Water balance assessment method

What does the water cycle represent?

The Stocktake and Analysis of Australia’s Water Accounting Practice project (SKM, 2006) has researched the status of water accounting in Australia and made recommendations to progress water accounting practice. One of the templates proposed for the water resource account is a combined surface water / groundwater water cycle report.

The water cycle diagram presents a conceptual model of the linkages between surface waters, the unsaturated zone and groundwaters and all the transfers and extractions from each. Those items identified as being critical to the mass balance are included in the water cycle report.

Example water cycle diagram, that presents a conceptual water balance for the Barron water management area, Queensland. The diagram illustrates the linkage between groundwater, unsaturated zone and surface water.

Download a copy of the balance water cycle report for Baron, Queensland (PDF, 101 kB)

Further information on the water balance assessment process:

• Time period being reported
• Water balance format
• Components of the water balance report
• Classifying data quality
• Water balance approach for capital cities

For individual results of water balance assessments go to Regional Water Resources Assessments

Time period being reported

The time period selected for the AWR 2005 was the 2004-05 financial year (1st July 2004 – 30th June 2005). This period was selected as it covers a single year, matches the financial cycle (in view of the diversity of the hydrologic cycle across Australia), and is the most recent period for which data was likely to be available. The period also corresponded with the reporting for the ABS Water Account 2004-05. Importantly, in addition the 2004-05 financial year marked the starting year of the National Water Initiative and was most suited to serve as a baseline.

An annual water balance provides a snapshot of the resource status, which may or may not reflect average conditions as these depend on the current and preceding climatic conditions. The variability in climate is discussed in relation to the water balances for each area to ensure that the information is presented in sufficient context to be meaningful.

The impact of temporal scales also varies enormously from the tropics to southern Australia. Depending on the component of the water balance being assessed its importance will also change. For example, groundwater recharge occurs over decades not years, as does flow into large storages in the arid regions of Australia which may fill once every 10 or more years. Contextual information to describe some of these factors has been included where available to ensure the status of the resource is not misinterpreted.

Some states or territories report on water cycles other than the financial year and so the collection of data on a financial year base may have implications on the usefulness and or repeatability of the AWR 2005 balances. Hence an outcome of this assessment is to identify and characterise these issues so that progress can be made towards achieving a national water accounting standard and methodology.

Water balance format

The key components of the water balance format are the initial and final storage volumes and the inflows and outflows, with the underlying assumption described by the following equation:

Inflow - Outflow = Change in storage

Outline summary of water balance format – Baron WMA, Queensland

Thus the surface and groundwater items are identified separately and transfers of water from surface to groundwater, or vice-versa are shown as outflows against, e.g. surface water with a corresponding inflow to groundwater. If the items do not balance the difference in volume has been assigned to an error term referred to as the “unaccounted for flow” which can be either an inflow, outflow or storage volume for groundwater or surface water.

Components of the water balance report

The water balance diagram above illustrates the key items to be considered when preparing the water balance report. These are broken down further to provide greater clarity in the definitions. Items which have a slightly altered definition to other publications or those that have been created for this assessment are discussed below. A large number of other definitions may be found in the glossary.

Water 2010 output definitions:

Runoff (Water 2010) = balancing item in the Water 2010 model. It is equal to the rainfall minus deep drainage and evapotranspiration volumes. The rainfall is calculated (from BoM gridded data sets); evapotranspiration and deep drainage were also calculated.

Deep drainage (Water 2010) = water that moves below the root zone, through the unsaturated zone. In many areas this does not equal the water that enters the saturated zone due to temporal and spatial variability which is not captured in the Water 2010 model. In some water management areas the deep drainage for an aquifer may actually occur in an area outside the water balance area and hence is not included within that areas balance. Where possible, groundwater recharge estimates have been used in the water balances. Groundwater recharge data is limited in Australia, especially at the catchment scale, and for this reason the Water 2010 deep drainage estimates are the primary source of the groundwater recharge values used in the water balances.

Further information on the Water 2010 project is available from the BRS website  and in Water Availability, National Perspective report (Appendix A) on the Publications page.

Other definitions

Recharge = component of rainfall that drains below the root zone of vegetation and joins the groundwater (i.e. the saturated zone). This is water that flows through the unsaturated zone and reaches the saturated soil or rock zone (i.e. the aquifer). Hence recharge is a sub-set of the deep drainage estimates from the Water 2010 model.

Water Use in the water balances loosely refers to any extractions, diversions or transfers out used for economic purposes. It generally does not include environmental or social/recreational uses of water which are also significant users in Australia.

Derived factors or definition of items for this water balance assessment

Total net inflows = Total surface water runoff and deep drainage to groundwater (groundwater recharge) and transfers into the water system (both surface and groundwater), for a defined area.

Total water resource = Total net inflows to surface and groundwater in a given year, for a defined area, plus the net volume of water in store at the start of the year.

Water utilisation = (extractions + diversions + transfers out) / (recharge + runoff + transfer in). This factor looks at the water coming in and out of the area for that year. It does not include storages which can make up any shortfall in inflows in some years and so in wet years may result in a % more than 100.

Net groundwater/surface water interaction = combination of baseflow components for the entire area including losing and gaining stream components. This means that the volume of groundwater / surface water interaction can be very low due to the upstream portions being gaining and the downstream being losing sections.

Total groundwater/surface water interaction volume = the absolute volumes of the baseflow components including losing and gaining portions of the streams to show the total volume interchanged between surface water and groundwater systems.

Double counting is considered as the volume of water that comes from groundwater to surface water (and vice versa) which can be allocated twice if groundwater and surface water entitlements are given for the same parcel of water. Double counting volumes quoted in this document refer to the total groundwater/surface water interaction volume (defined above).

Availability of data for the water balances – primary water balance components

The primary components of the water balance are shown in the table below. This includes only those items that had information available for them in more than 10 of the 51 water balances. The apparent lack of available data was not envisaged in the initial phase of the project when information was sought on the availability of data.

During the Discovery Phase of the AWR 2005 and as part of the Water Accounting Project, information was sought on the availability of the data that, desirably, would be used to construct the water balances. It was found that for several items either no data or limited data existed. These items were:

• Volume of water in soil moisture store (no data);
• Volume of water in snowpack (no data);
• Volume of water in river channels (data available in NSW).

Although these items continue to be shown in the water balance it is not expected that any data could be shown against them. They remain in the water balance sheet as a reminder of their existence and that in some instances they should not be overlooked. In all water balances the data components have been listed even when data was not available to highlight data availability issues and reliability of the water balances.

Water balance components that occur in more than 10 (i.e. 20%) of the water balances undertaken for AWR 2005

Classifying data quality

To assess the validity and accuracy of the water balance, data quality for each item was obtained. The data quality codes have been derived from the National Land and Water Resources Audit (2000), but with two additional categories (categories E and F) added.

Data reliability/accuracy classes used for AWR 2005

* Where this category is used respondents were also requested to provide an explanation.

Water balance approach for capital cities

Water balances for capital cities were compiled as part of AWR 2005, to address National Water Initiative (NWI) Clauses 90 to 92 which aim to facilitate urban water reform. These water balances were undertaken in a similar manner to the individual water management area water balances using the water balance format described above, with the main differences being:

• The entity boundary comprised the Surface Water Management Areas (SWMAs) that intersect the urban footprint (except for Perth which is based on Groundwater Management Units). Where water resources forming part of the supply system are located outside of this boundary, these resources are included in the water balance as transfers from outside of the entity. Similarly if transfers occurred out of the entity these are quantified.
• Additional items have been added, where available, to quantify distribution losses within the system, the extent of water recycling and use of stormwater; and discharge of effluent to marine or estuarine receiving waters.
• Water sourced from rainwater tanks or unlicensed bores is included where known, even though this item is likely to be insignificant in some cities.

The urban water resources (supply, use, and disposal) are itemised, where available, such that the actual “urban” footprint can be differentiated from the rural uses and supply. Environmental and social uses of the resource, such as outflows to coasts / lakes / wetlands and flows to rivers and aquifers, are also quantified where possible.

Further discussion of the water balance assessment method is provided in the Water Availability National Perspective report, which is available on the Publications  page.

For individual results of water balance assessments go to Regional Water Resources Assessments.

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