Let’s look at similar water storage and distribution systems to the New Bradfield Scheme, in California.
California is 400 km wide and 1200 km long. It has a land area of 423,970 km2 of which 36,421 km2 is irrigated. Agricultural production is $47 billion per year, a large proportion of which is irrigated or irrigation related.
California is largely desert and its population centres and agriculture watered by a number of long aqueducts exceeding 500 km in length.
For example, the California State Water Project, commonly known as the SWP, collects water from rivers in Northern California and redistributes it to the water-scarce but populous south through a 650 km length aqueduct, with pumping stations and power plants. About 70% of the water provided by the project is used for urban areas and industry in Southern California and the San Francisco Bay Area, and 30% is used for irrigation in the Central Valley.
By comparison, Northern Queensland has a land area of approximately 500,000 km². An extended Bradfield Scheme may provide irrigation for 30,000 km2 of land. This would be achieved by a system of aqueducts and dams, of similar length to similar to those in California. In return, we would expect agricultural production of $50 billion per year, similar to California.
In some ways Queensland is more suited than California, as due to good luck or blessing, the new Bradfield Scheme may entirely gravity fed through tunnels and aqueducts. The cost of water may therefore be considerably less than the cost of water in the SWP in California.
The construction of the aqueducts in the new Bradfield Scheme could be staged, developing first those areas that are closer to population centres and prepared for irrigation, and releasing the excess water into parched river systems. However, the irrigation of large areas black soil plains in the central state would open up the largest areas to new agriculture.