Part 2 Basin water accounts 2005/06 Part 2 presents an account of water in each of Victoria’s 29 river basins during 2005/06. Chapter 5 provides an outline of the accounts and identifies the key assumptions and limitations of the data. Chapters 6 to 34 deal with the basins in turn. Each contains: • a seasonal overview for 2005/06 including the main factors that have influenced water availability in the basin, significant events and a summary of 2005/06 in relation to the previous two years • a map of the basin • the current management arrangements for water resources within the basin • a summary of total water resources in the basin in relation to: • surface water • groundwater • recycled water • water for the environment • a water balance. State Water Report 2005/06 Page 43 Page 44 State Water Report 2005/06 Overview of methodology 5 Overview of methodology 5.1 Introduction This chapter outlines how the state water accounts are constructed. It presents a number of important assumptions and limitations of the data in the accounts that should be read in conjunction with the information in each basin summary. The reporting unit of the state water accounts is the river basin and surface water data is reported according to river basin boundaries as designated by the Australian Water Resource Council. This unit was chosen because it is consistent with Victoria’s surface water allocation framework which is predicated on achieving a sustainable balance between water that can be taken from the river for consumption and the water needs of the river itself for a healthy state acceptable to the community. Some data is not aligned with river basin boundaries and this data has been treated in various ways. For example, groundwater management units do not often fit neatly within river basins and require a different system of reporting. Where possible, groundwater is reported within a river basin to give an indication of total resource use. These accounts present information on diversions and extractions rather than use. Diversions may include (where relevant) urban diversions, irrigation district diversions, regulated licensed diversions, unregulated licensed diversions, environmental water diversions and small catchment dams. Because diversions are recorded at the point of offtake, not the point of end use, they include the volume of transmission losses that may occur prior to water being delivered to customers. Diversions are usually for consumptive uses, although some diversions are for other purposes, including environmental purposes. All information for each of the 29 basins is provided for the period 1 July 2005 to 30 June 2006. (Note that for 2004/05 the Glenelg and Wimmera basins were reported on the Wimmera Mallee system water year, 1 November to 31 October). The responsibilities for water management are reported in the state water accounts as they existed during the 2005/06 period. Any changes to responsibilities since June 2006 will be reflected in subsequent water accounts. The accounts are generally reported in megalitres (ML): one megalitre equals one million litres. Volumes of surface water and groundwater entitlements, entitlement transfer and use from surface water entitlements have been reported to the nearest megalitre as required to assess compliance. Groundwater use and recycled water iis reported to the nearest 10 ML. All other values in the report, such as catchment inflows, the surface water balance and small catchment dam usage have been rounded to the nearest 100 ML to reflect the uncertainty in these values. Gigalitres (GL) (one thousand megalitres) are only used where volumes are sufficient to express them in this way. The State Water Report does not provide information on water quality or environmental health of waterways unless it affects water availability and use. Details of river health programs are available from the relevant catchment management authorities. An assessment of the environmental health of rivers and streams in each of Victoria’s river basins is available in the Index of Stream Condition: the Second Benchmark of Victorian River Condition (Department of Sustainability and Environment, 2005) (www.vicwaterdata.net). 5.2 Data sources The state water accounts are compiled from information obtained from: State Water Report 2005/06 Page 45 Overview of methodology • responses to specific data requests from water authorities, the Department of Sustainability and Environment, major users and the Murray-Darling Basin Commission • water consumption and recycled water data collected from water authorities by the Essential Services Commission (ESC) • hydrologic information from selected streamflow monitoring sites • climate information provided by the Australian Bureau of Meteorology and Victorian water authorities from selected rainfall and evaporation monitoring sites • estimated relationships between water use and climate or hydrologic data, which is produced by water supply system modelling • water authority annual reports and related documents. 5.3 Comparison with 2004/05 This is the third year that the water accounts have been compiled. Water balance information for each basin in 2005/06 is presented alongside the values reported in 2004/05 for comparative purposes. Differences between 2005/06 and previous years are, in most cases, the result of changes in climatic conditions or water use. However, as noted below, some differences are due to improvements in estimation methodologies or data collection methods. 5.4 Methodology, key assumptions and limitations of data A number of key assumptions and limitations of the data presented should be borne in mind when interpreting the accounts. Qualifications on and interpretation of the data are provided, usually in the notes below each table. Notes are provided: • where qualification of the data, or further information is warranted • where the previous method to calculate or derive information has been revised • to explain large or significant differences between 2005/06 values and those of the previous year/s. 5.4.1 Surface water resources Surface water is always reported in the basin from which it is extracted. However, the report usually (but not in every case) indicates if water is transferred to another basin (or basins) where it is temporarily stored or used. For example, Coliban Water has a bulk entitlement that allows it to divert water from Lake Eppalock, which is located in the Campaspe basin, and then transfers that water to Bendigo, which is located in the Loddon basin. For the purposes of the water accounts, the accounting for that water is undertaken at the point of diversion (i.e. the Campaspe basin) and not the point of use (the Loddon basin). This similarly applies to Victoria’s major cross-basin irrigation supply systems. Information on water supplied to the Rochester Irrigation Area, located at the downstream end of the Campaspe basin, is presented in the Goulburn basin, where its source of supply is included. 5.4.2 Groundwater resources As noted in Chapter 2 the management of groundwater in Victoria is based on allocating resources within: • groundwater management areas (GMAs) • water supply protection areas (WSPAs) • unincorporated areas. Page 46 State Water Report 2005/06 Overview of methodology Groundwater is managed not only across areas, but also at different aquifer depths. In the Gippsland area, for instance, groundwater is drawn from different aquifers that lie at different depths. Each aquifer is reported individually in the water accounts. The concentration of bores and groundwater use varies considerably across groundwater management units, which often fall across more than one river basin. As a result, where less than 90% of the area of a GMA or WSPA falls within a river basin, it is not always accurate to apportion groundwater allocations and use on the basis of area alone. For the 2005/06 state water accounts, the total groundwater resource for the basin has been presented in the water accounts for each river basin only if the proportion of all GMAs or WSPAs within that river basin is greater than 90% by area. Where reported, this information will be located in Table x-3 of each basin chapter, for example, in Chapter 17 Snowy basin, the total groundwater resource is reported in Table 17-3. Although the total groundwater resource is not reported in Table x-3 if the above condition is not met, each GMA and WSPA contained within each basin are reported in the ‘Compliance with licensed groundwater volumes’ table, if the GMA or WSPA has more than 5% of its area in the basin. For example, in the Portland Coast basin chapter, Table 32-6 includes the Heywood GMA, Portland GMA, Condah WSPA and Yangery WSPA because all of these groundwater management units have more than 5% of their area within the Portland Coast basin (100%, 72%, 53% and 40% respectively). The Glenelg WSPA, which has 4% of its area within the Portland Coast basin has not been included in Table 32-6, because it is lower than the 5% threshold. As groundwater resources in unincorporated areas are largely undeveloped, currently resource information is not readily available in these areas. The interaction between surface water and groundwater introduces the potential for double counting of these resources. In determining permissible annual volumes for groundwater extraction throughout the state, varying degrees of consideration have been applied to groundwater-surface water interaction. The level of metering of groundwater resources is increasing as a result of initiatives in the Our Water Our Future action plan outlined in Part 3. Generally, metering within WSPAs is approaching 100% of the total number of licensed bores and this will enable more accurate reporting in the future. In non-metered areas, an estimate of use is provided for the 2005/06 report based on estimates from the relevant rural water authority. In 2004/05 a statewide average figure was used. 5.4.3 Recycled water The recycled water from towns with wastewater treatment plants has been assigned to river basins according to the point of discharge from the plant to the receiving waters.