The role of climate variability and climate change in NSW water sharing arrangements - Shahadat Chowdhury


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The role of climate variability and climate change in NSW water sharing arrangements - Shahadat Chowdhury

  1. 1. The role of climate variability and climate change in NSW water sharing arrangements Richard Beecham (Water Modelling) Shahadat Chowdhury (Water Modelling) Mark Harris (Planning and Policy)
  2. 2. OVERVIEW OF TALK <ul><li>Water resource plans in NSW </li></ul><ul><li>Climate variability in developing the plans </li></ul><ul><li>Climate variability versus climate change </li></ul><ul><li>Future planning challenges </li></ul><ul><li>This presentation is relevant to large regulated rivers in NSW. </li></ul><ul><li>Unregulated rivers and aquifers have different challenges. </li></ul>
  3. 3. DEVELOPMENT OF WATER PLAN <ul><li>Water Sharing Plans in NSW set the rules for sharing water between the environment and different consumptive users. </li></ul><ul><li>The plans are set for a ten year period. </li></ul><ul><li>Plan is designed for climatic conditions that are likely to occur. </li></ul><ul><li>Input: historical rainfall, evaporation, temperature and inflow over 1895 to 2010 </li></ul><ul><li>The impacts of various proposals are scrutinised using numerical river system models. </li></ul>
  5. 5. CURRENT PLAN’S RESPONSE TO CLIMATE STATE Simulated response of the Namoi Water Sharing Plan to climate variability Ref: NamoWSP2010.sqq % of long term mean 110 195 WETTEST DECADE 80 50 DRIEST DECADE 100 Diversions 100 1893-2008 Inflows PERIOD
  6. 6. CLIMATE VARIABILITY ISSUES <ul><li>Do we understand climate variability well? Is 120 year data sequence long enough to reflect the extremes? </li></ul><ul><li>Statistical generation of longer time series may help simulate rare events. </li></ul><ul><li>Some limited palaeo studies indicated longer dry spells than the one found in the historical record. </li></ul><ul><li>Should the plan cater for extremes with say 500 year frequency? </li></ul><ul><li>Would that be a too risk averse plan? </li></ul>
  7. 7. FUTURE WATER PLAN CHALLENGES <ul><li>More complex environmental or ecological targets. </li></ul><ul><li>The need to consider climate change (key directive). </li></ul><ul><ul><li>National Water Initiative </li></ul></ul><ul><ul><li>National Water Commission </li></ul></ul><ul><ul><li>Commonwealth Water Act 2007 </li></ul></ul><ul><li>Significant uncertainty in quantifying the hydrological impact of climate change. </li></ul><ul><li>Demanding accreditation test </li></ul><ul><ul><li>Eg. the Guide to the proposed (Murray Darling) Basin Plan. The reduction of decadal water extraction at the same rate to any reduction of available water over that decade. </li></ul></ul>
  8. 8. PLANNING IN STATIONARY CLIMATE 10 YR VARIABILITY PLAN LIFE Annual Rainfall (mm) Residual mass curve
  10. 10. MODELLING FUTURE CLIMATE <ul><li>Options to estimate future climate time series </li></ul><ul><li>Global Climate Model (GCM): 200 to 400 km grid </li></ul><ul><li>Regional Climate Model (RCM): 10 km grid </li></ul><ul><ul><li>GCM to RCM: statistical or dynamical </li></ul></ul><ul><li>Scaling of historical climate time series based on GCM </li></ul><ul><ul><li>current practice </li></ul></ul><ul><ul><li>computationally simple and provides long time series required for planning models </li></ul></ul>Scaling method GCM/ RCM
  11. 11. ISSUES WITH GCM/RCM PROJECTION <ul><li>Downscaling to be suitable for river system models, both in time and space scale. GCM to RCM </li></ul><ul><ul><li>Computationally expensive. </li></ul></ul><ul><li>Poor reproduction of historical rainfall including important low frequency events. </li></ul><ul><li>Large disagreements among different GCM’s rainfall projections. </li></ul><ul><li>Projections span from present to 2100. What should be the appropriate slice of time window to represent the climate variability during the life of the plan? </li></ul><ul><li>Selection of a short time window of GCM (10 year) means lower chance of experiencing long dry spells </li></ul>
  12. 12. SCALING METHOD <ul><li>Poorly represent spatial differences of climate projections and hence do not represent altered spatial dependence expected due to climate change. </li></ul><ul><li>Do not alter historical long dry spell length, very important aspect of water planning ! Hence underestimates extremes such as consecutive years of very low allocation. </li></ul>
  13. 13. Note the dry spells are of similar length!
  14. 14. THE WAY FORWARD <ul><li>“ Prediction is difficult, especially about the future” Niels Bohr </li></ul><ul><li>“… it is premature to make definitive statements about the levels of uncertainty in climate change impact ….”, Bates et al. (2010), Waterlines Report 28, NWC </li></ul><ul><li>Risk based approach of water planning in NSW: </li></ul><ul><ul><li>plan formulated using current knowledge (past climate variability) </li></ul></ul><ul><ul><li>test the robustness of the plan on plausible scenarios beyond the historical record </li></ul></ul><ul><ul><li>include contingency measures and triggers for unprecedented events </li></ul></ul><ul><ul><li>review plan at the 10 year planning cycle (use any new information). </li></ul></ul>
  15. 15. The End Author contacts: Richard Beecham, [email_address] Shahadat Chowdhury, [email_address] Mark Harris, [email_address]
  16. 16. SOME TYPICAL SIMULATION OUTCOMES IN THE PLANNING PHASE <ul><li>What is the mean diversion over long term? : Important for comparison to 1993/94 MDB cap on diversion. </li></ul><ul><li>How often flows are available for environmental requirement (eg. end of system, wetlands) ? : Environmental outcome. </li></ul><ul><li>How often water allocation becomes low (say < 20%)? : Irrigation reliability. </li></ul><ul><li>What is the longest dry spell in terms of allocation? : Industry viability. </li></ul><ul><li>What is the lowest storage level? : Security of essential supply. </li></ul><ul><li>… and many more </li></ul>
  17. 17. CLIMATE VARIABILITY VS CHANGE <ul><li>Climate change can be defined as non stationary state of the climate. </li></ul><ul><li>Stationary: long term mean, spread (highs to lows), skew, persistence, seasonality, chance of rare events (extremes). </li></ul><ul><li>Our current modelling practice assumes stationarity, past long time series represents best estimation of future variability. </li></ul><ul><li>Climate Change : non stationary climate and hence past long time series may not be adequate to define future variability. </li></ul>
  18. 18. CURRENT PLAN’S RESPONSE TO CLIMATE STATE Ref: NamoWSP2010.sqq Simulated response of the Namoi Water Sharing Plan to climate variability 1590 GL/a (240%) 260 GL/a (110%) 1390 GL/a (195%) Wettest decade 1947 to 1957 330 GL/a (50%) 190 GL/a (80%) 355 GL/a (50%) Driest decade 1936 to 1946 660 GL/a (100%) AVERAGE END of SYSTEM FLOW 235 GL/a (100%) AVERAGE DIVERSION 705 GL/a (100%) Full length 1893 to 2008 AVERAGE MAJOR INFLOW
  19. 19. CURRENT PLAN’S RESPONSE TO CLIMATE STATE Simulated response of the Lachlan Water Sharing Plan to climate variability Ref: LachW018.sqq 210 GL/a (210%) 335 GL/a (120%) 1480 GL/a (195%) Wettest decade 1948 to 1958 40 GL/a (40%) 265 GL/a (95%) 435 GL/a (60%) Driest decade 1901 to 1911 100 GL/a (100%) AVERAGE END of SYSTEM FLOW 280 GL/a (100%) AVERAGE DIVERSION 750 GL/a (100%) Full length 1898 to 2008 AVERAGE MAJOR INFLOW
  20. 20. FUTURE PLAN’S RESPONSE TO CLIMATE STATE The Guide to the Basin Plan proposed the accreditation tests shown in first two columns: *case study: Lachlan & Namoi 90-95% ~ 200% ~ 50% 100% MAJOR INFLOW * ~ 90% A% or less A % Driest decade 90-99% C % or less C % Climate change Full length ~ 110-120% B % or less B % Wettest decade 100% CURRENT IRRIGATION* 100% SURFACE WATER EXTRACTION 100% Full length AVAILABLE WATER