National Water Commission

The Carnarvon water management area is 44 km2 and is located 910 kms north of Perth at the mouth of the Gascoyne River. The boundary for the Carnarvon WMA covers the proclaimed Carnarvon Groundwater Area. However, to ensure that all of the groundwater resources of the Carnarvon area are accounted for, the Carnarvon Water Reserve has been included in the water balance.

The Department of Water is the agency responsible for managing all of Western Australia's water resources, assessing licence applications for water abstraction from private users, local governments and water service providers. The groundwater resources for Carnarvon have been subdivided into basin A (private supply), and Basins B-L (public supply). There are essentially no surface water diversions within the Carnarvon water management area (Water and Rivers Commission, 1999; Water and Rivers Commission, 2004).

Basin A is licensed by the Department of Water and provides water for irrigation to the plantations along the Gascoyne River from privately constructed wells. Basin B-L supplies town water, as well as irrigation supplies from a wellfield managed by Water Corporation in accordance with a license issued by the Department of Water. The Gascoyne Water Cooperative is under contract with the Water Corporation to manage the irrigation scheme (Water and Rivers Commission, 2004).



Figure 1 Canarvon water management area locality map

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The climate of the Carnarvon region is semi-arid, with hot summers and warm winters, high evaporation and seasonally variable rainfall. Rainfall and subsequent river flows tend to be generated by either summer tropical cyclone activity (January to March) or by mid-winter storms (May to September) (Water and Rivers Commission, 2004). For the 2004-05 period, rainfall within the Carnarvon WMA was 319 mm (Table 1), which is greater than the long term average rainfall of 236 mm and the ten year average of 285 mm (see Figure 2). Total evapotranspiration was 469 mm.

Runoff to the Carnarvon GMU is generated from the eastern Carnarvon Basin within the Kennedy Range and Yilgarn Province. River flow within the GMU is largely dependent on the highly variable rainfall of the area and hence aquifer recharge is sporadic and difficult to predict (WRC, 1999). There were no recorded streamflows in Gascoyne River at Nine Mile Bridge for the 2004-05 period (Figure 4).
The Carnarvon groundwater resources are contained in a regional, unconfined to semi-confined aquifer system. Figure 5 and Figure 6 illustrate representative monitoring bore hydrographs, chosen to illustrate any changes in the water levels in Basin A over the last 30 years (Water and Rivers Commission, 2004).

Bore 70418301 (Figure 5) is located on the western edge of Basin A. The hydrograph highlights the pattern of drawdown and recovery over the 30 year period as a result of groundwater pumping. Bore 70418315 (Figure 6) is located within the riverbed and is the easternmost bore in Basin A. The large peaks and troughs in the hydrograph indicate that this bore is located in close proximity to a production bore (Water and Rivers Commission, 2004).

The Lower Gascoyne River Groundwater Management Strategy outlines pumping restrictions for the Carnarvon Groundwater Area and the Carnarvon Water Reserve. During periods of river flow, licensees in Basin A are entitled to unrestricted access to both groundwater and surface water. During no-flow periods, licensees are restricted to their licensed annual entitlement. Groundwater abstraction from the Gascoyne River and its banks is to cease when the salinity level in any production bore exceeds 1,000 mg/L TDS (Basin A-L). In addition, groundwater abstraction is restricted to 10 ML/month from any one property and/or prolongation (Basin A only) (Water and Rivers Commission, 2004).

Table 1 Summary of Carnarvon water management area contextual information

Figure 2 Annual rainfall for the Carnarvon water management area (modified from BOM, 2006)




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




Figure 4 Streamflow gauge for the Carnarvon Water management Area for the Gascoyne River at Nine Mile Bridge (gauge 704139)




Figure 5 Representative bore hydrograph for the Carnarvon water management area (Basin A)




Figure 6 Representative bore hydrograph for the Carnarvon water management area (Basin A)

Summary of the total water resources in the basin

Table 2 Summary of water balance in the Carnarvon water management area, 2004-05

Figure 7 Water balance summary diagram for the Carnarvon water management area

Outcomes from the water cycle report

The following information becomes apparent from Table 2.

• There was no opening or closing balance of water in store. This was due to there being no major surface water storages within the Carnarvon water management area. In addition there were no defined management trigger levels from which renewable or non-renewable groundwater stores could be determined.
• The runoff estimate was extremely small and there was no estimated groundwater recharge volume.
• Surface water transfers in and out of the Gascoyne River dominated the water balance.
• The groundwater/surface water interaction volume was about 38 times greater than the captured rainfall (runoff plus recharge) indicating that much of the recharge to the groundwater was due to flooding of the Gascoyne River.
• Essentially all water use was from groundwater resources.
• Diversions and extractions were less than 1% of net inflows.
• Approximately 50% of groundwater extractions were from self-extracted water (e.g. private groundwater bores) which were not metered.
• 99% of surface water inflows were discharged to the ocean.
• There was a large unaccounted for groundwater outflow (40%).
• There was no surface water error term as it was assumed that all surface water not recharged to groundwater discharged to the ocean.
• The BRS Water 2010 recharge estimate was used for this catchment even though it was not accurate in Western Australia. The REG75 model was more accurate, however this model could not be used as the Carnarvon GMU is outside the area covered.
• There was minimal effluent recycling or returns from irrigation within this area.
• There was no information on farm dams within the Carnarvon water management area.
• Data could not be obtained for the following items in the water balance:
• Onstream minor and farm dams
• Offstream minor and catchment dams
• Groundwater storage volumes (renewable and non-renewable)
• Soil - unsaturated zone storage volumes
• River channels storage volumes
• Irrigation drainage returns
• Irrigation channel outfall returns
• Conveyance losses (seepage from channels)
• Inflow from aquifers outside of entity
• Aquifer flow out of entity
• Extraction to economy outside entity

Download 2004-05 water cycle report: