Abstract: Key findings for the Mitchell catchment: Introduction. The Mitchell catchment is approximately 72,000 km2 and flows into the Gulf of Carpentaria. It includes part of the Mareeba–Dimbulah Water Supply Scheme (MDWSS) (Figure 1-1) and supports a population of approximately 6000 people. There are no major urban centres. Pastoralism comprises over 95% of the catchment land use. The second largest land use, conservation reserves, covers about 3% of the catchment. Indigenous people have continuously occupied and managed the Mitchell catchment 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 Mitchell catchment has up to 3 million ha of potentially irrigable agricultural soils. Of this land area, 2.5 million ha are suitable for dry-season spray irrigation of cereals, cotton and soybean. The area suitable for furrow irrigation of the same crops is 1.3 million ha. There are 2.5 million ha and 1.3 million ha suitable for irrigation of sugarcane by spray and furrow irrigation, respectively. Just over 3 million ha are suitable for Rhodes grass with spray irrigation and 600,000 ha suitable for wet-season forage sorghum with spray irrigation. About 235,000 ha are suitable for aquaculture, 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.The total amount of beef produced each year by existing cattle breeding enterprises could be increased by using irrigated forages to overcome some of the productivity constraints inherent with reliance on native pastures. Access to standing green forage or high-quality hay (Figure 1-1) could increase weight gain in young cattle and enable early weaning of calves which, in turn, increases subsequent calving percentage by reducing nutritional pressure on lactating cows.Significant new instream storages are possible. The four most cost-effective major instream dams in the Mitchell catchment are capable of delivering approximately 2800 GL in 85% of years, which is sufficient water to irrigate 140,000 ha of sugarcane. This could generate an annual gross value of production of approximately $720 million, and the region would benefit from $1.5 billion of economic activity reoccurring annually and the generation of about 7250 jobs. Of the 2800 GL, 65% could be delivered by two potential dams, the Pinnacles dam site on the Mitchell River (2316 GL capacity) and Rookwood dam site on the Walsh River (1288 GL storage). These would yield 1248 GL and 575 GL, respectively, to agriculture in 85% of years. Offstream water harvesting could extract 2000 GL annually, with 85% reliability, which would be sufficient to irrigate 200,000 ha of cotton. Groundwater opportunities in the catchment are relatively small and localised, with the Bulimba aquifer offering up to a total of 5 GL/year with well-placed bores. Impacts and risks: Whether based on groundwater 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.Instream storages, such as the potential Pinnacles and Rookwood dams, require trade-offs that occur over both time and space. The upfront cost of the potential Pinnacles dam is estimated at $755 million and would generate an income stream that may contribute to the cost of construction. The dam would have a major impact on habitat immediately below the dam, and would potentially have an ongoing moderate impact downstream by affecting the perennial flow of the Mitchell River. Pumping water into offstream storages (water harvesting) was predicted to have a minor impact to the flow habitat of freshwater aquatic, riparian and marine ecosystems. Offstream water storages usually have lower impacts than major instream dams, partly because water extraction occurs during floods and is restricted in low-flow periods. Streams, wetlands and riparian areas remain of critical importance to Indigenous people. They have cultural significance and provide nutritional foo
Petheram, Cuan, Watson, Ian, Bruce, Caroline, Chilcott, Chris, 2018, Water resource assessment for the Mitchell catchment: 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 07 August 2022, https://www.nintione.com.au/?p=14889.