Northern Territory

Case studies to determine the economic feasibility
R Evans, L Lennon, G Hoxley, R Krake, D Yin Foo, C Schelfhout, J Simons
Publication Date (Web): 14 August 2017

Managed aquifer recharge (MAR) is a commonly used technique in many countries to artificially increase the recharge rate over the wet season and hence increase the groundwater storage available during the dry season. Considering northern Australia’s long dry season and relatively short wet season MAR has the potential to play a major role in water resource development.    
The fundamental advantages of MAR based developments over conventional water sources (typically large dams) are the cost of transporting water, lack of evaporative losses, scalability of MAR projects and the variability of the magnitude of the wet season. Conversely, impediments to irrigation based MAR schemes in northern Australia are believed to be primarily the economic feasibility of the total irrigation development, which includes a MAR component, not technical feasibility. When other water supply options are compared, MAR can be attractive both economically and environmentally.   

The implicit notion underpinning this project is that mosaic irrigation is preferred to large scale development. Mosaic irrigation is where individual relatively small scale irrigation developments are spread over large areas and hence in aggregate represent a significant area of irrigated agriculture.  Mosaic irrigation has been discussed by several authors (e.g. Grice et al., 2013) who point out the advantages over traditional large scale, predominantly surface water based development. The mosaic concept allows the development to gradually grow in proportion to income generation.

The Pilbara in Western Australia and the Daly catchment near Katherine and Central Australia (Ti Tree) in the Northern Territory were assessed to identify possible MAR based irrigation regions. Typically shallow weirs, infiltration trenches and injection bores were considered as the main MAR methods. Five sites were assessed in the Pilbara and six across the Northern Territory. After consideration of a range of factors such as the available source water, local hydrogeology, soil suitability and potential irrigation demand, six sites were considered technically feasible; three in the Pilbara and three in the Daly catchment. A cost benefit analysis was completed on the feasible sites to determine the capital and operation and maintenance costs of water supply and on-farm benefits.

Preliminary costings estimated in the Pilbara show that with a weir, the annualised (levelised) cost to supply water to irrigate range between $77 and $282/ML. The calculated net benefit of the irrigation configuration tested for fodder cropping is $76/ML.This is a small benefit, which when compared to the cost of water supply, makes the investment marginal. In the Daly catchment, preliminary costings show that with a weir, the annualised costs to supply water for irrigation ranges between $166 and $575/ML. The calculated net benefit of fodder cropping is $110/ML, however for horticulture (mango, Asian vegetables and melons) this ranges between $2,000 and $10,000/ML. All sites in Northern Territory were found to be economically justified given the estimated cash flows, including recharge trenches around Ti Tree with an annualised cost of $2,800/ML.

The prefeasibility assessment undertaken to date indicates that further more detailed investigations are justified and depending primarily on the capital cost of the MAR scheme and the crop selected may be technically and economically attractive.

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