Water Security Outlook December 2015

Transcription

1 Water Security Outlook December 2015 Page 1 of 24

2 1 Water Security Outlook In April 2012, Gippsland Water released its second Water Supply Demand Strategy (WSDS). The WSDS is Gippsland Water s principal water resources planning tool. In developing the WSDS, each water system was thoroughly examined to determine both the long term supply-demand outlook under a range of scenarios, as well as the system s resilience to short term droughts. A plan of actions was then developed to address those water supply systems that were identified as being vulnerable to droughts, or likely to suffer future shortfalls due to trends like climate change or growth. As the future is never certain, and can only be forecast using the best available science at a point in time, the WSDS was developed in line with the principles of adaptive management. This means that projects that are required urgently in the short term have been included in Water Plan 3 ( ), while those for which planning and delivery can be delayed, will be reassessed when the next WSDS is prepared in Adopting this approach ensures that actions are implemented when required to provide security of supply, without over investing. Critical to adaptive management is ongoing monitoring of system performance. The WSDS also details Gippsland Water s Drought Response Plan (DRP). The DRP sets out the management actions Gippsland Water may implement should a drought occur. These actions could include water restrictions, water trading, water carting, or other activities. Regular monitoring of water supply systems allows timely decisions to be made in relation to invoking these actions. The Water Security Outlook (WSO) is prepared each year in the lead up to summer to give customers an idea of the current status of Gippsland Water s systems and how they are performing with respect to both the longer term trends forecast in the WSDS, as well as in relation to the water restriction review points. It will indicate whether DRP actions may be necessary over the summer and whether any changes may be required to the longer term action plan. The WSDS included an example WSO which was adopted as the template for the 2012 WSO. For consistency, the same template was used in 2013 and 2014 and will be used again for this 2015 WSO. It must be emphasised that while Gippsland Water has considered relevant climate forecasts and taken care in presenting the information in this WSO, Gippsland Water cannot and does not guarantee any forecast outcome or event. There are many factors that could deliver a different outcome and many are beyond the control of Gippsland Water. 1.1 Gippsland Water Region Outlook This year has been a year of below average rainfall across the central Gippsland area, with the exception of some catchments in the far-east. Figure One shows rainfall as a percentage of the long term average across Victoria. It can be seen that up to December, the central Gippsland region had rainfall ranging from % of average depending on location. It is important to note that while rainfall for the whole year has been below average in most areas, the year has produced several average to above average months, with the annual total brought down by a few particularly dry months. In particular, June, September and October were well below average in both the Latrobe Valley and on the Mt Baw Baw plateau, Gippsland Water s primary Page 2 of 24

3 catchment. July and August fortunately delivered excellent rainfall on Mt Baw Baw with both months rainfall about 50% above average. Figure Rainfall year to date Figure Two shows rainfall deciles for the 2015 spring. Very much below average conditions can be seen across most catchments, with a decile of one. This means that only one in every 10 of all springs recorded was drier. The worst catchments this spring were the Aberfeldy River which is part of the catchment of the lower Thomson River, including supply to Heyfield, as well as the Merrimans Creek catchment which supplies Seaspray. These two catchments had their lowest rainfall on record. Figure Spring Rainfall Page 3 of 24

4 Most of Gippsland Water s catchments experienced their lowest streamflows in the summer of 2006/07. That Summer was preceded by nearly 10 years of below average rainfall (known as the Millennium Drought ) and 2006 itself had only two above average rainfall months at Mt Baw Baw and rainfall to the end of October was only 85% of that to the same time this year. In particular, winter this year was roughly twice as wet as the winter of So while Spring this year has yielded poor rainfall, and in some areas worse than 2006, the better winter, and the damper catchment conditions of the last five years when compared to the Millennium Drought, have led to better streamflows than at the same time in While not specific to the Gippsland area, Figure Four (over the page) shows the deviation of rainfall from average across Victoria for each season for the last 115 years. It can be seen that during the Millennium Drought there were some quite dry winters but also some average and slightly wetter winters. Indeed when the drought broke, winters didn t get much wetter. However, many of the dry winters during the drought followed exceptionally dry autumns. This meant that even if good rain fell in spring, a greater portion of the rain would be absorbed by dry soil in catchments, reducing river flows. Dry Springs like 2006 went on to yield the lowest streamflows on record. Furthermore, Figure Three, which shows total annual rainfall anomaly for Victoria (not including 2015 as the year hasn t yet finished) shows that the Millennium Drought had no intermediate wetter years. This data further reinforces the forecast that this summer is not likely to result in streamflows as low as some of the worse years of the drought, in particular 2006/07. This allows Gippsland Water to use experience from the drought as a probable worst case scenario. Figure 3 Annual Rainfall Anomalies for Victoria Page 4 of 24

5 Figure 4 Seasonal Rainfall Anomalies for Victoria Page 5 of 24

6 The relative dampness of catchments, when compared with the drought years prior to 2010, means that rainfall is more quickly converted into streamflow as it runs off catchments rather than being absorbed by dry soil. Figure Five and Figure 6Six show the effect of wetter catchments. Figure Five shows the cumulative rainfall on Mt Baw Baw, the source of the Tyers River, Moondarra Reservoir s primary tributary. It can be seen that while rainfall for 2015 has been tracking fairly averagely until September, with a deficiency beginning in October, corresponding streamflow has been tracking third highest (Figure Six) of the last decade during winter, and even with the poor spring rainfall, cumulative streamflow has remained close to average. The data shows that similar rainfall during the drought would have likely yielded lower streamflows (e.g. 2005). Figure 5 Cumulative monthly rainfall at Mt Baw Baw for recent years Figure 6 Cumulative monthly inflows to Moondarra Reservoir for recent years Page 6 of 24

7 Gippsland Water s main reservoir, Moondarra Reservoir, is currently (at the beginning of December 2015) at 95% storage. It was drawn down to around 83% during the summer. It can be seen that rainfall in May returned the storage to 100%, where it remained (Figure Seven) until November. Gippsland Water s share of Blue Rock Reservoir is currently at 99%. Figure 7 Moondarra Reservoir monthly storage volume for recent years The Bureau of Meteorology (BOM) prepares regular three month outlooks for temperature and rainfall. These outlooks provide an indication of the likelihood that either temperature or rainfall will exceed the long term median. The BOM monitor a range of climate indicators which are used to prepare a forecast. For example, if the forecast states that the probability of the Gippsland region exceeding the long term median rainfall for the next three months is 60%, that means that in six out of ten years when weather patterns are similar to those currently prevailing, the following three months were wetter than normal. It is important to note that the forecasts don t attempt to predict how much wetter. Temperature outlooks are prepared in a similar fashion. Figure Eight and Figure Nine show that for summer , the odds slightly favour drier conditions with neutral odds on temperature. Should warmer conditions prevail, this can have a double impact in drying out catchments and also increasing water demand. It is worth noting that past accuracy for both of these outlooks is only 50% so there is an equal chance of them being right or wrong. Page 7 of 24

8 Figure 8 Rainfall Outlook December 2015 to February 2016 Source: BOM Past accuracy ~50% Figure 9 Temperature Outlook December 2015 to February 2016 Source: BOM Past accuracy ~50% Page 8 of 24

9 Australia s weather is influenced by various global patterns. Some of the more important ones include El Nino/La Nina/Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Southern Annular Mode. These patterns tend to oscillate and can lead to wetter or drier weather. For example, in Gippsland, El Nino is associated with drier weather (particularly in winter and spring) and La Nina with wetter weather, although the relationship is not quite that simple and local weather can lag or lead changes in this global pattern. The very wet weather of is likely to be strongly related to the presence of the wet phase of all three of these global patterns. The presence and/or future likelihood of El Nino and La Nina are indicated by a range of indicators, both atmospheric and oceanic. In 2015, the BOM announced that an El Nino had commenced on 12 May 2015, after thresholds were exceeded in both tropical Pacific oceanic and atmospheric data, and climate models suggested the event would be sustained. This year s El Nino has been one of the strongest since and a positive IOD has reinforced El Nino impacts since late August. However, in early November this positive IOD has diminished, and typically the IOD has little influence on Australia s climate between December and April. The strength of the El Nino/IOD this year was only moderated by the overall warmer than usual Indian Ocean with the October sea surface temperature anomaly for the Indian Ocean the highest positive anomaly for any month on record. This outcome reduced the impact of El Nino/IOD during winter as was seen in Gippsland during July and August. Currently the strong El Niño persists in the tropical Pacific Ocean. International climate models suggest that El Niño sea surface temperatures are approaching their peak, and will decrease in the first quarter of The models suggest a return to neutral probably won t occur until at least autumn 2016, but El Nino impacts on Australia should decline prior to this. In the past, El Ninos that persist into summer have led to both wetter and drier summers depending on the rate of El Nino decline. However, on average an El Niño in summer has a slight drying influence across the southeast of Australia. The BOM also provides outlooks for streamflow in several rivers in Gippsland, recently expanding the number of catchments for which these outlooks are provided. Rivers that supply Gippsland Water systems that are included in the BOM outlooks include the Tanjil, Macalister and Thomson Rivers. In addition, an outlook is provided for the Latrobe River which is not a source for Gippsland Water town systems but some of its tributaries are. Presented below are outlooks for the Tanjil River (inflow to Blue Rock Reservoir), upper Thomson River (upstream of Thomson Reservoir and indicative of flow off the Baw Baw Plateau) and Latrobe River (several tributaries supply towns like Moe and the upper Latrobe catchment abuts the Tarago catchment). The outlook for Macalister River has not been shown as Gippsland Water already has a 100% allocation for this system for the year. Figure 10 shows that the most probable outcome for the Latrobe River is low flow over summer. Figure 11 shows even greater odds of low flow for the Thomson River. Figure 12 shows the Tanjil River outlook for which the odds for low or median flows are similar. High flows are unlikely in all catchments over summer. The flow exceedance plots (Figure 13, Figure 14 and Figure 15) show that for all three rivers, the probability of exceeding any given total streamflow this summer is lower than in an average year. Only the Tanjil River displays similar flow exceedance probabilities to an average year, especially for flows that have an Page 9 of 24

10 exceedance probability of greater than 50%, i.e. those flows that are more likely to occur. Figure 10 Latrobe River Flow Forecast Terciles Figure 11 Upper Thomson River Flow Forecast Terciles Page 10 of 24

11 Figure 12 Tanjil River Flow Forecast Terciles (upstream of Blue Rock Reservoir) Figure 13 Latrobe River Exceedance Probability Forecast Page 11 of 24

12 Figure 14 Upper Thomson River Exceedance Probability Forecast Figure 15 Tanjil River Exceedance Probability Forecast Page 12 of 24

13 The summer outlook for Gippsland Water s systems takes into account a large range of factors. These include: For Gippsland Water s larger systems (Warragul/Drouin, Latrobe Valley including Moe, Thomson-Macalister towns, and Sale), the current storage and aquifer (as applicable) position is excellent for this time of year except for the Warragul/Drouin system which has minimal storage capacity. Small systems are more difficult to forecast but each is discussed in the following section with respect to its own current position compared with the average and the worst years. A dry spring has been experienced, particularly October, but this has followed a relatively good July and August. When compared with the 2006/07 Summer which produced very low streamflows, this year has not followed nearly a decade of drought and has had better overall year to date rainfall. BOM outlooks for summer are now showing fairly equal odds for a hotter or cooler than normal summer and only slightly greater odds towards a drier summer. The El Nino/positive IOD effects are likely to have contributed to the dry spring conditions although the warm Indian Ocean probably helped with the good rainfall in July and August. The positive IOD is now weakening and El Nino typically has less impact over summer and BOM forecasting suggests it may weaken as Summer progresses. Streamflow forecasts show low flows are most likely but in the Tanjil River (and therefore quite likely in the Tyers River Moondarra Reservoir inflow) median flows also show reasonable odds. These points suggest conditions this summer are not likely to be as severe as during 2006/07 and Gippsland Water s main systems are currently better placed to buffer dry conditions. Gippsland Water managed all systems during 2006/07 at a maximum of Stage Three water restrictions, with the exception of Maffra, Coongulla, Boolarra, Seaspray and Thorpdale which incurred Stage Four restrictions. Although some systems did not require Stage Three restrictions, those were applied uniformly across the region in that year. It is extremely unlikely that Maffra and Coongulla will require restrictions for water quantity reasons this year due to an already full allocation. Boolarra has been connected to the Latrobe system since 2012 and the Latrobe system is currently in an excellent position. Seaspray now has a 30 ML basin that can provide nearly a year of supply, although water quality reliability (potential for algal blooms) in that basin is still being confirmed. Therefore restrictions may be required in the case of water quality problems at Seaspray. Thorpdale is currently being supplied by water carting due to water quality problems in Easterbrook Creek (manganese) and this small system is relatively easy to manage with water carting. This is being managed without water restrictions. The performance of any one of Gippsland Water s systems will depend on the conditions that manifest as well as the current system status such as reservoir storage levels. The following sections provide a system by system outlook. It is important to note that water restrictions and/or other water supply contingency measures may need to be introduced to manage conditions other than drought. Water quality incidents such as the floods of 2007 that followed the 2006/07 summer Great Divide Fires that brought large amounts of suspended solids (soil) into rivers, Page 13 of 24

14 can lead to the inability to treat water to a potable standard, or at least a reduction in the rate of treatment and the ability to meet demand. Therefore even supply systems that are secure from a water quantity perspective are not guaranteed to be immune to restrictions. Page 14 of 24

15 1.2 System Specific Outlooks Briagolong System Towns Supplied Current Status Current aquifer level (m) year average aquifer level at this time of year (m) Aquifer level at this time in drought year (m) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Briagolong Briagolong Current aquifer levels are higher than the 10 year average for this time of year. Water restrictions have not been necessary in the last eight years. In 2007 restrictions went as far as Stage Three. It is unlikely that restrictions will be required this Summer unless particularly dry conditions develop or irrigation usage significantly increases. Demand for was relatively low. The 2012 WSDS identified possible problems with bulk metering in this system. A bulk flow meter was replaced in to ensure that part of the metering system was functioning correctly. Since then, an intermittent error was found in the electronics that allow remote monitoring of the meter. A manual correction was made to the meter read to correct for this error. A project is underway to replace the faulty electronics. The demand is thus deemed accurate and the high readings of the last two years are suspected over-reads. Demands will continue to be closely monitored with the new equipment. With relatively low growth, this system is deemed secure for the short to medium term. Gippsland Water continues to work with Southern Rural Water to monitor Page 15 of 24

16 aquifer trends. Page 16 of 24

17 1.2.2 Erica - Rawson System Towns Supplied Current Status Raw water basin level Weeks of supply at average summer demand Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Erica - Rawson Erica and Rawson 9% as the large basin is offline for structural and water quality assessments. The smaller basin remains online. 2-3 The volume in storage in the smaller basin is currently sufficient for about 2 3 weeks at Summer demand levels. However this assumes no stream inflow which is extremely unlikely. The stream has been highly reliable for at least 25 years. It is unlikely that restrictions will be required this Summer except in extreme conditions. Demand in has been below the forecast and similar to previous years. The 2012 WSDS proposed no actions for this system as it has a history of being highly reliable. No changes to this recommendation are proposed. Page 17 of 24

18 1.2.3 Latrobe System Towns Supplied Current Status Latrobe Volume in storage (ML) 61, year average storage volume for this time of year (ML) Traralgon, Tyers, Glengarry, Toongabbie, Cowwarr, Rosedale, Morwell, Churchill, Yinnar, Boolarra, Traralgon South, Hazelwood North & South, Jeeralang & Jumbuk, Moe, Yallourn North, Newborough, Trafalgar, Yarragon, Darnum, Willow Grove 52,936 Storage volume at this time in dry year (ML) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend 44,528 The Latrobe system is currently at 97% and significantly higher than during the drought years at the same time of year. Gippsland Water s share of Blue Rock Reservoir also increased during 2014 with the purchase of an additional 3.87% of storage and inflow. It is unlikely that restrictions will be required this Summer. Demand during shows a continuation of a declining trend. In particular, the scaling back of the Energy Brix operation has been a major reason for this decrease. Gippsland Water proposes no changes to the strategy recommended for this system in the 2012 WSDS. Page 18 of 24

19 1.2.4 Mirboo North System Towns Supplied Current Status Average stream flows over last month (ML/d) Nine year average stream flows at this time of year (ML/d) Streamflows at this time in a dry year (ML/d) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Mirboo North Mirboo North Streamflows are currently higher than the nine year average and higher than at this time during the 2006/07 drought year. Restrictions were not required in the last eight years and it is deemed unlikely that restrictions will be necessary this summer unless exceptionally dry conditions develop. Demand during was similar to recent years and below forecast. The 2012 WSDS forecast this system to be secure until 2040 and proposed no actions. In light of recent demands, no changes to this outlook are proposed at this stage. Page 19 of 24

20 1.2.5 Sale System Towns Supplied Current Status Sale Current aquifer level (m) year average aquifer level at this time of year (m) Sale, Wurruk 10.4 Aquifer level at this time in drought year (m) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend 9.8 While aquifer levels are close to those during the Millennium drought, the last five years since the end of the drought has shown a halt in the ongoing decline of the aquifer that began in the late 1990s. This is thought to be a product of some recharge during wet years (2011 and 2012) as well as more moderate irrigation extraction in recent years. Gippsland Water will continue to monitor trends. Current bore drawdown levels are well above the pump levels and it is deemed unlikely that restrictions will be required this summer. Demand in was slightly below the previous two years. Even if demands return to earlier higher levels, this system has significant reserves. No changes to the strategy recommended in the 2012 WSDS are planned. Page 20 of 24

21 1.2.6 Seaspray System Towns Supplied Current Status Average stream flows over last month (ML/d) 10 year average stream flows at this time of year (ML/d) Streamflows at this time in a dry year (ML/d) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Seaspray Seaspray Flows for the last month have been lower than the average of the last 10 years but are currently sufficient to meet town demand. Merrimans Creek is subject to rapid streamflow fluctuation. The raw water basin is at 87% full. As the basin is relatively new, Gippsland Water has limited experience in its water quality performance. Algae has recently developed and is currently being tested and managed to ensure safe drinking water. A range of actions have been identified with water carting being a possibility. Restrictions may be required as summer progresses depending on demand and source water quality. Demand in was the lowest in 15 years. Note that the high demand in was due to the raw water basin being filled. Bulk Entitlement rules changed on 1 July These changes include a higher annual diversion limit, a winter (July October inclusive) minimum passing flow before diversions can be made, as well as a greater allowable diversion rate during that period. This change is factored into the long term yield trend shown above. Page 21 of 24

22 1.2.7 Tarago System Towns Supplied Current Status Tarago Warragul, Nilma, Drouin, Buln Buln, Rokeby, Neerim South, Noojee Average stream flows over last month (ML/d) 48 7 year average stream flows at this time of year (ML/d) 55 Streamflows at this time in a dry year (ML/d) 26 Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Streamflows in the Tarago River are slightly below average at present. Gippsland Water has also not used any of its supplementary 400 ML in Tarago Reservoir. The Tarago system is currently in a relatively good position. Based on performance in previous years, water restrictions are considered to be unlikely this Summer. However, as this large system has been experiencing significant growth and has minimal storage reserves, restrictions may be necessary if particularly dry conditions eventuate during Summer. Demand for was lower than the last two years and below that forecast in the 2012 WSDS. The relatively mild summer of is likely to have led to this relatively low demand despite ongoing growth in this system. Gippsland Water is planning to continue with the strategy outlined for this system in the 2012 WSDS, which includes the Warragul Moe interconnection. Page 22 of 24

23 1.2.8 Thomson - Macalister System Towns Supplied Current Status Current Allocation (%) 100 Average allocation at this time of year (%) Allocation at this time in dry year (%) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Thomson - Macalister Maffra, Stratford, Boisdale, Heyfield, Coongulla, Glenmaggie As Gippsland Water currently has a full allocation, it is unlikely that restrictions will be required this Summer. Demand in was slightly lower than recent years and below forecast. The 2012 WSDS forecast this system to be secure for the short to medium term and in light of recent demand trends, Gippsland Water doesn t propose any changes to the strategy recommended in the WSDS. Page 23 of 24

24 1.2.9 Thorpdale System Towns Supplied Current Status Average stream flows over last month (ML/d) 10 year average stream flows at this time of year (ML/d) Streamflows at this time in a dry year (ML/d) Summer Outlook Likelihood of water restrictions or other actions Longer Term Trend Thorpdale Thorpdale Streamflows are currently below average but this system is being supplied by water carting from other systems due to high manganese levels in Easterbrook Creek leading to poor drinking water aesthetics (taste/colour/odour). Water restrictions may be necessary to manage high demand should it eventuate. The 2012 WSDS identified short term risks for this system both from a drought resilience perspective and in terms of long term yield vs demand trends. Demand for was lower than in recent years and lower than the forecast. Gippsland Water has been working with the community of Thorpdale with a view to putting in place the most appropriate strategy to address risks for this system. Page 24 of 24