Water resource assessment for the Darwin catchments: A report to the Australian Government from the CSIRO Northern Australia Water Resource Assessment, part of the National Water Infrastructure Development Fund: Water Resource Assessments

Water resource assessment for the Darwin catchments: A report to the Australian Government from the CSIRO Northern Australia Water Resource Assessment, part of the National Water Infrastructure Development Fund: Water Resource Assessments Report

  • Author(s): Petheram, Cuan, Chilcott, Chris, Watson, Ian, Bruce, Caroline
  • Published: 2018
  • Publisher: CSIRO

Abstract: Key findings for the Darwin catchmentsIntroduction In the Darwin catchments the Finniss, Adelaide, Mary and Wildman rivers flow through extensive coastal and marine floodplains into the Arafura Sea. Land use in the 30,000 km2 that make up the Darwin catchments is dominated by conservation and natural environments (38%), extensive grazing (32%) and dryland and irrigated cropping (7%). About 140,000 people live in the 2% of the landscape comprising urban and peri-urban development.Indigenous people have continuously occupied and managed the Darwin catchments for tens of thousands of years and retain significant and growing rights and interests in land and water resources, including crucial roles in water and development planning and as co-investors in future development.Agriculture and aquaculture opportunitiesThe Darwin catchments have up to 1 million ha of potentially irrigable agricultural soils. Of this land area, 800,000 ha are suitable for trickle-irrigated crops such as mangoes ( Figure 1-1), whereas about 90,000 ha are suitable for flood-irrigated crops such as rice. A further 420,000 ha of land is moderately suitable for aquaculture, including species such as prawns and barramundi, grown in lined ponds. For all of these uses the land is considered moderately suitable with considerable limitations and would require careful soil management.Groundwater is the Darwin catchments’ most important consumptive water resource. Aquifers in the Darwin Rural Water Control District (DRWCD) currently provide an estimated 25 gigalitres (GL) for the purpose of irrigated agriculture, horticulture, public water supplies and local domestic use. New groundwater resources outside of the DRWCD (35 GL) could, if allocated, enable an additional 7800 ha of trickle-irrigated vegetable production, which could add $320 million and 345jobs to the regional economy.Significant new instream surface water storage is possible. Potential dams at Mount Bennett on the Finniss River (343 GL capacity) and the upper Adelaide River (298 GL capacity) could releaseapproximately 436 GL for agriculture in 85% of years. This could support 40,000 ha of mangoes or 60,000 ha of trickle-irrigated vegetables, enabling just 2% of the area of the catchment to add $2.3 billion and 2500 jobs to the regional economy.Offstream water harvesting is possible on the Margaret River in the Adelaide catchment (200 GL) and the McKinley and Mary rivers in the Mary catchment (400 GL). This could provide water sufficient to trickle irrigate 50,000 ha of vegetables, although the proximity of irrigable soils to locations suitable for water storage may be a limitation of this area.Impacts and risksWhether based on groundwater, instream dams or offstream storage, irrigated agricultural development has a wide range of potential benefits and risks that differentially intersect diverse stakeholder views on ecology, economy and culture. The detailed reports upon which this catchment report is based provide information that can be used to quantify the trade-offs required for agreed development plans.The general impacts of potential new groundwater-based developments may include a reduction in spring flows and an increase in the depth to groundwater beneath groundwater-dependent vegetation. These impacts can be reduced with good planning, such as evidence based water allocations and appropriately siting groundwater bore infrastructure. In some areas with existing groundwater development, there may be opportunity to intentionally recharge water to underlying aquifers, referred to as managed aquifer recharge (MAR), for subsequent recovery or to provide environmental benefit. Instream storages, such as the potential upper Adelaide River dam, require trade-offs that occur over both time and space. Construction of large instream dams provide water that is generally secure across many years. This requires significant upfront investment ($190 million) that is intended to generate a future income stream that may contribute to the cost of investment. Instream dams significantly disrupt their immediate upstream and downstream environments but, when located high in the catchment, as with the potential upper Adelaide River dam, have negligible to minor impacts on coastal floodplains and their related mosaic of nationally important wetlands. Streams, wetlands and riparian areas remain of critical importance to Indigenous people. They have cultural significance and provide nutritional food.Pumping water into offstream storages (water harvesting) generally has less impact on freshwater aquatic, riparian and marine ecosystems than major instream dams, in part because water extraction occurs mainly during floods and is restricted during low-flow periods. Offstream storages are readily scaled to match the availability of financial and physical capital. They are not usually capable of securing water for more than 1 year and, as a result, they ‘make good wet seasons better’ rather than reliably ‘making dry seasons wet'.

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Petheram, Cuan, Chilcott, Chris, Watson, Ian, Bruce, Caroline, 2018, Water resource assessment for the Darwin catchments: A report to the Australian Government from the CSIRO Northern Australia Water Resource Assessment, part of the National Water Infrastructure Development Fund: Water Resource Assessments, Report, viewed 10 August 2022, https://www.nintione.com.au/?p=14890.

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