Ord
Resources and Systems

Surface water

Purpose of note

The purpose of this note is to provide a consolidated report on the surface water store within the region during the 2010–11 year. Information on all water flows to and from the surface water store are presented here, including between store flows and transfers that are not presented in the Water accounting statements.

Background

A description of the Ord region's surface water resources is provided in the Contextual information under Physical information and then  Surface water.


Water in store

Table 1 below shows that the total surface water store increased during the 2010–11 year in the Ord region.


Table 1. Surface water store volume at the start and end of the 2010–11 year

 

30 June 2011 (ML)

30 June 2010 (ML)

9 Surface water

 

 

1.1 Storages

15,078,047

8,809,407

1.2 Unregulated river

1.3 Regulated river

70,444

15,555

1.4 Lakes and wetlands

Total

15,148,491

8,824,962

  

The volume of water in rivers, lakes and wetlands could not be quantified due to a lack of available data. However, it is considered that the volume of water held in channels and lakes is relatively small in size compared to the volume held in storages.

The location of each storage within the Ord region, and the volume of water in each storage as a percentage of capacity at the end of the 2010–11 year, is shown in Figure 1.


Figure 1. Location map of the storages within the Ord region. The % full volume on 30 June 2011 for each storage is also shown
Figure 1. Location map of the storages within the Ord region. The % full volume on 30 June 2011 for each storage is also shown

The water volume in almost all the storages within the Ord region at the end of the 2010–11 year is greater than that at the start (1 July 2010) (see 1.1 Storages). All storages within the Ord region were more than 90% of capacity on 30 June 2011 (Figure above). Lake Argyle was 139% of capacity.

The large increase in surface water storage during the reporting period is primarily attributed to high inflows into the storages during the 2010–11 year (see Contextual information then Climate overview).

Water flows

A schematic diagram representing all the inflows and outflows associated with the surface water store in the Ord region is provided in Figure 2. The inflow and outflow volumes for the surface water store during the 2010–11 year are given in Table 2.


Figure 2. Schematic diagram of water inflows and outflows for the surface water store within the Ord region during the 2010–11 year. Dotted lines indicate natural flows while solid lines represent flows induced by human activities. Line numbers are provided next to the flows,
Figure 2. Schematic diagram of water inflows and outflows for the surface water store within the Ord region during the 2010–11 year. Dotted lines indicate natural flows while solid lines represent flows induced by human activities. Line numbers are provided next to the flows,



Table 2. Volume of inflows and outflows for the surface water store during the 2010–11 year. Line items in italics indicate between-store flows. These flows are not presented in the Water Accounting Statements as they occur within the region. 

 

Volume (ML)

9 Surface water inflows

 

9.1 Precipitation on surface water

1,485,020

9.3 Groundwater discharge

9.4 Rainfall runoff

37,140,631

9.5 Point return from irrigation scheme

9.6 Overbank flood return to river channel

9.9 Discharge from urban water system

591

Total 9 Surface water increases

38,626,242

17 Surface water outflows

 

17.1 Evaporation from surface water

2,080,765

17.2 Outflow to outside region

18,104,939

17.3 Groundwater recharge

17.5 Overbank flood spilling

17.11 Surface water allocation diversion

12,339

17.12 Surface water allocation diversion – urban system

430

17.13 Surface water allocation diversion – irrigation scheme

161,155

Total 17 Surface water decreases

20,359,628

Balancing item – surface water store

11,943,085

Change in surface water storage

6,323,529

Opening surface water storage

8,824,962

Closing surface water storage

15,148,491

 

No inter-region transfers of water into or out of the Ord region occurred.

Allocation diversions

Most of the allocation diversion from surface water is for irrigation scheme water supply (see 17.13 Surface water allocation diversion – irrigation scheme), which accounts for approximately 93% of the total allocation diversion.

Balancing item – surface water store

This volume represents the difference between the measured opening and closing balances of the surface water store, after physical inflows and outflows have been applied. 

The balancing item – surface water store is calculated according to Table 3.

Table 3. Surface water store inflows, outflows and balancing item for the 2010–11 year

 

Account

Volume (ML)

 

Opening balance (30 June 2010)

8,824,962

add

Total 9 Surface water inflows

38,626,242

less

Total 17 Surface water outflows

20,359,628

less

Closing balance (30 June 2011)

15,148,491

 

Balancing item – surface water store

11,943,085


The calculation of the water balance on the surface water store yielded a balance difference of 11,943,085 ML. This is approximately 80% of the total surface water store volume at the end of the 2010–11 year and less than one third of the total surface water inflows during the 2010–11 year.

It is likely that the balancing item difference is primarily attributed to errors associated with the rainfall runoff, which is a large source of surface water increase (Line item 9.4), errors associated with outflow to outside region, which is a large source of surface water decrease (Line item 17.2) and the lack of an estimate of the surface water recharge to groundwater.

The rainfall runoff volume is derived from a rainfall-runoff model and it is reasonable to expect a 10–20% uncertainty around the estimated runoff volume (+/– 7,430,000 ML). The surface water outflow to sea is based on measured flow data collected at the most downstream station along a river. There is no adjustment made for the contributing area below the gauging station. As such, outflow to sea is likely to be underestimated by 10–20% (+/– 3,620,000 ML). No estimate was made of surface water recharge to groundwater.

Storage inflows and outflows

Inflows to and outflows from storages are shown in the table below.

All the volumes given in Table 4 relate to water flows into and out of the surface water storages, which are the water assets actively managed in the region. These flows may be different from the flows reported in the Statement of Physical Water Flows or in tables above, which refer to flows affecting the surface water store as a whole.

Table 4. Volume of inflows and outflows for storages in the Ord region during the 2010–11 year

Volume (ML)
Opening storage

8,809,407

41 Storage increases
41.1 Precipitation on storages

1,475,297 

41.3 Runoff into storages

20,908,921

Total 41 Storage increases 22,384,218
42 Storage decreases
42.1 Evaporation from storages

2,067,968

42.3 Leakage from storages

-

42.4 Spillage from storages

-

42.5 Releases from storages

12,086,397

42.6 Diversions from storages

-

Total 42 Storage decreases

14,154,365

Closing storage

15,078,047

Net change in volume

1,961,213