Research Library Resource management technical reports Natural resources research 2-2010 Modelling the impacts of a mixed forestry plantation on groundwater resources in the Beaufort River area, Western Australia Paul Raper Richard J. George Dr Follow this and additional works at: https://researchlibrary.agric.wa.gov.au/rmtr Part of the Agriculture Commons, Environmental Indicators and Impact Assessment Commons, Environmental Monitoring Commons, Fresh Water Studies Commons, Hydrology Commons, Natural Resources Management and Policy Commons, Soil Science Commons, and the Water Resource Management Commons Recommended Citation Raper, P, and George, R J. (2010), Modelling the impacts of a mixed forestry plantation on groundwater resources in the Beaufort River area, Western Australia. Department of Primary Industries and Regional Development, Western Australia, Perth. Report 358. This report is brought to you for free and open access by the Natural resources research at Research Library. It has been accepted for inclusion in Resource management technical reports by an authorized administrator of Research Library. For more information, please contact [email protected]. ISSN 1039-7205 Modelling the impacts of a mixed forestry plantation on groundwater resources in the Beaufort River area, Western Australia GP Raper and RJ George February 2010 RESOURCE MANAGEMENT TECHNICAL REPORT 358 Disclaimer The Chief Executive Officer of the Department of Agriculture and Food and the State of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it. Copyright © Western Australian Agriculture Authority, 2010 Copies of this document are available in alternative formats upon request. 3 Baron-Hay Court, South Perth WA 6151 Tel: (08) 9368 3333 Email: [email protected] www.agric.wa.gov.au Header Contents Page Acknowledgments .................................................................................................... ii Summary ................................................................................................................... iii 1. Introduction and background ......................................................................... 1 2. Methods ............................................................................................................ 2 2.1 Study area ............................................................................................... 2 2.1.1 Hydrogeology ....................................................................................... 3 2.1.2 Groundwater pumping .......................................................................... 6 2.1.3 Soils ...................................................................................................... 8 2.2 Conceptual groundwater model ........................................................... 8 2.3 Numerical groundwater model ............................................................. 10 2.3.1 Model calibration .................................................................................. 13 2.3.2 Scenarios .............................................................................................. 13 3. Results ............................................................................................................. 16 3.1 Calibration—Base case ......................................................................... 16 3.2 Reduced rainfall scenarios ................................................................... 19 3.3 Plantation impact scenarios ................................................................. 20 3.3.1 Zero Recharge under planted area ...................................................... 20 3.3.2 Low groundwater use scenarios ........................................................... 21 3.3.3 High groundwater use scenarios .......................................................... 26 3.3.4 Combined reduced recharge -plantation groundwater use scenarios . 30 3.4 Estimated salt accumulation under plantation ................................... 34 3.5 Summary of predicted impacts .............................................................. 34 4. Discussion and recommendations ................................................................ 36 4.1 Groundwater monitoring ....................................................................... 36 4.2 Further groundwater modelling ............................................................ 37 5. References ....................................................................................................... 39 Appendix 1. Proposal for Monitoring and Review Program ............................... 41 Appendix 2. Bore details ........................................................................................ 44 i February 2010 Acknowledgments The authors would like to acknowledge the valuable inputs of Stewart Crombie (FPC) for the provision of field data collected by FPC staff and Colin Fairclough (DAFWA, Bunbury) for groundwater monitoring. Jane Kowald sited the DAFWA bores drilled to the south of the Beaufort River palaeochannel aquifer and supervised the drilling. We would like to thank the landholders for their co-operation in providing information on bore locations and pumping to FPC and directly to us, and also for permission to monitor bores on their properties. We would also like to thank Don Bennett (DAFWA, Bunbury) and Ryan Vogwill (Department of Environment and Conservation, Kensington) for their insight in reviewing early drafts of this report and contributing substantially to it meeting the purpose for which it was intended. Similarly, we wish to thank Peter Morrell and Geoffrey Lush for their feed back on our verbal presentation of these results. ii Impacts of a plantation on groundwater resources, Beaufort River, Western Australia Summary During 2008 and 2009, the Western Australian Forest Products Commission (FPC) established a large mixed species plantation covering most of a 1400 ha property on Leggoe Road at Beaufort River, in the Upper Great Southern region of Western Australia. The property falls within the Shire of Woodanilling, although its western boundary adjoins the Shire of West Arthur. A numerical model of the Beaufort River plantation and surrounds was established using the US Geological Survey groundwater model MODFLOW (McDonald and Harbaugh, 1988). The purpose of the modelling was to forecast the likely impacts of the plantation on the local groundwater system and provide guidance on siting bores required to monitor the impacts of the plantation on access to groundwater resources by neighbouring landholders. The model closely predicted observed groundwater heads in the surficial aquifer and in particular under and adjacent to the plantation. The predicted pattern of areas of shallow groundwater also matched the Land Monitor salinity map. The model predicted dry areas in the surficial aquifer on the valley flanks. Due to a lack of data, we modelled only three of the thirteen bores known to be used for water supplies and have not taken into consideration planned irrigation developments proposed for neighbouring properties. A range of reduced recharge and plantation impact scenarios where run. A scenario in which the maximum rate of groundwater uptake by the plantation was set to 300 mm/y and the maximum extraction depth was 6 m was considered to represent the most likely impact of the plantation on groundwater levels in the surficial aquifer. The model predicted that for this scenario, the actual plantation water use from the surficial aquifer would be 50 mm/y (700 ML/y), and that the maximum watertable reduction would be 4.5 m relative to the base case; the spatial distribution of watertable reduction is shown in Figure 19 (page 26). At this level of impact, shallow soaks, close to the plantation, used to supply stock water are likely to be impacted but landholders using bores at current rates are not likely to be significantly affected. Other scenarios, predicting greater impacts are possible but less likely due to variability in the depth to which the trees are likely to be able to access groundwater. A scenario in which the maximum rate of groundwater use was set to 500 mm/y, with a maximum extraction depth of 6 m resulted in a plantation water use from the aquifer of 54 mm/y (755 ML/y). Despite the relatively small actual increase in water use, this scenario would result in a yield reduction at the bore supplying the Beaufort Meats abattoir on Leggoe Road. These results highlight the role of monitoring, evaluation and need for adaptive management plans. Finally, the model predicted that there would be a reduction in area with a shallow watertable (salinity and risk) as a consequence of water use by the plantation. The model forecast that as a result of lowered water levels this area may extend some 800 m beyond the plantation. As MODFLOW does not explicitly model the unsaturated zone or surface water flows, this result is considered indicative of the order of magnitude of the potential impacts only. Recommendations for further drilling and groundwater monitoring, to be used to verify the model predictions and detect significant negative impacts on groundwater levels are presented. Recommended sites for additional bores are shown in Figure 32 (page 40). Furthermore, the FPC currently monitors thirteen bores around the plantation using automatic data loggers, several of these bores are slotted in either the