Indicator: Water balance

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What the results tell us for Tumut

See also: | Water Use |

For maps showing the water balance for catchments over the entire Region, Contact us for details of the full report for the Australian Capital Region (available only on CD-ROM).

Council does not allocate water to different users. The current status of streams within the Shire is provided by data from the Stressed Rivers Assessment Reports (DLWC, 1999) (Table 1).

One of the eight unregulated subcatchments which fall either partly or wholly within Tumut Shire is considered to be under high hydrologic stress, which implies that demand for water already equals or exceeds supply. These subcatchments cover 60 100 hectares of the Shire, which represents 16% of the Shire's total area. One of the subcatchments had insufficient river flow and water extraction data to be assessed.

Approximately 66% of Tumut Shire falls within unregulated subcatchments. Not all subcatchments listed in Table 1 lie wholly within the Shire. Subcatchments are of differing size, in terms of area and volume of water flowing through them.

About the data

Assessments of hydrologic stress and data on the number of water extraction licences, licensed irrigation area, river flow and average water extraction rates in unregulated streams have been made and collected by the NSW Department of Land and Water Conservation (DLWC). The data are reported in the Stressed Rivers Assessment Report for the Murrumbidgee catchment. The reports were published by the DLWC, in Sydney, 1999. Details about the data sources and assessment process are contained in each of these reports. A brief summary about some of the data follows:

Hydrologic stress rating - The stress rating is determined by proportioning estimated water extraction against the estimated streamflow. Four stress categories were used, these being; low, medium and high stress or unresolved (for those subcatchments where inadequate data were available). Low stress indicated that 0 - 30% of the flow was extracted, medium stress indicated that 40-60% of the flow was extracted and high stress indicated that 70-100% of the flow was extracted.

River flow - Flow percentiles are reported for the month of maximum demand. The flows were estimated using information available through DLWC's flow gauging network. For those rivers which don't have local gauging stations on them, a range of hydrologic predictive techniques was used to derive flow estimates.

Average water extraction rates - Estimates of peak monthly water extractions were made using the surface water returns lodged by licence holders. The dataset is incomplete as not all water usage cards are returned to DLWC. Volumes of extraction have been adjusted to account for this.

The Stressed Rivers Assessment Reports provide data on unregulated catchments only. DLWC has defined unregulated streams as "those which do not have major rural dams, primarily for irrigation supply downstream and therefore most water users rely on natural flows for their water supplies" (DLWC, 1999:1). Flows in unregulated streams however, can be and are still affected by town water supply, hydro-electric dams, weirs and other water diversions.

Description: What does 'water balance' measure?

Which data are collected?

• average and per capita water allocations in relation to total available catchment yield (or streamflow) for each catchment in the Region

Why do we report this indicator?

Because of the competing demands for water in many areas, authorities now tend to set aside an amount of water, or allocation, to ensure that the amount of water used does not exceed the amount available from a catchment.

The volume of water allocated is of concern to State of the Environment reporting because of the pressure potentially placed on human and natural systems if the amount of water allocated exceeded the amount available (called over-allocation). There is more potential for this in years of relatively low rainfall, or in areas experiencing rapid population growth.

Catchments in which not all the water available has been allocated, and in which environmental flow guidelines are being met, have the potential for future development of the water resource. This could occur by population growth, or by changes in landuse such as development of higher water use industries such as irrigated agriculture.

Identifying those groups of users (i.e. sectors such as domestic, industrial, rural, and environmental) with greater water allocations (either total or per-capita) also helps identify the more significant sources of pressure on the water resource, and potentially, where the greatest efficiencies in water use could be attained.