Publication Date

8-2022

Series Number

425

Publisher

Department of Primary Industries and Regional Development

City

Perth

ISSN

1039-7205

Abstract

‘Cockatoo Sands’ is a common name for the Cockatoo Sands family of soils (comprising red to yellowish-red sands, sandy earths, and loamy earths) that have formed from quartz sandstone colluviums in relatively isolated patches throughout the East Kimberley region of Western Australia and the Northern Territory. Cockatoo Sands are recognised as potentially suitable for irrigated agriculture because they are generally well drained and not subject to waterlogging or inundation. These characteristics allow them to be cultivated and prepared for planting various crops during the wet and dry seasons of northern Australia.

Expanding agricultural production onto the Cockatoo Sands around Kununurra will increase opportunities by increasing the overall scale of agriculture, allowing year-round agricultural enterprise, and new crops and market opportunities.

In 2016, about 8,000 ha of suitable Cockatoo Sands soils were assessed close to Kununurra and the Ord River Irrigation Area. A further 34,947 ha of suitable Cockatoo Sands soils were identified on the Bonaparte Plains in 2019. However, because the Bonaparte Plains area is 50–100 km from the Ord River, irrigation water for any development here would need to come from groundwater or other water sources nearby.

This report describes the method, data, and analyses used to determine the hydrogeological and groundwater physicochemical conditions of the Bonaparte Plains area, in relation to the potential for irrigated agriculture development on the suitable Cockatoo Sands. Climate, watertable depth, shallow watertable extent, watertable dynamics, aquifer physical properties, groundwater chemistry and water balance data are reported in the context of the opportunities, potential hazards and generic risks to land and water resource conditions that may arise following irrigated agriculture development.

Underlying the Cockatoo Sands, the Point Spring Sandstone Formation aquifer is extensive and contains high quality groundwater. However, compared to the area of suitable soils, the aquifer has limited potential to provide a sustainable supply of groundwater for irrigation. Preliminary water balance modelling indicated that the net outflow to the ocean is 4.4 GL/y. This means that at the likely maximum allocation (50% of net outflow), the supply available for irrigation could be only 2.2 GL/y, equivalent to 150–300 ha of irrigation at application of 750–1,500 mm/y.

Scenario modelling forecast that suitable locations for irrigation were limited by the need to minimise depth to groundwater for economic reasons and the potential pumping could induce aquifer drawdowns that can extend to the wetlands fringing the Cockatoo Sands. Scenario modelling forecast that annual wet season cropping on Cockatoo Sands located above irrigation areas could reduce the extent of the aquifer drawdown by supplying additional recharge. The modelled application of this system was shown to minimise the potential head reduction impacts on wetlands at most of the locations modelled.

However, we stress that the investigation and modelling was not of a suitable scale and complexity as to be suitable for water allocation planning, support of a water licence application, or for the detailed planning of pumping optimisation that will need to be undertaken by proponents of specific developments.

Generally, there is a low on-site risk of land and water resource degradation associated with irrigated agriculture development on Cockatoo Sands.

However, groundwater extraction for irrigation could pose site-specific off-site risks to the permanent wetlands and springs, which are biologically diverse and culturally significant. These off-site risks, which include eutrophication and reduction of the volume of groundwater discharge, could be mitigated by careful location of the development areas and the siting and design of any production bore fields. The modelled application of areas of dryland agriculture, to increase recharge above irrigated areas, was shown to minimise the potential head reduction impacts on wetlands at most locations modelled.

In addition, uncertainty remains regarding the location of the seawater interface. Therefore, the risks associated with seawater intrusion remain poorly defined.

Potential developers and environmental regulators can use the data reported here, in combination with the land capability report (Smolinski 2019), as a basis for further investigations of enterprise-specific opportunities and risks, and for developing specific management plans.

Number of Pages

305

Keywords

Bonaparte Plains, Cockatoo Sands, hydrogeology, aquifer, groundwater chemistry, irrigated agriculture, Western Australia

Disciplines

Agriculture | Environmental Monitoring | Geology | Hydrology | Natural Resources Management and Policy | Soil Science | Water Resource Management

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