DPIRD 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, 2000–2007 and 2007–2012.
Between the periods 1991–2000 and 2000–2007, the proportion of bores with rising levels fell from 60% to 40% and the proportion of bores with falling levels increased from 6% to 29%.
For 2007–2012, 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 covered 21% of the region each. Hydrozones with mainly falling groundwater covered 6%, and there was no data 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.
Note: The figures below are numbered as in the published report, and any references or citations can be found in the published report.
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.
We 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|
Secondary salinisation within the south-west agricultural region of Western Australia is a form of hydrologically induced land degradation brought about by a change in the water balance caused either by clearing or by irrigation for agriculture. In dryland agricultural areas, shallow-rooted annual crops and pastures intercept and transpire significantly less water than the native vegetation they replaced, leading to an increase in recharge, rising groundwater levels and the development of shallow watertables in areas where often none existed previously.
Rising groundwater levels mobilise soluble ions, primarily sodium and chloride, naturally stored at high concentrations in the regolith, particularly on the Yilgarn Craton. These salts accumulate within the root zone when watertables approach the soil surface and groundwater evaporates.
In addition to the effect of clearing on dryland agricultural areas, applying fresh to brackish irrigation water to areas of the coastal plain leads to salt accumulation in the root zone where subsoil drainage is poor. Parts of the irrigation area are also underlain by inherently saline, poorly drained soils.
Soil salinisation not only reduces agricultural production but also damages rural and townsite infrastructure, renders water resources unusable and threatens native ecosystems. The State of the Environment Report: Western Australia 2007 (EPA 2007) found that some 450 plant and 400 animal species are at risk of global or regional extinction because of salinisation and associated hydrological changes.
Land salinisation is considered one of WA’s most significant environmental issues. According to the Department of Environmental Protection (1997), salinity occurs over a significant part of the agricultural area and it has severely damaged the natural environment and reduced agricultural productivity.
The last salinity risk assessment undertaken in WA (2005) concluded that salinity either currently affects or threatens large areas of agricultural land and many sites containing high value infrastructure. However, despite the extent and effects of salinity, the State of the Environment Committee (2011) noted that salinity was not a priority issue under the Caring for our Country program.
The purpose of the work presented in this report is to:
- determine the dominant groundwater trends within the south-west agricultural region
- relate groundwater trends to spatial patterns of changes in rainfall
- assess the salinity risk for the region
- determine the suitability and capability of the available data to assess the salinity risk.
This report provides a brief overview of the physiography and climate of the south-west agricultural region, followed by a description of the method used to analyse groundwater trends and the hydrozones on which all analyses are summarised. The results of this groundwater trend analyses are compared to the results published in 2008 to provide a summary of groundwater trends in the region over the whole period (1975–2012) for which data is available. The results are then related to the rainfall observed in the region over the analysis periods and the salinity risk is assessed for each hydrozone. The suitability of the available data for assessing a salinity risk is also discussed.
For this analysis, the south-west agricultural region of WA is defined as the entire area south and west of the clearing line. The clearing line marks the boundary between freehold land that has been substantially cleared for broadacre agriculture and leasehold land that is used for pastoral grazing of native vegetation.
The region is bound to the west by the Indian Ocean and to the south by the Southern Ocean (Figure 1). Within the south-west agricultural region, there are large forested areas that include unallocated Crown land and areas managed for conservation, state forests and water catchment protection. The Perth metropolitan region is excluded from the region.
The region is about 25 million hectares and the cleared portion that is used for agricultural production is about 16 million hectares (64%).
For information relating to previous assessments of groundwater trends, extent of dryland salinity and salinity hazard and risk assessments, refer to the full report.
Characteristics of the study area
Much of the south-west agricultural region lies on Archean granitoid rock of the Yilgarn Craton (Figure 2.1). On the Yilgarn Craton, the regolith profile is typically 30 to 50 metres (m) of gritty clay saprolite formed by in situ weathering of the crystalline basement rock. The weathered profile is occasionally covered by 10 to 30m of mixed sediments in sheets or palaeochannels. The western edge of the Yilgarn Craton is defined by the Darling Fault, which extends some 700km north–south through the entire region.
West of the Yilgarn Craton, the Perth Basin is an elongate trough containing up to 12 000m depth of Permian to Early Cretaceous sediments. The Permian sediments are dominated by glacial tillite and shales. The Mesozoic sediments are dominated by felspathic sandstone and host extensive regional aquifers such as the Yarragadee.
In the northern-most extent of the Perth Basin within the region, the Northampton Complex is essentially a large outcrop of Proterozoic crystalline gneissic basement rock. It is partially capped in western areas by thin sequences of Jurassic sediments that form the flat-topped Moresby Range near Geraldton.
At the south-west tip of the region, the Leeuwin Complex is a narrow strip of Proterozoic granitic basement similar to the Northampton Block.
Along the south coast, the Albany–Fraser Orogen is exposed along the southern margin of the Yilgarn Craton and is characterised by high-grade gneisses and granitoid intrusions. Near the coast, the Albany–Fraser Orogen is partially capped by sediments assigned to the Bremer Basin. The Bremer Basin consists of numerous small depressions filled with Eocene sediments of the Plantagenet Group.
The Stirling Range Formation straddles a portion of the contact between the Yilgarn Craton and the Albany–Fraser Orogen, and its most obvious expression is the Stirling Range which runs from west of Cranbrook to Ellen Peak, south-east of Borden. The formation consists of a Middle Proterozoic sequence of metamorphosed sandstone and shale laid down in shallow water.
The region has generally subdued relief and the landscape is largely plateau with ranges of low hills (for example, Darling Range, Stirling Range). There are low scarps that are surface expressions of geological faults, notably the Darling Scarp formed by the Darling Fault (Figure 2.1).
The Meckering Line, originally delineated by Jutson in 1934, is a north-north-west to south-south-easterly trending zone marking a major transition in landform and drainage characteristics in the region (Figure 2.1). To the west of the Meckering Line, valleys are relatively narrow-floored and steep-sided with high gradients. To the east, valleys are much broader with flat floors, the drainage is generally sluggish and intermittent, and chains of salt lakes (playas) are common.
The soils of the Yilgarn Craton are formed mainly on laterite, truncated lateritic profiles, bedrock weathered in situ, colluvium and alluvium. On the catchment divides, soils are mainly sandy gravels with some pale deep sands. Grey sandy duplex soils, often with alkaline subsoils, are found on the valley slopes. Alkaline grey shallow loamy and sandy duplex soils, calcareous loamy earths and saline wet soils occur on the valley floors.
On the west and south coasts, the soils are derived from sedimentary sequences. They are often deep calcareous or alkaline sands or sandy duplex soils. Acid sands, clays and loams are common in low-lying coastal areas.
Previous studies of groundwater trends or salinity risk in the south-west agricultural region have used soil-landscape zones as the spatial unit for analyses. The Land Monitor project also used areas based on soil-landscape zones to develop the rules used to define areas of salinity hazard from Landsat TM data. Soil-landscape zones are areas defined on geomorphologic or geological criteria and are of the order of 103 to 104 square kilomtres (km2), which is suitable for regional perspectives. These zones reflect state-scaled regions with similar geomorphology, relief and farming system attributes. Furthermore, they align well with the gradient of mean annual rainfall (MAR) from the coast to the interior (Figure 2.3).
For this study, the concept of hydrozones is used as the spatial unit. Hydrozones are based on soil-landscape zones. However, there are several instances where adjacent soil-landscape zones are underlain by the same hydrogeological unit and where this occurs, soil-landscape zones are aggregated. In several cases, adjacent soil-landscape zones share contiguous distinct geological boundaries, such as major faults and, from a hydrological point of view, belong to the same functional unit. Hydrozones are therefore defined to coincide with soil-landscape zones, except where hydrogeological boundaries dictate that several soil-landscape zones belong to a contiguous hydrogeological unit.
The climate of the south-west agricultural region ranges from temperate dry-summer or Mediterranean climates to arid, according to the updated Köppen-Geiger climate classification.
The mean annual rainfall (MAR) ranges from more than 1200 millimetres per year (mm/y) in the Darling Range south of Perth and on the far western south coast, down to 280mm/y in the east. About 80% of the rain falls between April and October over most of the region. Summer rainfall is highly variable and often associated with the southern passage of tropical cyclones (Indian Ocean Climate Initiative [IOCI] 2012).
Mean annual pan evaporation ranges from less than 1000mm/y in the far south-west, to more than 2800mm/y in the north-east. In the arid and much of the temperate areas, mean annual pan evaporation exceeds rainfall in most, if not all months. Having evaporation in excess of rainfall is one of the factors that predispose the region to salt accumulation within the soil profile.
Mean monthly maximum summer air temperatures exceed 35°C in places and mean monthly minima are as low as 4°C.
According to the IOCI, the May to July rainfall in the western portion of the region has decreased since the 1970s and this rainfall reduction has intensified and expanded geographically over the past decade. This rainfall reduction is generally accepted to include both natural climate variability and anthropogenic components and is expected to continue (IOCI 2012).
Mean annual temperatures in the region have increased over the past 50 years. However, summer maxima have decreased along the south coast and in the east of the region (IOCI 2012).
The IOCI (2012) raises the question of the appropriate period of the historical rainfall record to use as a baseline for comparing recent rainfall decline and the expected future decline. This question remains unresolved. The summary below is based on data accessed from the Patched Point Dataset, compiled by the Queensland Department of Science, Information Technology, Innovation and the Arts, and uses 1910–1974 as a baseline period.
The MAR for 1975–1990 was below the long-term mean over much of the south-western portion of the region, but there was an equivalent area with above average rainfall (Figure 2.8). This pattern was repeated in the 1991–2000 period, though the area with above average rainfall was larger.
From 2001 to 2007, annual rainfall was much less than the long-term mean over most of the region. The percentage decrease was highest and most widespread in the north. The exception was along the eastern fringe of the cleared agricultural area and along the eastern south coast where MAR remained above average.
During 2008–2012, the rainfall deficit (relative to the long-term mean) was lower in the north of the region and more pronounced in the central west and south-west. Although the distribution of areas with above average rainfall changed in the eastern portion of the region, most of the eastern south coast remained wetter than average.
Groundwater trend analyses
Each bore site was analysed for the period 2007–2012, which complemented the analyses for pre–1990, 1991–2000 and 2001–2007 performed by George et al. (2008). The analysis published by George et al. (2008) covered the 2000–2007 period up to observations made up to mid–2007. The additional analyses presented here continued from that period, that is, mid–2007 to mid–2012. The rainfall change maps (Figure 2.8: see full report) however, are based on analysis of whole calendar year data.
To determine groundwater trends, lines of best fit were drawn through the data for the period of record and the gradient calculated. For bores with high variability between seasons, the line of best fit was aligned to the summer minima. Bores were included in the analyses if they were remote from any likely effects of salinity management treatments (drains, trees, perennial pastures) and met a minimum standard of five years duration with at least 20 monitoring observations. The criteria of a minimum of 20 observations was relaxed for some of the bores drilled during the 2007–2008 program (DAFWA 2008) where a clear trend could be identified.
Where nested bores exist at a monitoring site, the trend and depth for the deepest bore was reported, as it is most likely to reflect the status of the groundwater system responsible for mobilising salts stored within the regolith. To maintain a methodology consistent with the analyses reported by George et al. (2008), the effects of rainfall variability were not separated from linear trends in groundwater levels in this study.
Data for some bores monitored by DPaW, not included in the analysis by George et al. (2008), were available for inclusion in this study and rates of change in groundwater level for the 1991–2000 and 2001–2007 periods were calculated for those bores. These results were then pooled with the results of George et al. (2008) for those analysis periods.
Groundwater trends were categorised as either rising, stable or falling. Low (less than ± 0.03m/y) calculated rates of change in groundwater level were assigned to the stable category. Groundwater levels at most bores are measured quarterly or biannually to the nearest 0.01m or 0.005m, and a 0.03m/y threshold was adopted to account for measurement error.
A representative hydrograph that matched the dominant trend and seasonal variability was selected for each hydrozone and plotted with the accumulated monthly residual rainfall (AMRR) for the closest rain gauge. AMRR is calculated as the cumulative sum of the difference between the observed monthly rainfall and the mean rainfall for that particular month (Weber & Stewart 2004, Ferdowsian et al. 2001).
All monthly data from the first full year of record for each rain gauge was used to calculate the AMRR. An AMRR plot that covers the full period of record will always start and finish at zero. If AMRR is increasing, monthly rainfall is consistently exceeding the long-term mean, indicating above mean rainfall. If AMRR is decreasing, monthly rainfall is consistently below the long-term mean, indicating drier periods.
Rainfall data was accessed from the Patched Point Dataset compiled by the Queensland Department of Science, Information Technology, Innovation and the Arts.
Groundwater trends by hydrozone
The groundwater trends for 2007–2012 are presented for each hydrozone. In each case, the results for earlier analysis periods (1991–2000, 2001–2007), reported by George et al. (2008), are included in the summary tables. The results for the pre–1990 period reported by George et al. (2008) are not shown because of the relatively small number of bores for which data was available.
A brief description of each hydrozone is given which includes an overview of the geology, physiography and main groundwater flow systems (Coram 1998).
Groundwater trend by depth scatter plots are included to provide a complete picture of groundwater trends for the hydrozone over the full period of record. The scatter plots show the rate of change in groundwater level over each of the analysis periods, plotted against the depth to groundwater at the last observation date for each bore. Artesian bores plot above the abscissa on these graphs. The numbers of bores for which results are plotted are shown on each plot because the clustering of bores around the origin makes it difficult to differentiate individual results.
A representative hydrograph with AMRR is shown for each hydrozone. The AMRR provides site-specific, temporal information on rainfall variability compared to the long-term regional information in Figure 2.8. The AMRR plot on each graph does not start and finish at zero because the time series of rainfall observations is significantly longer than the time series of groundwater observations.
Maps showing the location of bores, classified by groundwater trend for the 1991–2000, 2001–2007 and 2007–2012 periods are presented for each hydrozone. Where appropriate, the Land Monitor average height above valley floor (AHAVF) salinity hazard area is also shown to provide context on the location of bores in each of the trend categories.
The Kalbarri Sandplain was assessed as having moderate risk of future salinity because there are rising groundwater trends where the watertable is shallow.
Groundwater was either falling or stable where groundwater was more than 15m below the surface. Rising groundwater trends were observed in the north of the hydrozone, where the watertable is less than about 15m deep, despite below average rainfall.
The electrical conductivity of groundwater in this hydrozone was generally low – less than 400 milliSiemens per metre (mS/m) and the quality of water is usually suitable for most dryland farming purposes.
This is the first assessment of groundwater trends within the Kalbarri Sandplain Hydrozone since 11 monitoring sites were established in late 2007.
For detailed information about this hydrozone see section 3.1.1 and section 4 of the full report.
The Northampton Block Hydrozone was assessed as having low risk of future salinity despite the rising trends due to well-defined surface drainage, enhanced topographic relief and groundwater discharge as baseflow into streams.
Water suitable for most dryland farming activities can be found however is often discretely located according to geology. Most of the bores monitored in the hydrozone have electrical conductivity (EC) values below 1000mS/m. EC values above 3000mS/m (half as saline as seawater) have been observed but are rare.
For detailed information about this hydrozone see section 3.1.2 and section 4 of the full report.
The East Binnu Sandplain Hydrozone was assessed as having a high risk of future salinity. Rising groundwater trends dominate throughout the hydrozone, despite the reduced rainfall since 2000 (Figure 3.9).
Our investigation indicates that a significant salinity problem could develop in this hydrozone due to the rising and shallow watertables (<10m). Groundwater in this hydrozone is at best brackish and is often far too saline for any agricultural use.
This is the first assessment of groundwater trends in the East Binnu Sandplain since monitoring began with the installation of monitoring sites in 2004.
For detailed information about this hydrozone see section 3.1.3 and section 4 of the full report.
The Irwin Terrace Hydrozone was assessed as having a moderate risk of future salinity.
Rising groundwater trends are associated with sites located within areas of sandplain soils. Falling trends tend to be associated with sites located in areas of heavier soil types. Stable trends are observed where groundwater is shallow (<2m) and the sites are affected by salinity.
Groundwater quality ranges from brackish (500mS/m, 2750mg/L) to hypersaline (7000mS/m, 38 500mg/L).
Groundwater monitoring began in 1995 with the installation of five monitoring sites. This report is the first assessment of groundwater trends at 10 additional sites installed in 2007–2008.
For detailed information about this hydrozone see section 3.1.4 and section 4 of the full report.
The Arrowsmith Hydrozone was assessed as having a low risk of future salinity.
Rising groundwater trends in the regional groundwater system are observed in central and eastern parts of the hydrozone, generally where localised perched watertables overlie the regional aquifer. The greatest rates of rise are observed where watertables are deepest. Water suitable for dryland farming activities can be found throughout the hydrozone.
For detailed information about this hydrozone see section 3.1.5 and section 4 of the full report.
The Dandaragan Plateau Hydrozone was assessed as having a high risk of future salinity. Consistently rising groundwater trends are dominant and recurring throughout the hydrozone. Water is generally suitable for dryland agricultural purposes.
For detailed information about this hydrozone see section 3.1.6 and section 4 of the full report.
The Northern Zone of Ancient Drainage Hydrozone was assessed as having a moderate risk of future salinity. Existing dryland salinity (primary and secondary) is common in this hydrozone. Palaeochannels of deep sediment are known to occur within salt lake systems and can provide high yielding but hypersaline groundwater resources.
Perched aquifers in deep sands on hillslopes are common and often contain small supplies of fresh groundwater that are suitable for stock and that can be accessed via soaks or low-yielding windmills. Saline hillside seeps often occur at the downslope end of perched aquifers where the salts are concentrated by evaporative discharge.
Since 2007, there has been more rising groundwater trends, although falling and stable trends remain in some areas.
For detailed information about this hydrozone see section 3.1.7 and section 4 of the full report.
The Northern Zone of Rejuvenated Drainage Hydrozone was assessed as having a moderate risk of future salinity.
Groundwater is more saline close to drainage lines and perched aquifers containing low salinity water suitable for livestock are common but are of very limited supply and volume.
Most bores in low landscape positions have shallow, stable groundwater levels that fluctuate seasonally in response to rainfall and evaporation. However, one-third of bores still display rising groundwater trends. In mid to upper landscape positions, most bores with groundwater levels deeper than 5m have falling or stable trends.
Overall, relative to the 2001–2007 period, there is a small increase in the proportion of bores with rising groundwater trends and a decrease in the proportion with stable trends, despite continued below average rainfall.
For detailed information about this hydrozone see section 3.1.8 and section 4 of the full report.
The Southern Cross Hydrozone was assessed as having a low risk of future salinity.
Groundwater within the hydrozone is generally too saline for dryland agricultural purposes.
In 2007, 15 groundwater monitoring sites were established in the southern part of the hydrozone. This is the first assessment of groundwater trends within this hydrozone.
Falling groundwater trends are evident in all lower-catchment bores and variable groundwater trends are observed in mid and upper catchment locations.
For detailed information about this hydrozone see section 3.1.9 and section 4 of the full report.
The South-eastern Zone of Ancient Drainage Hydrozone was assessed as having a moderate risk of future salinity. This zone contains extensive palaeodrainages through broad, flat valley floors with numerous salt lakes. Groundwater is generally poor quality.
Groundwater monitoring in the South-eastern Zone of Ancient Drainage Hydrozone has occurred since the mid–1990s with 70 bores currently monitored. In the current analysis, rising trends were observed in bores in mid and upper landscape positions and in some lower-catchment bores, falling trends were apparent.
For detailed information about this hydrozone see section 3.1.10 and section 4 of the full report.
The South-western Zone of Ancient Drainage Hydrozone was assessed as having a high risk of future salinity. It is an ancient, gently undulating plateau with sluggish drainage systems that only flow in very wet years.
Groundwater occurs in saprolite and palaeochannel aquifers and these groundwater systems and are mainly saline.
The current analysis shows that bores in the southern portion of the hydrozone adjacent to areas of salinity hazard continue to rise, despite below average rainfall. Most mid and lower-catchment bores in the north-eastern portion of the hydrozone also have rising groundwater trends. In the north-western portion, many bores have stable trends.
For detailed information about this hydrozone see section 3.1.11 and section 4 of the full report.
The Southern Zone of Rejuvenated Drainage Hydrozone was assessed as having a moderate risk of future salinity. The zone is characterised by gently undulating rises to low hills with broad valley floors, lakes and associated dune systems. The main stream channels are continuous and flow in most years. Groundwater quality is mainly brackish to saline.
The proportion of bores with rising groundwater trends fell in 2007–12 because of ongoing low rainfall. However, there is a small but significant group of bores within and adjacent to salinity hazard areas with rising trends.
For detailed information about this hydrozone see section 3.1.12 and section 4 of the full report.
The Eastern Darling Range Hydrozone was assessed as having a moderate risk of future salinity. The zone consists of undulating to rolling terrain formed by the dissection of a lateritic plateau and narrow valley floors incised into the underlying granitic basement rocks of the Yilgarn Craton.
Winter discharge from perched aquifers associated with deep sands can provide relatively fresh stock water during periods of low rainfall. Otherwise, groundwaters are generally brackish to saline, with marginal quality groundwater found occasionally.
Since 2007, the proportion of bores with rising groundwater levels has decreased in response widespread below average rainfall. However, there are still bores within and adjacent to areas of salinity hazard with rising trends.
For detailed information about this hydrozone see section 3.1.13 and section 4 of the full report.
The Western Darling Range Hydrozone was assessed as having a low risk of future salinity.
The hydrozone is an undulating lateritic plateau derived from the basement rocks of the Yilgarn Craton. Major river systems, such as the Avon, Blackwood, Murray and Collie, have eroded the plateau, forming deeply incised valleys. Groundwaters in the hydrozone range from fresh to saline but are predominantly brackish.
Five surveillance bores are monitored in the far south of the hydrozone. Four of the bores have had stable groundwater trends since they were installed in 1992. One bore has had rising groundwater since it was installed in 1994.
For detailed information about this hydrozone see section 3.1.14 and section 4 of the full report.
The Coastal Plain Hydrozone was assessed as having a low risk of future salinity.
The hydrozone occupies part of the Perth Basin which is dominated by unconsolidated sediments and limestone over sedimentary rocks. Major aquifers are located within the hydrozone with groundwater quality ranging from fresh to saline, with the fresher groundwaters found in the main sedimentary aquifers. Groundwater is shallow over much of the hydrozone but groundwater trends are stable, responding to seasonal rainfall.
Groundwater remains less than one metre below the surface in low-lying areas in the northern portion of the hydrozone, despite below average rainfall.
For detailed information about this hydrozone see section 3.1.15 and section 4 of the full report.
The Donnybrook Sunkland Hydrozone was assessed as having a very low risk of future salinity.
Land salinity in the Donnybrook Sunkland Hydrozone is currently limited to very small areas adjacent to drainage lines.
For additional information about this hydrozone see section 3.1.16 and section 4 of the full report.
The Leeuwin Hydrozone was assessed as having a very low risk of future salinity.
Land salinity in the Leeuwin Hydrozone is limited to very small areas adjacent to drainage lines and small areas on the Whicher Scarp.
For additional information about this hydrozone see section 3.1.17 and section 4 of the full report.
The Scott Coastal Hydrozone was assessed as having a very low risk of future salinity. The hydrozone consists of coastal sand dunes and plain with swamps.
There are no significant areas of land salinity in the Scott Coastal Plain Hydrozone.
For additional information about this hydrozone see section 3.1.18 and section 4 of the full report.
The Warren–Denmark Southland Hydrozone was assessed as having a moderate risk of future salinity.
Groundwaters are generally low quality, ranging from brackish to saline. When groundwater rises occur, localised hillside seeps and salinity become evident.
In 2007–2012, the proportion of bores with rising groundwater trends has fallen to 25% following a period of below average rainfall. The proportion of bores with stable trends has been steadily increasing.
For detailed information about this hydrozone see section 3.1.19 and section 4 of the full report.
The Albany Sandplain Hydrozone was assessed as having a low risk of future salinity. Most of the area consists of broad plains with numerous lakes and depressions that become seasonally inundated. Sand and rounded gravel aquifers at the base of the Werillup Formation can provide good yields of groundwater with a high potential for fresh water resources close to the coast.
The proportion of bores with rising groundwater trends fell to 47% during the 2007–2012 period. During the same period, the proportion of bores with falling groundwater trends increased to 30%.
For detailed information about this hydrozone see section 3.1.20 and section 4 of the full report.
The Stirling Range Hydrozone was assessed as having a low risk of future salinity.
The mountainous Stirling Range National Park is a large feature of the hydrozone. At the base of the footslopes to the north of the range there are many large lakes, with very little run-off leaving the area because of the flat landscape. Groundwater in these internally drained aquifers is saline to extremely saline.
During the 2007–2012 analysis period there was an increase in the proportion of bores with rising trends to 50%.
For detailed information about this hydrozone see section 3.1.21 and section 4 of the full report.
The Pallinup Hydrozone was assessed as having a moderate risk of future salinity.
Groundwaters are mainly saline and the extent of salinity is limited to creeklines and hillside seeps.
The monitoring network was expanded in 2008 to 17 bores and 35% of these had a rising trend during the most recent analysis.
For detailed information about this hydrozone see section 3.1.22 and section 4 of the full report.
The Jerramungup Hydrozone was assessed at having a moderate risk of future salinity.
Groundwaters are predominantly saline.
The proportion of bores with falling groundwater trends has increased since 2007 in response to decreasing annual rainfall.
For detailed information about this hydrozone see section 3.1.23 and section 4 of the full report.
The Ravensthorpe Hydrozone was assessed as having a moderate risk of future salinity.
Groundwater flow systems are mainly local in scale and groundwater is saline. The aquifers either discharge into low-lying areas, or form hillside seeps.
During 2007–2012, all monitored bores with groundwater levels deeper than 10m had rising trends and bores with shallow groundwater levels had stable trends.
For detailed information about this hydrozone see section 3.1.24 and section 4 of the full report.
The Esperance Sandplain Hydrozone was assessed as having a high risk of future salinity.
The groundwater salinity in the monitoring bores ranges from fresh to saline, with over 60% of bores having an EC of 2000mS/m or less.
Since 2007, about half of the bores have rising groundwater trends. Bores with a rising trend are either located west of Esperance in the Munglinup area, or in the east of the hydrozone.
For detailed information about this hydrozone see section 3.1.25 and section 4 of the full report.
The Salmon Gums Mallee Hydrozone was assessed as having a moderate risk of future salinity.
Groundwaters are saline to extremely saline throughout the hydrozone.
During 2007–2012, over half the bores had a rising groundwater trend, mainly in the west of the hydrozone where annual rainfall has been higher than long term average.
For detailed information about this hydrozone see section 3.1.26 and section 4 of the full report.
Groundwater trends summary
See section 3.2 of full report.
Regional changes in groundwater trends since 2007
See section 3.3 of full report.
Factors affecting groundwater trends at the hydrozone scale
See section 3.4 of full report. Headings include the impact of native vegetation, land use impacts and hydrological equilibrium.
Salinity risk assessment
For information relating to determining salinity risk from groundwater trends, salinity risk for hydrozones, salinity risk summary, timing of salinity and uncertainty in salinity risk assessments, see section 4 of full report.
Groundwater trends were determined for 1500 surveillance bores distributed across the south-west agricultural region of WA. The objective of the analysis was to update the last groundwater trend assessment made in 2007 (George et al. 2008) and to make a salinity risk assessment for the region at the most appropriate scale for the available data. Agricultural land was the principal asset considered in the risk assessment.
The proportion of bores exhibiting rising trends underwent a reduction between the 1991–2000 and 2001–2007 assessment periods due to lower than average rainfall over much of the region. Conversely, the proportion of bores exhibiting falling trends increased between 1991–2000 and 2001–2007.
Trends in groundwater levels are variable (that is, bores within the hydrozone have roughly equal numbers of falling, rising and stable groundwater trends) in nine of the 23 hydrozones, which make up 50% of the area of the south-west agricultural region.
Groundwater levels were mostly rising in most hydrozones in the Northern Agricultural Region, the South-western Zone of Ancient Drainage Hydrozone in the Central Agricultural Region and the, Jerramungup, Ravensthorpe, Esperance Sandplain and Salmon Gums Mallee Hydrozones in the Southern Agricultural Region. The hydrozones in which rising trends dominated make up 21% of the land area of the region.
Stable groundwater trends dominate the Eastern Darling Range Hydrozone, the Northern and Southern Zones of Rejuvenated Drainage Hydrozones and the Coastal Plain Hydrozone on the west coast. Together these hydrozones make up 21% of the region. Groundwater levels are mostly falling in two hydrozones that constitute 6% of the south-west agricultural region.
Despite an increase in the proportion of bores exhibiting falling trends during 2007–2012, groundwater levels have continued to rise in, and adjacent to, areas of salinity hazard in lower landscape positions over much of the region.
The results of the groundwater trend analyses were the primary input into a salinity risk assessment for the south-west agricultural region. The salinity risk assessment also relied on the Land Monitor (Caccetta et al. 2010) AHAVF salinity hazard map and AOCLP salt-affected area map for 1996–98, and an understanding of the local hydrogeology. The risk assessment methodology was based on a matrix of likelihood and consequence adapted from Spies and Woodgate (2005).
The groundwater trends and salinity risk assessments are reported for hydrozones that are regions of similar hydrogeological, climate, landscape and farming system attributes. The risk assessments are presented in Table 4.4 (see full report).
The salinity risk map (Figure 2) shows that the highest salinity risk occurs in:
- Portions of the Northern and Central Agricultural Regions of the region, where groundwater levels have continued to rise, despite several decades of below average rainfall.
- In the east of the Southern Agricultural Region, where rainfall has been above average for the past decade and watertables are already shallow.
The significant proportion of bores with rising trends in and adjacent to areas of salinity hazard, indicates that, over most of the region, the impact of reduced rainfall on groundwater trends is still less than the impact of clearing. Climate variability therefore appears to be a moderating, rather than driving factor in the risk of dryland salinity in the south-west agricultural region.