National Water Commission

The Lachlan River is located within the Murray-Darling drainage division in central New South Wales. Although the complete Lachlan River catchment includes regulated and unregulated areas, only the regulated area was assessed for the water balance.

The Lachlan River is a tributary of the Murrumbidgee River, which in turn is a tributary of the Murray River. It should be noted that the Lachlan only contributes flows to the Murrumbidgee in flood times and flows are generally not significant unless it is a major flood. The Lachlan River has several tributaries itself, including the Abercrombie and Belubula Rivers.

About 13% of irrigation within the Lachlan River – Regulated WMA is carried out in the Jemalong Irrigation District [http://jemalongirrigation.com.au/index.html], the majority of irrigation in the WMA is private irrigation (i.e. not delivered by an irrigation authority).

The primary water storage in the WMA is Wyangala Dam and there are 4 additional major storages (above 1,000 ML in capacity), including Carcoar Dam. Two of these major storages, Lake Brewster and Lake Cargelligo, are major off-stream storages. There are a large number of groundwater bores in the catchment with a large proportion of water sourced from self-extracted groundwater. Major towns in the area include Cowra, Forbes and Condobolin.

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 Lachlan CMA is responsible for managing the native vegetation, soil and other natural resources in the catchment in conjunction with the community, local government, industry and State Government. State Water [http://www.statewater.com.au] 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 treat the bulk water and deliver it to urban users in the major towns within the catchment as well as managing the wastewater collection and treatment network (e.g. Cowra Shire Council.)



Figure 1 Lachlan River - Regulated water management area locality map 

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Rainfall across the Lachlan River - Regulated WMA was below average in 2004-05, at around 88% of long term average rainfall. The 10 year average was 98% of the long term average (see Figure 2). Some parts of the Lachlan River stopped flowing in 2004-05. Both towns and stock and domestic/high security irrigation allocations started at 40% and 20% respectively and were increased to 50% and 30% after inflows to Wyangala Dam in September (State Water, 2006).

In the Lower Lachlan catchment, an agreement was put in place to allow groundwater pumping in excess of sustainable yield to carry the community through the current drought (and associated low river entitlement availability). The rate of pressure level decline (as shown in Figure 6) was within that predicted for the current rate of abstraction and is expected to slow or reverse in some areas once the more normal long term sustainable yield pumping is restored.

Table 1 Summary of the Lachlan River – Regulated water management area contextual information

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




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




Figure 4 Modelled pre development streamflow hydrograph for the Lachlan River - Regulated water management area for the Lachlan River at Forbes (site 412004)




Figure 5 Modelled pre development streamflow hydrograph for the Lachlan River - Regulated water management area for Wyangala Dam inflows




Figure 6 Representative bore hydrograph for the Lachlan River - Regulated water management area (Bore name: Cowra - Back Ck South)

Summary of the total water resources in the basin

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

Table 2 Summary of water balance in the Lachlan River – Regulated water management area, 2004-05

Figure 7 Water balance summary diagram for the Lachlan 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 Lachlan River catchment. There was a large amount of data available, including detailed modelling, for the regulated area, 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, recharge and groundwater extractions.
• Water use within the regulated area was predominantly for agriculture as there are few urban areas in the catchment.
• 86% of water used in the regulated area was sourced from groundwater in 2004-05, with the remainder from surface water sources. It should be noted that water use in this catchment may be predominantly from groundwater sources during periods of drought (as in 2004-05); however it is likely to revert back to predominantly surface water supply when surface water resources are available.
• Metered data was available for surface water diversions and groundwater extractions in this catchment
• The interaction between surface water and groundwater (predominantly seepage from streams to groundwater) was 69,100ML in 2004-05, or 20% of the total captured rainfall (sum of runoff and recharge).
• There was about 22% 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 9% of capacity in July 2004 and finished at 7% of capacity in June 2005.
• The volume of on farm storage in farm dams / minor storages may be significant; however there was no data available on volumes in storage or used.
• The volume of groundwater in storage 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 Lachlan 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 was used as the balancing item).
• Data could not be obtained for the following items in the water balance:
• Minor catchment dams (may be a large part of the water balance).
• Soil - unsaturated zone storage volumes
• Urban treated effluent (returns to the river)

Download 2004-05 water cycle report: