We use water accounting models to simulate flows in 78 Tasmanian rivers, covering the majority of Tasmania’s land surface (Fig 5.2). While we describe overall changes to the 78 rivers, we focus on describing impacts on water allocations for the Rubicon, South Esk, and Little Swanport Rivers (Table 5.3). These rivers are important agricultural catchments in Tasmania, span a range of hydrological and climatological conditions, and have already been introduced in Chapter 4. Further description of the remaining 74 catchments is given by Bennett et al. (2010).
Table 5.2 Run-of-river power stations described in this study
Storage Name Capacity (GL) Power generation scheme
Lake Cethana 50 Mersey & Forth Rivers
Lake Pieman 300 Pieman River
Lake Trevallyn 12 Great Lake-Trevallyn
Meadowbank Lank 60 Derwent River
Sixty-two of the 78 river models (tan colour in Fig 5.2) were developed for the CSIRO Tasmania Sustainable Yields (TasSY) study. An additional sixteen catchments were added for this study (green coloured catchments in Fig 5.2). These 16 catchments are free-flowing rivers in remote areas that have negligible water extractions for human use and thus do not require complex water accounting rules. Accordingly, for these catchments river flow is equal to the aggregated runoff within the catchment area. The 62 TasSY river models were created byLing et al. (2009a, 2009b, 2009c, 2009d, 2009e). The river models aggregate runoff from the Simhyd runoff simulations (Chapter 4) to subareas and account for infrastructure (diversions and dams), water demands and water management rules current at 31 December 2007. Subareas vary in size, but are usually in the range of 25-50 km2. Subareas are delineated according to points of interest to DPIPWE water managers. Each subarea is a calculation point, accounting for subarea runoff, any inflow from upstream, and any extractions, diversions and storages. The resulting outflow from a subarea is passed to downstream subareas along drainage lines.
Water extractions for agriculture and other uses are not consistently monitored in Tasmania. Water use in the TasSY models was estimated from DPIPWE water licensing information held on its Water Information Management System (WIMS) database. When water is allocated in Tasmania, both the volume of the allocation and the months during which this water may be extracted are specified. The actual rates of extractions vary, and are not monitored. The TasSY models assume that water allocation holders attempt to extract water at a single, average rate over those months when they are entitled to extract their allocation. This may be a crude simplification of how these systems are operated. Farm dams smaller than 1 ML do not need to be licensed in Tasmania. Estimates of the number of unlicensed small farm dams were made for the TasSY models using aerial photographs and Google Earth images. For calculation purposes, all unlicenced extractions are summed for each subarea, and the river models treat them as a single extraction from each subarea. Unlicensed water extractions are included in water extraction estimates for each sub-area, as described by Ling et al. (2009a, 2009b, 2009c, 2009d, 2009e). Infrastructure, water demands and water management rules vary from catchment to catchment. Details of each of the 62 TasSY river models are given by Ling et al. (2009a, 2009b, 2009c, 2009d, 2009e).
Fig 5.2 Catchments modelled for this study. Tan colour shows regions modelled with Tasmania Sustainable Yields (TasSY) river models. Regions modelled with Temsim are shown in blue. Green-coloured catchments are river models developed for this study. White areas are not covered by the river models.
Tasmanian water allocations are assigned a ‘surety’, which corresponds to the priority each allocation is accorded when demand for water is greater than supply. The TasSY models account for all water allocations and the priority they are accorded. To assess impacts on irrigators, we describe impacts on ‘Surety 5’ water allocations. Surety 5 is ascribed to water allocations for irrigation. It is accorded a lower priority than water allocated for domestic consumption. Surety 5 water is expected to be available at about 80 percent reliability. We describe changes in water allocations by calculating reliability of supply, as follows:
(5.1)
where QAllocated is the total volume of water allocated for a given period, and QSupplied is the total volume of water
supplied over the same period. The TasSY models are configured so QSupplied can never exceed QAllocated. This
Table 5.3 River summary characteristics (after Ling et al., 2009a, 2009b; Ling et al., 2009d)
River Catchment area
(km2)
Rainfall (mm/yr)
Streamflow (GL/yr) Extractions (GL/yr)
Little Swanport 875 640 127 3.1
Rubicon River 641 932 167 11.4
South Esk 3351 805 842 37.9
even if allocations are supplied in all other years. However, we calculate reliability over 30-year periods, reducing the impact of a single dry year on results. Further, acute droughts can have severe impacts on irrigators, particularly in regions where supply is generally reliable, and our calculation of reliability reflects these impacts.
5.3.1
Major irrigation storages
We describe changes to four major irrigation and water supply storages (Table 5.4). Changes to a further 10 storages are described by Bennett et al. (2010). Storages are modelled according to operating rules current at 31 December 2007. To be able to compare historical to future river flows, these operating rules were applied for the entire 140-year simulation (1961-2100). For example, construction of the Meander Dam was completed only in 2007, but the river models assume it has been in place since 1961. We calculate reliability of supply from these storages for water allocations of all sureties, following equation 5.1.