1.1.2 Geography


The Northern Inland Catchments bioregion covers an area of approximately 248,000 km2, of which the Namoi river basin covers 42,000 km2 and the Namoi subregion covers approximately 29,300 km2. The Namoi subregion is smaller than the Namoi river basin because the eastern part of the river basin does not overlie a coal-bearing geological basin. The Namoi subregion landscape is characterised by highlands in the east and south and a broad floodplain in the west with the highest point being 1400 m AHD on the flanks of Mt Kaputar in the north-east. Soils are typically clay and sandy- or clay-loams.

Drainage is dominated by the Namoi River, which flows east to north-west and west, and its tributaries and distributaries, the Mooki River, Coxs Creek, Pian Creek and Turragulla Creek. There are many ecologically important small lagoons, natural and artificial wetlands and floodplain woodlands, which can be inundated during a 1-in-2 year event. Lake Goran is listed as a wetland of national significance.

Approximately 27,000 people live in the Namoi subregion, concentrated along the Namoi River, mainly between Gunnedah and Narrabri. The subregion intersects four local government areas. Prior to European settlement the river basin was inhabited by the Gomeroi people.

In 2001, native vegetation was estimated to cover 62% of the Namoi river basin, the remainder having been cleared for cropping and grazing. Of the remnant vegetation, much has been substantially altered. In the subregion, nature conservation areas cover 2065 km2, approximately 7% of the total area. About 40% of the subregion is used for grazing, 23% for dryland cropping and horticulture, 10.5% for forestry and 5% for irrigated crops such as cotton and wheat. The economic value of agricultural production in 2010-11 was estimated to be $1.12 billion (ABS, 2012), of which 25% was from irrigated cotton.

Water is extracted from surface water and groundwater resources for irrigation, livestock and domestic use. The Namoi river basin as a whole uses approximately 2.6% of the total surface water diverted for irrigation in the Murray–Darling Basin and 15.2% of the total groundwater resource that is extracted from the Murray–Darling Basin (CSIRO, 2007, p.14). Water is allocated through state licences. Some water requirements for the Namoi subregion are met from the upstream dams, such as the Keepit and Split Rock dams on the Namoi River and the Chaffey Dam on the Peel River. In the lower Namoi, lagoons and major weirs are regulated to supply water for domestic, irrigation and stock use. Water is also extracted from some creeks and rivers (such as Pian Creek and Gunidgera Creek). Some of the lower areas are semi-arid but experience significant flooding at times. Other natural hazards include bushfires, storms and flooding. Flash flooding also occurs in other parts of the river basin. High winds, heavy rain and associated landslips have also been known to occur.

The climate is typically cooler and wetter in the east and hotter and drier in the west. Annual average potential evapotranspiration is highest in the north-east of the subregion, and lower in the west and south-west. In contrast, average annual actual evapotranspiration exhibits a strong decreasing gradient from east to west. Annual rainfall varies from about 500 mm to 1100 mm with most rainfall typically occurring in the summer months. Climate modelling investigations by CSIRO (2007), Chiew et al. (2009) and Vaze et al. (2008), summarised in the Namoi Catchment Action Plan (Namoi CMA, 2012a), suggest a drier and warmer future in this area. Estimates of future climate show a 5% decrease in rainfall and a 1 °C increase in temperature by 2030 relative to 1990, based on a moderate emissions scenario. Although annual rainfall will decrease, the modelling suggests that summer rainfall events will be more intense. A warmer, drier climate will mean an increase in potential evapotranspiration and drier soils, and has the potential to change flood behaviour in the Namoi River, although this latter change has not been modelled. These changes could cause land degradation and erosion. The more intense summer rainfall events and drier winters will “more likely than not increase the risk of dryland salinity” (Namoi CMA, 2012a p. 89). Wind erosion is expected to increase, and as surface water availability reduces, groundwater resources will be in greater demand while also being subject to reduced recharge (Namoi CMA, 2012a p. 89).

Last updated:
5 January 2018