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National Water Account 2016

Ord: Water stores

These notes provide a water balance for each of the region's water stores for the 2015–16 year. Relatively poor rainfall during January–February, the typically wettest months of the year, contributed to low runoff across the region and a drop in storage volumes. Despite these climate conditions, there was a decrease in water use during the year.

 


 

Surface water store

Lake Argyle, Ord region. Source: istock © czardases

The volume of water in the Ord region's surface water store decreased during the 2015–16 year from 9,942,970 ML at 1 July 2015 to 8,072,764 ML at 30 June 2016 (Table S5).

 

Table S5 Water balance for the surface water store
 2016
ML
2015
ML
Opening surface water store9,942,97010,944,857
Inflows4,306,3386,467,901
Outflows(5,182,218)(6,453,883)
Balancing item(994,326)(1,015,905)
Closing surface water store8,072,7649,942,970

 

A schematic diagram representing all surface water inflows and outflows during the 2015–16 year is provided in Figure S3.

 

Figure S3 Schematic diagram of water inflows and outflows for the surface water store during the 2015–16 year
Figure S3 Water inflows and outflows for the surface water store during the 2015–16 year

 

Surface water inflows

The largest natural water inflow in the region was runoff, and this was almost 40% less than the previous year. The drop in runoff reflects the relatively poor rainfall conditions experienced across the region for the second successive year (see Climate and water). Interestingly, precipitation marginally increased compared with the previous year. More than 95% of the total volume of precipitation on surface water in the region occurred at Lake Argyle. The remaining natural inflow was discharge: groundwater, which could not be quantified.

The two water transfers in the region—discharge: wastewater and point return: irrigation—are less climate-dependent and change little from year to year. 

 

Surface water outflows

The largest natural water outflows was river outflow. The outflow to sea was more than 20% less than the previous year, reflecting the poor rainfall across the region for the second successive year.

Evaporation during the 2015–16 year was less than that which occurred during the previous year, which may be attributed to the decreased storage volume (and hence surface area of the storages) during the year. More than 95% of the total volume of evaporation from surface water in the region occurred at Lake Argyle. The remaining two natural surface water outflows were leakage: landscape and recharge: groundwater. Leakage to landscape is a very small outflow and varies little annually; recharge to groundwater could not be quantified accurately due to a lack of available data.

Total surface water diverted in the region (172,912 ML) decreased marginally from the previous year. Allocated diversion: irrigation scheme (147,822 ML) made up approximately 85% of the total allocated diversion. Allocated diversion: individual users (24,657 ML) and allocated diversion: urban system (433 ML) accounted for approximately 14% and 1% of the total diversion, respectively. The volume of allocated water diverted from each river is shown in Figure S4.

 

Figure S4 Map of allocated water diversions during the 2015–16 year; percentage of allocation diverted is also shown
Figure S4 Allocated water diversions during the 2015–16 year; percentage of allocation diverted is also shown

 

For a more detailed description of the water usage in the region, and the associated entitlements, see the Surface water rights note.

 

Surface water balancing item

The calculation of the surface water balance (Table S5) yielded a balance of –994,326 ML, which is more than 10% of the total surface water store volume at 30 June 2016 and more than 20% of the total surface water inflows during the year. The negative balancing item indicates that either the inflows are too high or the outflows are too low.

It is likely that the balancing item is primarily attributed to uncertainties associated with the runoff estimation (a large source of surface water increase). Runoff is estimated from a rainfall–runoff model (see Methods) and it is reasonable to expect a relatively high uncertainty around this volume.

 

Groundwater store

Windmill in the outback, Ord region. Source: istock © Totajla

The volume of water in the Ord region's groundwater store is assumed to remain unchanged throughout the 2015–16 year (Table S6).

 

Table S6 Water balance for the groundwater store
 2016
ML
2015
ML
Opening groundwater store
Inflows8,5729,333
Outflows(8,572)(9,333)
Balancing item00
Closing groundwater store

 

A schematic diagram representing all groundwater inflows and outflows during the 2015–16 year is provided in Figure S5.

 

Figure S5 Schematic diagram of water inflows and outflows for the groundwater store during the 2015–16 year
Figure S5 Water inflows and outflows for the groundwater store during the 2015–16 year

 

Groundwater flows

The only water movements to and from aquifers in the region that can be quantified are the allocated extractions for individual users and urban supply. In the absence of available data, it is assumed that aquifer recharge from the landscape (8,572 ML) is equal to these extractions, which is based on the assumption that the volume of extraction in the long term leads to no change in aquifer levels.

Groundwater movement across the region boundary as well as surface water–groundwater interactions are considered to occur within the region; however, these flows could not be quantified.

Total groundwater extracted in the region (8,572 ML) decreased marginally from the previous year. Allocated extraction: individual users (6,740 ML) and allocated extraction: urban system (1,832 ML) accounted for approximately 80% and 20% of the total extraction, respectivelyFor a more detailed description of the water usage in the region, and the associated entitlements, see the Groundwater rights note.

   

Groundwater balancing item

As shown above, the only groundwater outflows that could be quantified were the entitled extractions for individual users and urban supply. It is assumed that the total groundwater inflow (recharge from landscape) was equal to these extractions, so the groundwater store is assumed to balance (i.e. the balancing item is zero).