Pilbara–Gascoyne
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Pilbara–Gascoyne 11 Pilbara–Gascoyne ...................................................... 2 11.5 Surface water and groundwater ....................... 26 11.1 Introduction ........................................................ 2 11.5.1 Rivers ................................................... 26 11.2 Key information .................................................. 3 11.5.2 Flooding ............................................... 26 11.3 Description of the region .................................. 4 11.5.3 Storage systems ................................... 26 11.3.1 Physiographic characteristics.................. 6 11.5.4 Wetlands .............................................. 26 11.3.2 Elevation ................................................. 7 11.5.5 Hydrogeology ....................................... 30 11.3.3 Slopes .................................................... 8 11.5.6 Water table salinity ................................ 30 11.3.4 Soil types ............................................... 9 11.5.7 Groundwater management units ........... 30 11.3.5 Land use ............................................. 11 11.6 Water for cities and towns ............................... 34 11.3.6 Population distribution .......................... 13 11.6.1 Urban centres ....................................... 34 11.3.7 Rainfall zones ....................................... 14 11.6.2 Sources of water supply ....................... 34 11.3.8 Rainfall deficit ....................................... 15 11.6.3 Geraldton ............................................. 36 11.4 Landscape water flows .................................. 16 11.4.1 Rainfall .................................................. 17 11.4.2 Evapotranspiration ............................... 20 11.4.3 Landscape water yield ......................... 23 Pilbara–Gascoyne 11 Pilbara–Gascoyne 11.1 Introduction Spatial and temporal patterns in landscape water flows are presented as well as an examination of This chapter examines water resources in the the surface and groundwater resources. The chapter Pilbara–Gascoyne region in 2011–12 and over recent concludes with a review of the water situation for decades. It starts with summary information on urban centres. The data sources and methods used the status of water flows, stores and use. This is in developing the diagrams and maps are listed in the followed by descriptive information for the region Technical Supplement. including the physiographic characteristics, soil types, population, land use and climate. 2 Australian Water Resources Assessment 2012 11.2 Key information Table 11.1 gives an overview of the key components of the data and information in this chapter. Table 11.1 Key information on water flows, stores and use in the Pilbara–Gascoyne region Landscape water flows Region average Difference from 1911–2012 Decile ranking with respect to the long-term annual mean 1911–2012 record Rainfall 338 mm +29% 9th—above average Evapo- transpiration 341 mm +36% 9th—above average Landscape water yield 19 mm +73% 10th—very much above average Streamflow (at selected gauges) Flooding: Minor flooding limited to two gauges in the far northern rivers in single gauges in the midstream part of the Gascoyne and Ashburton rivers Surface water storage (comprising all of the region’s major surface water storages) Total 30 June 2012 30 June 2011 Change accessible accessible % of total accessible % of total accessible % of total capacity volume capacity volume capacity volume capacity 63 GL 36 GL 57% 62 GL 98% -26 GL -41% Groundwater (in selected aquifers) Salinity: Large areas of saline groundwater (≥3000 mg/L) along the coast and in alluvial aquifers surrounding major river beds Urban water use (Geraldton) Total sourced in 2011–12 Total sourced in Change Restrictions 2010–11 7.8 GL 8.1 GL –0.3 GL (-4%) Permanent Water Conservation Measures Australian Water Resources Assessment 2012 3 Pilbara–Gascoyne 11.3 Description of the region Most of the region is in a relatively natural state, much of which is used for grazing (See Figure 11.1). The Pilbara–Gascoyne region is the central western Dryland agriculture accounts for approximately 3% corner of Western Australia. It covers a long coastal of the land use in the region. Irrigation is limited and section and a dry inland section bordered by the intensive land uses such as urban areas account North Western Plateau and South Western Plateau for 0.03%. Section 11.7 has more information on regions in the east, and the South West Coast region agricultural activities in the region. in the south. The region covers about 478,000 km² of land area. River basin areas vary in size from The region has an arid subtropical climate, with a 18,000 km² to 91,000 km². temperate Mediterranean climate predominantly occurring in the south. Rainfall is generally low and The area encompasses two major plateaus: the variable. Irregular monsoonal rain occurs in the Pilbara plateau in the north and the Gascoyne north. Subsections 11.3.7 and 11.3.8 provide more plateau in the southeast. The western part of the information on the rainfall patterns and deficits across region includes coastal and inland dunes and alluvial the region. floodplains, with some low hills in the southwest. Seasonal or persistent aridity has resulted in low The generally flat landscapes ensure high rainfall vegetation cover and intermittent river systems. infiltration rates. Rivers are sparse and drain internally Subsections 11.3.1–11.3.4 give more detail on the or towards the Indian Ocean. The major rivers only physical characteristics of the region. generate substantial amounts of flow during and after high rainfall periods. The Pilbara–Gascoyne region has a population of approximately 117,000 people, which is 0.5% of The hydrogeology of the region is dominated by the nation’s total population (Australian Bureau of the large area of outcropping Palaeozoic fractured Statistics [ABS] 2011b). basement rock of low permeability. The groundwater systems in this rock typically offer restricted low Major population centres are shown in Figure 11.1 volume groundwater resources. A more detailed and include the city of Geraldton as well as the description of the rivers and groundwater status in regional centres of Karratha, Port Hedland (including the region is given in section 11.5. South Hedland) and Carnarvon. Further discussion of the region’s population distribution and regional urban centres can be found in subsection 11.3.6 and section 11.6 respectively. Python Pool, Millstream–Chichester National Park, Western Australia | Tourism Australia 4 Australian Water Resources Assessment 2012 Figure 11.1 Major rivers and urban centers in the Pilbara–Gascoyne region Australian Water Resources Assessment 2012 5 Pilbara–Gascoyne 11.3.1 Physiographic characteristics flat-topped hills of igneous and metamorphic rocks interspersed by stony plains on granite. Further south The physiographic map in Figure 11.2 shows areas are dissected plateaus and ranges of sandstone, with similar landform evolutionary histories (Pain et quartzite and volcanic rocks. There are also some al. 2011). These can be related back to similar geology alluvial lowlands with sand, stony and hardpan plains. and climatic impacts which define the extent of The Western Coastlands province occupies 29% erosion processes. The areas have distinct physical of the region and comprises mainly plains of sand, characteristics that can influence hydrological stone and calcrete with dunes, some low ridges, processes. The Pilbara–Gascoyne region has three plateaus and hills of sandstone and shale. physiographic provinces, namely the Pilbara, Western Coastlands and the Yilgarn Plateau. The Yilgarn Plateau province occupies 21% of the region and comprises sand and hardpan plains with The Pilbara province occupies 50% of the region. In outgoing drainage and salt lakes, broken by ridges of the north along the coast are flood and deltaic plains metamorphic rocks and granite. with tidal flats and some metamorphic, volcanic and granitic hills. Inland from this are dissected Figure 11.2 Physiographic provinces of the Pilbara–Gascoyne region 6 Australian Water Resources Assessment 2012 11.3.2 Elevation The low-lying coastal zone covers a large part of the region and is only intercepted by the Cape Range. Figure 11.3 presents ground surface elevations in the It contains limestone plateaus exceeding 300 m Pilbara–Gascoyne region. Information was obtained in altitude. from the Geoscience Australia website (www. The eastern border of the region is not an obvious ga.gov.au/topographic-mapping/digital-elevation-data. water divide in a topographical sense. Many smaller html).The region encompasses two major plateaus: and larger internally draining basins exist on both the Pilbara plateau in the north and the Gascoyne sides of the border. plateau in the southeast. The western part of the region includes coastal and inland dunes and alluvial floodplains, with some low hills in the southwest. In the north of the region, the Hamersley Range form a dominant landscape feature. Peaks exceed 1,200 m above sea level, the highest in Western Australia. Figure 11.3 Ground surface elevations in the Pilbara–Gascoyne region Australian Water Resources Assessment 2012 7 Pilbara–Gascoyne 11.3.3 Slopes The flat stretch to the north of the ranges is the Fortescue River and includes the Fortescue Marsh Areas with steep slopes provide higher run-off wetland. generating potential than flat areas. The Pilbara– The Gascoyne plateau