Groundwater yields in south-west Western Australia

Transcription

1 Groundwater yields in south-west Western Australia Summary of a report to the Australian Government from the CSIRO South-West Western Australia Sustainable Yields Project December 2009 Summary 2

2 About the project Scenarios assessed In 2007 and 2008, CSIRO produced a series of reports examining the likely water yield of surface water and groundwater catchments in the Murray-Darling Basin as a result of future climate changes and possible land management changes such as afforestation and farm dams. On 26 March 2008, the Council of Australian Governments (COAG) agreed to expand the CSIRO assessments of sustainable yields to northern Australia, Tasmania and south-west Western Australia (SWWA). For the first time, Australia will have a comprehensive scientific assessment of water yields in all major water systems across the country, which allows a consistent analytical framework for water policy decisions across the nation. The CSIRO South-West Western Australia Sustainable Yields Project has estimated the likely water yields of all major fresh, marginal and brackish surface water and groundwater systems between Geraldton and Albany under the same climate and development scenarios as used in the other three projects, except that the historical climate data were of shorter length (the 33-year period from 1975 to 2007). The project has also estimated future water demands and compared these with likely future yields from all water resources under all scenarios. For the first time: A consistent set of future climate inputs have been compiled for use in surface water and groundwater models over the main water catchments in SWWA. Recharge to aquifers has been estimated under climate scenarios using a consistent methodology over all major aquifers in the region. Groundwater modelling has been undertaken over an area of about 20,000 km 2 to estimate future groundwater levels and water balances under climate and development scenarios. The project has reported the results in three main reports titled: Surface water yields in south-west Western Australia Groundwater yields in south-west Western Australia Water yields and demands in south-west Western Australia. This report presents modelled groundwater yields across the project area. In the surface water report, runoff projections under the future climate are presented for every major stream, and projected inflows to all major dams across the project area are reported. The water yields and demands report presents estimates of future water demands by major water user groups and compares these with possible future surface water and groundwater yields to identify areas where water shortages may occur and where there may be impacts on water dependent ecosystems. Companion technical reports provide more detail on the methods and results. The assessments reported here have been reviewed by expert staff within the Department of Water Western Australia, a Technical Reference Panel, external reviewers, and a Steering Committee with representatives from the Australian and Western Australian governments. The assessments of current and future water availability were made by considering a range of climate and development scenarios. The historical climate scenario was based on the climate of the historical past (1975 to 2007). This period was chosen because the climate in SWWA changed in about 1975 resulting in substantially reduced streamflows. This scenario was used to assess water yields should the climate in the future prove to be similar to that of the historical past. This scenario was used as the baseline against which other scenarios are compared. Current levels of surface water and groundwater development were used. The recent climate scenario was based on the climate of the recent past (1997 to 2007). This scenario was used to assess water yields should the climate in the future prove to be similar to that of the recent past. Current levels of surface water and groundwater development were used. The future climate scenario used 15 global climate models with three estimates of temperature changes (due to global warming) to provide a spectrum of possible ~2030 climates. From this spectrum three were selected for reporting, representing a wet extreme, median and dry extreme future climate. Current levels of surface water and groundwater development were used. The future climate with future development scenario used the same climate time series as the median future climate scenario, but future levels of development were used (by increasing groundwater abstractions to full allocation limits). Acknowledgments The South-West Western Australia Sustainable Yields Project was undertaken by CSIRO under the direction of the Australian Government Department of the Environment, Water, Heritage and the Arts. Important aspects of the work were undertaken by the Department of Water Western Australia. The Water Corporation and the Western Australia Department of Agriculture and Food provided expert advice and data. A contract to develop a groundwater model for part of the region was undertaken by URS Australia Pty Ltd. Additional technical input was provided by CyMod Systems, Jim Davies and Associates, Resource Economics Unit and Geographic Information Analysis. Valuable feedback was received during the review process from Tony Jakeman, Andy Pitman, Don Armstrong, Peter Davies and Murray Peel. Disclaimers Derived from or contains data and/or software provided by the Organisations. The Organisations give no warranty in relation to the data and/or software they provided (including accuracy, reliability, completeness, currency or suitability) and accept no liability (including without limitation, liability in negligence) for any loss, damage or costs (including consequential damage) relating to any use or reliance on the data or software including any material derived from that data or software. Data must not be used for direct marketing or be used in breach of the privacy laws. Organisations include: Department of Water Western Australia, Bureau of Meteorology, Water Corporation and the Western Australia Department of Agriculture and Food. CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensation, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. Data are assumed to be correct as received from the organisations. CSIRO 2009 all rights reserved. This work is copyright. Apart from any use as permitted under the Copyright Act 1968, no part may be reproduced by any process without prior written permission from CSIRO. 2 Groundwater yields in south-west Western Australia >> > Cover: Sunrise over Lake Joondalup, Wanneroo, WA (CSIRO)

3 Overview of the project area The geographic extent of the South-West Western Australia Sustainable Yields Project includes all fresh, brackish and marginal surface water basins from Gingin Brook, north of Perth, to Albany, and groundwater resources in the Perth and Collie basins and the west Bremer Basin near Albany in the south-east (Figure 1). This area covers all current and anticipated future water resources in SWWA suitable for irrigation, domestic water supplies and industries that require low salinity water. Inland water supplies are either limited or too saline for these uses. As a result these inland areas are supplied with water from the project area. The project area covers about 62,500 km 2 and contains over 1.9 million people or 89 percent of the population of Western Australia. It is concentrated over the highest rainfall part of the south-west of the state and includes all of the state s irrigation areas with the exception of the Gascoyne (Carnarvon) and the Ord (Kimberley). Groundwater is a major source of water in the Perth and Collie groundwater basins and near Albany (Figure 1). The Perth Basin comprises the flat sandy Swan and Scott coastal plains, and more elevated and clayey plateaux such as the Blackwood and Dandaragan (Figure 2). The dissected western Darling Plateau contains most of the usable surface water resources. Groundwater in this area is contained in clayey weathering zones and becomes more saline to the east. Interactions between surface water and groundwater occur in both the Collie and Perth basins. >>Coastal sand dunes near Bunbury, WA (CSIRO) >>Figure 1. Groundwater resources and surface water resources in the project area >>Figure 2. Main geomorphologic regions in the project area December

4 Key findings About 74 percent of all water used in the project area is from groundwater. Almost all increased demand for water is being met by groundwater so its proportion of total use has been increasing. A future drier and hotter climate is likely to affect groundwater levels differently depending on vegetation cover, soil type, depth to watertable and level of abstraction. The sandy coastal plain soils (where perennial vegetation has been cleared) are expected to be fairly resilient to all but extreme climate change and high abstraction. As watertables fall, less groundwater is lost to drainage and evaporation. Areas with perennial vegetation and clayey soils are expected to be most impacted by climate change. Interactions between surface water and groundwater at several locations may change in amount and type as a result of lower water levels in rivers and their surrounding aquifers. Climate Up to 80 percent of the annual rainfall in the project area occurs between May and October. Temperatures are also at their lowest during this period, making the rainfall more effective in terms of producing runoff and recharge. There is a strong south-west to north-east gradient in rainfall (Figure 3) with the highest rainfall in south-west coastal parts and along the Darling Range east of the Darling Scarp (Figure 2). Annually, it exceeds 1200 mm in these areas and is less than 350 mm in the north-east. The mean annual areal potential evapotranspiration (APET) varies from 1650 mm in the north to 1180 mm in the south (Figure 4). When mean annual rainfall deficit is calculated (by subtracting APET from rainfall), almost all except the extreme south of the project area has a negative moisture balance (Figure 5). >>Rainfall in parklands near Perth (CSIRO) All historical climate data for this project are for the period 1975 to 2007 because a climate shift occurred in SWWA in the mid-1970s. Mean annual rainfall for this historical climate period is 10 to 15 percent lower than the long-term mean. All water managers use rainfall data from after 1975 or even more recently for planning purposes. > >Figure 3. Mean annual rainfall >> Figure 4. Mean annual areal potential evapotranspiration >>Figure 5. Mean annual rainfall deficit (rainfall less areal potential evapotranspiration) 4 Groundwater yields in south-west Western Australia

5 Groundwater models suitable for estimating the impact of climate change and development are required for the Northern Perth Basin and the Albany Area to extend the results of this project. >>Lake Cave near Margaret River, WA (CSIRO) Factors affecting recharge The main land uses in the groundwater part of the project area are dryland agriculture (56 percent); native vegetation including forests and national parks (38 percent); urban (3 percent); softwood plantation forestry (1 percent); open water and estuaries (1 percent); and irrigation and other summer wet areas (1 percent). Over two-thirds of the Northern Perth Basin is under dryland agriculture but less than one-third of the Southern Perth Basin has this high recharge land use. The proportion of rainfall that becomes recharge is greatly affected by soil type, vegetation cover and depth to watertable. It is highest when soils are sandy; there is little or no perennial vegetation; and the watertable is neither very shallow (so that potential recharge is rejected and Groundwater use evaporation losses are high) nor very deep (so that roots can intercept it before it reaches the watertable). Much of the Swan Coastal Plain is occupied by dunal sands with clayey soils fringing the scarp and major rivers. Most of the plain has been cleared of vegetation, except on the Gnangara groundwater system north of Perth and the coastal strip between Jurien Bay and Dongara. The depth to watertable is 3 m over 22 percent of the southern half of the Perth Basin (comprised of the Central Perth Basin, Peel-Harvey Area and Southern Perth Basin), whilst 14 percent of the area is between 3 and 6 m and 10 percent is between 6 and 10 m (Figure 6). These areas support groundwater dependent ecosystems. Given these circumstances, recharge under the Swan Coastal Plain is generally high except where watertables are very high, there is perennial vegetation or the soils are clayey. Recharge under the Scott Coastal Plain is also favoured by high rainfall, sandy soils and agricultural clearing in some areas but is even more limited by high watertables, especially in the west. The Dandaragan Plateau and Arrowsmith Region are largely cleared for dryland agriculture and have moderately deep watertables which encourage recharge. However, soils of these regions are more gravelly and clayey than the coastal soils and the region s rainfall is lower. Most of the Blackwood and Darling plateaux are covered with native vegetation or plantations. Their soils are more gravelly and clayey than in all other areas, and watertables tend to be deep. Therefore despite high rainfall, recharge is significantly inhibited. Annual groundwater use in the project area is estimated to currently be about 850 gigalitres (GL) which is about 74 percent of all water used. The main uses are for drinking water supplies in Perth and towns, and self-supply for the irrigation of public and private lawns and gardens, horticulturalists, industry and commerce. Unrestricted demand projections for 2030 are between 970 and 1380 GL/year indicating that some industries and user groups might be required to increase water use efficiencies, find alternative sources or be limited by a lack of groundwater. Abstraction throughout the project area is very non-uniform because the aquifers are variable in extent, depth and salinity, and demand is uneven (Figure 7). The area near Perth has the highest use because of the needs of drinking water, peri-urban horticulture, private and public parks, and gardens and industry. This area has deep aquifers, especially north of the Swan River, which are now fully exploited. Concentrations of use also occur on the Swan Coastal Plain south of Perth associated with irrigation and industrial demands. >>Sprinklers watering a crop at Wanneroo, WA (CSIRO) December

6 >>Figure 6. Depth to watertable in the southern half of the Perth Basin >>Figure 7. Groundwater abstractions in the Perth Basin 6 Groundwater yields in south-west Western Australia

7 Change in groundwater levels between 2008 and 2030 all scenarios except the dry extreme future climate and the future development scenario. Vegetated parts of the Scott Coastal Plain have lower groundwater levels under the dry extreme climate. Groundwater levels under most of the Blackwood Plateau fall by 3 m or more under all scenarios because of the perennial vegetation cover and clayey soils. One small area in the central south of the plateau is estimated to have higher groundwater levels. >>Water level gauge at Perry Lakes, Perth (CSIRO) Results from the three groundwater models (PRAMS, PHRAMS and SWAMS) which cover the 20,000 km 2 southern half of the Perth Basin were combined to estimate the change in groundwater levels between 2008 and 2030 under climate and development scenarios (Figure 8). The effect of vegetation, soil type, abstraction and depth to watertable on groundwater levels is evident. Groundwater levels continue to rise under the Dandaragan Plateau despite this being the driest modelled area because the area is cleared, the watertable is relatively deep, soils are reasonably permeable and abstraction is modest. Groundwater levels under the western Swan Coastal Plain are estimated to fall by less than 3 m by 2030 in all except the Gnangara Mound (within the Gnangara groundwater system) which is vegetated and has relatively high levels of abstraction (Figure 7). This is due to sandy soils and lack of vegetation. However, in areas with high watertables, potential recharge is lost through drainage and evapotranspiration. Despite the reduction in rainfall, the shallow aquifers refill each winter under Groundwater models and other methods used Four regional groundwater models were used to estimate the impact of climate and development on groundwater levels by 2030: The Perth Regional Aquifer Modelling System (PRAMS) was used in the Central Perth Basin (Figure 1). A new groundwater model, the Peel-Harvey Regional Aquifer Modelling System (PHRAMS), was developed for the Peel-Harvey Area. The South West Regional Aquifer Modelling System (SWAMS), recalibrated and coupled with a vertical flux model in this project, was used in the Southern Perth Basin. The Collie Basin model, recalibrated and coupled with a vertical flux model in this project, was used in the Collie Basin. In the Northern Perth Basin and Albany Area, where suitable groundwater models were not available, the impact of climate on recharge was estimated using the Water Atmosphere Vegetation Energy Solutes (WAVES) model and the impact on groundwater levels was estimated using the HARTT hydrograph analysis technique. Most of the results presented are for the southern half of the Perth Basin and the Collie Basin where groundwater models could be used. The effect of future development (increasing groundwater abstraction to allocation limits) was assessed for the Southern Perth Basin (Figure 9). Under the median future climate, groundwater levels decline by less than 3 m over most of the area which is partly due to abstraction already being at the maximum limit. The main areas where levels decline are the northern Swan Coastal Plain, the Peel-Harvey Area and the Scott Coastal Plain. Some of these areas have rising groundwater levels under the median future climate. Groundwater levels in the Collie Basin fall over most of the area under all scenarios due to both climate and abstraction for coal and power production (Figure 10). Areas where groundwater levels are increasing coincide with open coal pits that are gradually filling with groundwater. Levels are least affected where the south and east Collie River branches cross the sedimentary basin. This is because groundwater that previously discharged to the river diminishes. Under the dry extreme future climate and under future development the river may discharge to the aquifer. It is likely that groundwater in the Northern Perth Basin will continue to rise in areas with sandy soils and under cleared agricultural land, especially in the south of this region. Under perennial vegetation, groundwater levels decline by up to 2 m under all future climate scenarios. The Albany Area appears similar to vegetated sandy areas on the Swan and Scott coastal plains with stable or rising levels under the historical climate, stable levels under the recent climate, and falling levels under all future climate scenarios. December

8 Groundwater level changes between 2008 and 2030 (cont.) >>Figure 8. Change in groundwater levels between 2008 and 2030 in the southern half of the Perth Basin under climate and development scenarios 8 Groundwater yields in south-west Western Australia

9 >>Figure 9. Change in groundwater levels in the southern half of the Perth Basin under future development relative to the median future climate >>Figure 10. Change in groundwater levels between 2008 and 2030 in the Collie Basin under climate and development scenarios December

10 Surface groundwater interactions Surface groundwater interactions were evaluated for the Blackwood and Capel rivers in the Southern Perth Basin, Gingin Brook in the Central Perth Basin, and the Collie River in the Collie Basin. In the Southern Perth Basin the modelled mean annual groundwater discharge to the Blackwood River is about 33 GL under the historical climate. Under all other climate and development scenarios, fresh groundwater discharge to the brackish river reduces by 3 to 27 percent relative to the historical climate (Table 1). The Capel River is used for irrigation and has important environmental flows. Under the historical climate, groundwater discharge to the river is about 11 GL/year which may reduce by between zero and 19 percent under all other scenarios (Table 1). The largest reduction in groundwater discharge occurs after groundwater abstraction is increased to allocation limits (under future development). Sections of Gingin Brook with gaining and losing flow are estimated to be 20 and 9 km long respectively, with baseflow constituting about 67 percent of total flows. The average groundwater discharge into the brook is between 30 and 58 GL/year under climate and development scenarios (Table 1). Under the dry extreme future climate, groundwater discharge to the brook may reduce by about 48 percent relative to the historical climate. The impact of increased abstraction under future development is only 2 GL/year. The Collie River surface water and groundwater interact both seasonally and spatially throughout the Collie Basin. Over the 2008 to 2030 period, river losses to groundwater increase under the median and dry extreme future climate in the Cardiff sub-basin, resulting in the river becoming a losing stream (Table 2). With only a small length of the river overlying the Premier sub-basin, the river exchanges with groundwater are very small but follow a similar pattern. Leakage of river water may have implications for groundwater quality as both branches of the Collie River contain brackish to saline water at different periods of the year. >>Aerial view of groundwater lake north of Perth, WA (CSIRO) >>Table 1. Mean annual groundwater discharge to creeks and rivers in the Southern Perth and Central Perth basins Scenario Groundwater discharge ( ) Southern Perth Basin Central Perth Basin Blackwood River Capel River Gingin Brook Percent change from the historical climate Groundwater discharge ( ) Percent change from the historical climate Groundwater discharge ( ) Percent change from the historical climate GL/y percent GL/y percent GL/y percent Historical climate Recent climate 32 3% 11 0% 39 33% Wet extreme future climate 29 12% 11 0% 56 3% Median future climate 26 21% 10 9% 42 28% Dry extreme future climate 24 27% % 30 48% Future development 25 24% % 40 31% >>Table 2. Mean annual groundwater discharge to the Collie River in the Collie Basin Collie River leakage within River head reduction Scenario Premier sub-basin Cardiff sub-basin GL/y % Historical climate 0% Recent climate 10% Wet extreme future climate 0% Median future climate 20% Dry extreme future climate 40% Future development 20% Groundwater yields in south-west Western Australia

11 Limitations The main results produced in this report are estimates of groundwater levels by 2030 under climate and development scenarios. An assessment of the confidence in these estimates is shown in Figure 11. This assessment is based on the amount and quality of groundwater data; the level of understanding of the area s hydrogeology and its conceptualisation within a groundwater model; the maturity and calibration accuracy of the groundwater model; and whether water balance components or future groundwater level estimates could be checked by more than one method. The highest confidence is in the Swan Coastal Plain near Perth where there is an extensive bore network and the groundwater model has been checked and modified over the last seven years. The south-west groundwater model is also thought to be reasonable for the coastal plain and western Scott Coastal Plain. The new model for the Peel-Harvey Area has only a medium to low level of confidence as it has not been evaluated under enough situations nor had independent estimates made of the water balance. The Dandaragan Plateau has a low level of confidence because of a paucity of longterm monitoring bores. Finally, the two areas without suitable models have a very low level of confidence: the Northern Perth Basin and the Albany Area. >>Figure 11. Confidence in the estimates of future groundwater levels in parts of the project area that contain groundwater resources December

12 Where to from here The Northern Perth Basin has underutilised groundwater resources but they are located in an area that is coming under increasing pressure from mining and construction development. This area constitutes almost half of the onshore Perth Basin and would greatly benefit from having a groundwater model for assessing the future impacts of climate and development. The Central Perth Basin contains Australia s most important groundwater resource, satisfying over half of the domestic water needs of about 1.9 million people as well as supporting horticulture, industry and the irrigation of parks and gardens. Reliable estimation of the groundwater resources that will be available in the future under a drying climate is very important and requires good hydrological data and a reliable groundwater model. The model for this area has improved significantly in recent years and is suited for regional and subregional assessments of the impacts of climate, land use changes and abstraction. The model is poorly calibrated in the Dandaragan Plateau and northern Swan Coastal Plain. It could be improved with better monitoring data, especially of the deep aquifers; better quantification of rainfall recharge under key land uses; a better representation of the drainage network and wetlands; an accurate measure of abstraction; a better representation of the hydrogeology of poorly mapped onshore and offshore formations and faults; and an improved mapping of coastal and western boundaries. The results of these groundwater analyses have been converted into possible future water yields and, in combination with surface water yields, have been compared with expected future water demands in the report titled Water yields and demands in south-west Western Australia. For further information: CSIRO Water for a Healthy Country Flagship Project Leader Dr Don McFarlane Phone: (08) don.mcfarlane@csiro.au Web: MCK218 Dec 2009 SWSY_Groundwater_Summary.indd Web: Enquiries More information about the project can be found at This information includes the full terms of reference for the project, an overview of the project methods and the project reports that have been released to-date. More information on the Australian Government s Water for the Future plan can be found at Printed on recycled paper