Characteristics of the study area
Geology
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 30–50m of gritty clay saprolite formed by in situ weathering of the crystalline basement rock. The weathered profile is occasionally covered by 10–30m 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.
Figure 2.1 Geology and physiography of the south-west agricultural region
Landform
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.
Soils
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.
Hydrozones
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 103-104km2, 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.
Figure 2.2 Hydrozones in the south-west agricultural region and the location of bores used in this groundwater trend analysis
Climate
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 MAR ranges from more than 1200mm/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.
Climate trends
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 issue 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 following summary 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 the period 1910–74 as a baseline period.
The MAR from 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 2001-2007, annual rainfall was much less than the long-term mean over most of the region; 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 2008–12, 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.
Figure 2.8 Percentage change in annual average rainfall 1975–90, 1991–2000, 2001–07 and 2008–12, compared to the long-term average of 1910–74