Ord
Resources and Systems

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.

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

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

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).


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,

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.


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.

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.

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