National Water Commission

The Namoi River is located within the Murray-Darling Drainage Division in northern New South Wales. Although the complete Namoi River catchment includes regulated and unregulated areas, only the regulated area was assessed for the water balance.

The Namoi River – Regulated WMA included the regulated Namoi, Peel and Upper Manilla Rivers downstream of Split Rock Dam. The Namoi River is a tributary of the Barwon River, which in turn is a tributary of the Darling River. It has several tributaries, including the Macdonald, Peel, Manilla and Mooki Rivers and Coxs Creek. Irrigation within the Namoi River – Regulated WMA is predominantly private irrigation (i.e. not delivered by an irrigation authority). Keepit, Split Rock and Chaffey Dams are the primary water storages in the WMA and the major towns include Tamworth, Gunnedah and Narrabri. The main purpose of Chaffey Dam is to provide town water supply to the Tamworth region and to guarantee its long term security of water supply.

The Department of Natural Resources (DNR) is the licensing authority and manages both surface and groundwater diversions for urban, commercial/industrial, irrigation and environmental purposes. The Namoi CMA is responsible for managing the native vegetation, soil and other natural resources in the catchment in conjunction with the local community, local government, industry and State Government. State Water manages the major water supply storages in New South Wales and supplies bulk water to the irrigation authorities, town water supply authorities, mines, electricity generators and other industry by releasing water from its dams into rivers to be accessed by downstream water users. State Water also delivers water for private irrigators and stock and domestic users and is responsible for delivering environmental flows, in accordance with rules as set out in NSW Water Sharing Plans, on regulated rivers in the state. Local councils have the responsibility to treat the bulk water and deliver it to urban users in the major towns within the catchment and also manage the wastewater collection and treatment network (e.g. Tamworth Regional Council.



Figure 1 Namoi River - Regulated water management area locality map

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Rainfall across the Namoi River - Regulated WMA was above average in 2004-05, at around 120% of long term average rainfall and the 10 year average was 110% of the long term average (see Figure 2 and Figure 3).

Environmental flows were made from the Keepit Dam for the first time in 2004-05. The valley received heavy rain in December, allowing upper Namoi users 100% allocation and increasing the initial 5% allocation for Peel Valley users to 65% (State Water, 2006).

Table 1 Summary of the Namoi River - Regulated Water Management contextual information

Figure 2 Annual rainfall for the Namoi River - Regulated water management area (modified from BOM, 2006)




Figure 3 Monthly rainfall and evaporation for the Namoi River - Regulated water management area (modified from BOM, 2006 & Climate Atlas)




Figure 4 Modelled pre development streamflow hydrograph for the Namoi River - Regulated water management area for the Namoi River at Keepit Dam (site 419041)




Figure 5 Modelled pre development streamflow hydrograph for the Namoi River - Regulated water management area for the Namoi River at Mollee (site 419039)




Figure 6 Representative bore hydrograph for the Namoi River - Regulated water management area (Bore name: Gunnedah – Carroll)

Summary of the total water resources in the basin

The total volumes of water available and supplied from water resources in the Namoi River – Regulated WMA are shown in Table 2.

Table 2 Summary of water balance in the Namoi River - Regulated Water Management, 2004-05

Figure 7 Water balance summary diagram for the Namoi River – Regulated water management area

Outcomes from the water cycle report

The following information becomes apparent from Table 2.

• The water balance was completed for the regulated area of the Namoi River catchment. There was a large amount of data available for the regulated area, including detailed modelling, and hence the data quality for this water balance was relatively high. However, the balance may be misleading without considering the entire hydrological catchment (i.e. the combined regulated and unregulated areas). This is an opportunity for improvement in future assessments.
• The error in the water balance was 0%. This was due to the unaccounted for flows being incorporated with other items in the water balance. For surface water the balancing item was ‘Other losses’, the groundwater balancing item was the closing storage volume. Other jurisdictions reported unaccounted flows as an error term.
• The water balance was dominated by rainfall, surface water outflow, diversions, groundwater extractions and losses from surface water.
• Water use within the regulated area was predominantly for agriculture as there are few urban areas in the catchment.
• About 60% of water used in the regulated area was sourced from surface water in 2004-05, with the remainder from surface water sources. It should be noted that the majority of water use in this catchment may be from groundwater sources during periods of drought; however it is likely to revert back to predominantly surface water supply when surface water resources are available.
• The interaction between surface water and groundwater (predominantly seepage from streams to groundwater) was 61,000ML in 2004-05, or 6% of the total captured rainfall (sum of runoff and recharge).
• There was about 20% less surface water available in storage at the end of 2004-05 than at the start, including the water stored in river channels. The major surface water storages had a 24% reduction in volume over the year, in real terms the storages started at 32% of capacity in July 2004 and finished at 24% of capacity in June 2005. This includes estimates of major off river storages.
• The volume of on farm storage in farm dams / minor storages was not available explicitly, however an estimate of the volume of rainfall / runoff harvesting in the catchment was provided and is likely to include the inflows to the majority of farm dams. The estimate of rainfall / runoff harvesting in the catchment is approximately the same as the inflows to major storages in the Namoi River – Regulated area.
• The volume of groundwater stored was estimated by DNR by calculating the volume in the aquifer(s) to a depth of 20m below the ground surface (assuming a saturated thickness, estimating the water table drawdown depth from bore levels, and using the area and aquifer porosity). For the Namoi River – Regulated area, the volume of groundwater at the end of the year was determined from the volume at the start of the year plus any unaccounted for groundwater errors (i.e. it has been used as the balancing item).
• Data could not be obtained for the following items in the water balance:
• Minor catchment dam volume / usage and evaporation (majority included in the rainfall / runoff harvesting estimate).
• Soil - unsaturated zone storage volumes
• Urban treated effluent (returns to the river)
  

Download 2004-05 water cycle report: