The 2 parts of this investigation were:
What we did
We assessed and described 55 000 hectares (ha) of predominantly red sands and sandy loams – locally referred to as Cockatoo Sands – on the Bonaparte Plains, about 70 kilometres (km) north of the Ord Irrigation Area, Kununurra (Figure 1).
We did this using:
- a preliminary data analysis
- several field trips to assess and describe soils and collect samples
- chemical laboratory analyses of soil samples
- soil classifications and map preparation
- a slope analysis
- a digital soil map
- a land capability assessment.
What we found
- We identified 34 947ha of Cockatoo Sands with a moderate to high capability for irrigated agriculture.
- The landscape is almost level, with most slopes less than 2%.
- Soil pH is mostly neutral, with topsoil pH in the range 5.5 to 6.5, and subsoil pH in the range 6.5 to 7.0.
- Most of the soils have low nutrient retention, and low water-holding capacity.
- Subsoils are dense, massive, and may require deep-ripping for plant establishment.
- Water erosion risk is high because of the soil structure and monsoonal climate.
The upper 30–60 cm of the soil profile has loamy sand to clayey sand soil textures, which are highly permeable, rapidly drained, and have low nutrient retention capacity. This means that these soils are susceptible to nutrient leaching and subsequent acidification. Irrigation management will need to minimise drainage below rooting depth, and minimise nutrient leakage. Suitable technology is fertigation with soil moisture monitoring.
The underlying sandy loam subsoils are compact, with soil bulk density in the range of 1.7 to 1.8 tonnes per cubic metre (t/m3). We recommend deep-ripping these soils prior to seeding deep-rooted annual and perennial crops.
Water erosion is a major risk because the region has a monsoonal climate. Under intense rainfall, sheet and rill erosion is likely to develop, particularly on long slopes of greater than 1% gradient. We recommend developers plan and implement systems to prevent erosion: earthworks, and crop rows of suitable length and slope (seek specialist advice).
What we did
- In 2013, we did a desktop review and field reconnaissance of existing bores, mineral exploration drilling, outcrop geology and groundwater outcrop within the area.
- We constructed nested deep monitoring bores and a test-pumping bore, in the geographical centre of the Bonaparte Plains area of Cockatoo Sands.
- During 2015 and 2016, we did a more intensive census of operating and abandoned bores providing water to livestock, cased mineral exploration holes and groundwater outcrops in the area.
- An airborne electromagnetic survey (AEM; SkyTEM312 system) of the area was completed in December 2015 (Figure 2).
- We used information from the 2013 drilling, the bore census and the AEM to plan a drilling program conducted in 2017.
- A water balance model was then constructed and a range of scenarios were run to explore impacts of land-use change and abstraction on the aquifer.
For more information
Download the full reports:
- Investigations of the potential for irrigated agriculture on the Bonaparte Plains: bore completion report, 2nd edn, Resource management technical report 414, Department of Primary Industries and Regional Development, Perth.
- Investigations of the potential for irrigated agriculture on the Bonaparte Plains: land capability assessment report, Resource management technical report 410, Department of Primary Industries and Regional Development, Perth.
- Investigations of the potential for irrigated agriculture on the Bonaparte Plains: hydrogeology, aquifer properties and groundwater chemistry, Resource management technical report 425, Department of Primary Industries and Regional Development, Perth.