Data reliability and gaps

The water balance is principally composed of six groups of components, with there being 71 components in total. The groups are:

• Opening Storage
• Inflows to Surface Water
• Inflows to Groundwater
• Outflows from Surface Water
• Outflows from Groundwater
• Closing Storage

In addition to compiling the water balances for AWR 2005, the team identified data gaps; determined the reliability of the data and information, and documented issues in obtaining the data. Through compiling the balances the following questions have been answered: [Internal page links]

• What components had limited data available?
• What additional components were required for the water balance?
• What were the significant components of the water balance?
• What was the reliability of the water balances?
• What changes to databases will improve compilation of water balances in the future?

What components had limited data available?

Seventy-one different components make up the water balance, of which there was no data on eight of the following components:

• Non-renewable groundwater storages
• Soil-unsaturated zone storage volume
• Snowpack storage volume
• Desalinisation as a surface water inflow source (note: Perth Desalinisation plant became operational on the 20th November 2006 and so would be included in future water balances for Perth)
• Groundwater system gains from septic tanks
• Extraction to ASR from Groundwater (most ASR sources are surface water based from either floodwaters, urban drainage or other surface or re-use sources)
• Extraction to economy outside of entity from Groundwater (i.e. groundwater is extracted and used on site or within the entity)

Data could not be obtained for the following items in most of the water balances:

• River channels storage volumes (only supplied in NSW)
• Groundwater storage volumes (renewable and non-renewable) (only supplied in NSW)
• Seepage from streams to groundwater (and vice versa)

What additional components were required for the water balance?

Generally there were no additional components to the water balance after the initial trial was undertaken; however there were several sub-components of the original water balance items added to the balance where the information was available, (e.g. rainfall runoff to rainwater tanks in the capital city water balances).

Water Balance components and sub-components

What were the significant components of the water balance?

The most significant components of the water balance shown as a proportion of the total inflow volume are:

• Major surface water storages (33% of inflows)
• Groundwater discharge to surface water (9% of inflows)
• Surface inflow from other entities (13% of inflows)
• Irrigation diversions (15% of inflows)
• Streamflows out of entity (40% of inflows)
• Groundwater discharge to evapotranspiration (16% of inflows)

Major water balance components by volume and % of total recharge and runoff

What was the reliability of the water balances?

These major components have been grouped and plotted according to the average reliability from the 51 water balances and the average volume in the water balances. Groundwater storage volumes are the least reliable where the information has been provided, followed by the runoff and groundwater recharge volumes (mainly provided from Water 2010 model), and evapotranspiration. All of these components of the water balance cannot be controlled or impacted on by water management policies and so it is even more important that they are better understood.

The other main components of lower reliability and lesser volumes are groundwater / surface water interaction volumes which are impacted on by extractions and diversions, and the extraction and diversion volumes. This emphasises the need to better understand groundwater processes and to be able to estimate runoff, recharge and evapotranspiration to a higher accuracy to manage the resource better.

Water balance reliability plot showing average reliability of each term and average volume for those terms for the AWR 2005 water balances.

Note that the largest component of the water cycle is evaporation, which was not included in the 51 water balances as they only comprised the “terrestrial” phase of the water cycle. Hence the evapotranspiration component of the 51 water balances has been identified in about two thirds of the balances (evaporation from major storages is noted in 33 balances and groundwater evapotranspiration is noted in 25 balances). The reliability of the groundwater evapotranspiration values though is very low as in many cases it is based on a percentage of rainfall or recharge and is not a measured or calculated value.

The other significant components of the water balance consist of mainly streamflow gauged information (streamflows out of entity and surface inflow and transfers from other entities).

Each component of the water balance was given a reliability rating from A to F based on the data source and type of analysis used.

Reliability categories (A to F) by Volume for the AWR 2005 water balances

The data quality indicated by these ratings shows that most of the information is from basic analyses or estimates, 20% of the data is category C or D, and some data is high quality from modelled estimates (16% of the information supplied). More than 50% of the data fields could not be filled in due to no information being available or the field not being applicable to that water management area. In some cases the data was not available in the timeframe in which the water balances had to be compiled. This also indicates that the information from which the water balances are assessed is not readily available and is difficult or time consuming to access.

The water balances also have an error term (or balancing item) which gives a good indication of the potential error in the balance. This varied from 0% in NSW where they assessed the water balance for each area and included any balancing item within the terms of the balance, to 50 to 70% in Queensland where the runoff component was so large that it created a large error term. Other balances were generally in the order of 20-30% errors indicating that the B (±25%) and C (±50%) reliability categories were indicative of the level of accuracy of the balances. Error terms for each of the balances are given in the summary tables, along with a reliability assessment. The individual water balance assessments are available on the Regional Water Resources Assessments page

Generally, where there was little data the water balance either had a small error as there was no information to report on, or was very large as the runoff and recharge components could not be accounted for. In many cases evapotranspiration (ET) is a significant component of the water balance and is not well known. Recent advances in technology with satellite imagery may enable more accurate estimates of ET to be made. (see section on Evapotranspiration for more information).

Those water balances where modelled information was available (either surface water or groundwater models), the water balances had a high level of accuracy. This however, does not always mean that all components can be correctly attributed. It may just be the case that models always have to be balanced.

What changes to databases will improve compilation of water balances in the future?

The water balances are primarily based on analysed, interpreted and modelled data (25% of the 45% of fields which had data were category A or B which indicates modelled or monitored data with a reliability of ±10% and ±25% respectively). Models were used for many of the water balances components, for example:

• Groundwater models used in Western Australia (Perth, Gnangara Mound, South West Yarragadee) and for the Great Artesian Basin assessment;
• REALM models were used for Victorian water balances (Broken, Ovens, Goulburn Rivers); and
• Complex integrated models used in NSW for groundwater and surface water components.

The data classification types (C to F) have an implication for future water balances in that the water balances will not be able to be created from the type of information normally held in a database (e.g. licensing information, metered use, streamflows, bore levels, etc). In most cases it will only be possible to use this data if tools to analyse the database information are available in conjunction with the database.

Users of the currently available databases are aware of the limitations of users relying solely on this data source when all the quality information may not be available for the data set, or data may be missing, or the data may be misleading because of the way the information is collated. For example in some groundwater licensing databases it is possible to have multiple bores linked to the one license and use either part or all of that licensed entitlement. In the database, each bore may be given the total entitlement volume, which would imply greater allocation of the resources than actually might occur.

Any future database for water resource information must carefully consider the reliability of the information it presents and protocols for entering data into a common database. The system should also ensure that quality codes and metadata are presented with the dataset to reduce misinterpretation.

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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