a few other areas.
Figure 2 Salinity risk assessment for hydrozones in the south-west agricultural region, 2007–12

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.

Table 1 Key to Figure 1 and Figure 2
Key Hydrozone
1 Kalbarri Sandplain
2 Northampton Block
3 East Binnu Sandplain
4 Irwin Terrace
5 Arrowsmith
6 Dandaragan Plateau
7 Northern Zone of Ancient Drainage
8 Northern Zone of Rejuvenated Drainage
9 Southern Cross
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
15 Coastal Plain
16 Donnybrook Sunkland
17 Leeuwin
18 Scott Coastal Plain
19 Warren-Denmark
20 Albany Sandplain
21 Stirling Range
22 Pallinup
23 Jerramungup Plain
24 Ravensthorpe
25 Esperance Sandplain
26 Salmon Gums Mallee

Pages

Contact information

Paul Raper
+61 (0)8 9780 6295
John Simons
+61 (0)8 9083 1128
Russell Speed
+61 (0)8 9956 8561