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Water User Sectors and Priorities

Water User Sectors and Priorities. Contents. 1. Overview of water user sectors 2. Water user criteria 3. Yield reliability 4. Restricting water users 5. Practical. 1. Overview of water user sectors. 6 Main User Sectors: SA. + Environment. Requirements.

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Water User Sectors and Priorities

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  1. Water User Sectors and Priorities

  2. Contents 1. Overview of water user sectors 2. Water user criteria 3. Yield reliability 4. Restricting water users 5. Practical

  3. 1. Overview of water user sectors

  4. 6 Main User Sectors: SA + Environment

  5. Requirements • Different users require water at different assurances of supply • Usually stated as a percentage of reliability • Demand provided for 98 out of 100 years is provided at a 98% assurance of supply • Ie. 2 of 100 years won’t be supplied, or 1 in 50 years

  6. Reliability • Assurance vs Risk of failure

  7. Resources and Assurance • Resources decrease as assurance increases • Can supply more water if users are prepared to accept a lower assurance, higher risk of failure • Eg. A dam that can supply 100 million m3/a at an assurance of 99% could supply 150 million m3/a at an assurance of 90%

  8. Typical user Assurance Requirement Rank • Environmental • Afforestation • Power Generation, Bulk Industrial • Urban & Rural (primary users) • Irrigation

  9. Distribute Requirements over Assurance Criteria • Concept of “fairly” allocating a resource amongst competing users • Some users need high assurance • Other users prefer more water at lower assurance • Each users requirement has portions distributed into various assurance criteria • Known as the priority classification table

  10. 2. Water user criteria

  11. Example of user criteria breakdown 3 user criteria 1 in 10 years 1 in 50 years 1 in 100 years • 3 users of a resource • Urban: 40 million m3/a • Industrial: 20 million m3/a • Irrigators: 100 million m3/a

  12. Example of user criteria breakdown • 3 users of a resource • Urban: 40 million m3/a • Industrial: 20 million m3/a • Irrigators: 100 million m3/a

  13. Example of user criteria breakdown Percentage Volume: million m3/a

  14. Water user criteriaExample: Mgeni River system Description of water requirement components Percentage of total requirement Percentage allocated to indicated priority class 1:200 years (0.5 %) 1:100 years (1.0 %) 1:50 years (2.0 %) 1:20 years (5.0 %) Losses Wet industry Dry industry Domestic Total Priority class: 24.5 16.3 12.2 47.0 100.0 100 70 70 40 63 - 10 15 20 13 - 10 5 20 12 - 10 10 20 12 H MH ML L

  15. 3. Yield reliability

  16. Long Term Yield Curve Y-axis = demand imposed on system X-axis = reliability of Supply (%)

  17. Definitions • Rn • Ep • RI = Long-term risk of failure = [Failure sequences + 1] / [Total sequences] = (19 + 1) / 41 = 0.488 = 48.8 % = Long-term reliability of supply = 1 – Rn = 51.2 % = Recurrence interval of failures = 1 / (1 – Ep1/n) = 90.18 years

  18. Long Term Yield Curve 3.2 million m3/a at 1 in 200 years reliability 4.7 million m3/a at 1 in 20 years reliability

  19. Practical • Can the Water Resources system represented by the long term yield curve handed out provide the required assurances for the requirements represented by the following priority Classification Table? • Demands are: • Urban: 10 million m3/a • Irrigation: 40 million m3/a • Industrial: 5 million m3/a

  20. Long Term Curve for Practical

  21. Long Term vs short Term Yields • Long Term: System yield over the long term, used for allocations, quoting availability of resource, planning • Short Term: Takes storage into consideration, supply more over short term is system storages high, used more for operations, short term decision making

  22. S1 S2 Short-term yield reliability • Represent yield-reliability characteristics over short term (up to 5 years) • Individual set for each defined subsystem D1 D2 D3

  23. Dependent on: • Hydrology • Physical characteristics of system • Operating rule • Uncontrolled water requirements (e.g. afforestation and diffuse irrigation) Note: If any of the above changes, curves must be updated!

  24. Independent of: • Controlled water requirements • Selected priority classification • Inter-subsystem support definition Note: Curves need not be updated for a change in any of the above

  25. Impact of starting storageStart at 80 % of live FSV Target draft = 80 million m3/a Yield / target draft (million m3/a) Sequence number

  26. Impact of starting storageStart at 60 % of live FSV Target draft = 80 million m3/a Yield / target draft (million m3/a) Sequence number

  27. Impact of starting storageStart at 100 %of live FSV Firm yield line

  28. Impact of starting storageStart at 80 %of live FSV Firm yield line

  29. Impact of starting storageStart at 60 %of live FSV Firm yield line

  30. Impact of starting storageStart at 40 %of live FSV Firm yield line

  31. Impact of starting storageStart at 20 %of live FSV Firm yield line

  32. Impact of starting storageStart at 10 %of live FSV Firm yield line

  33. Impact of starting storageFamily of firm yield lines 100 % 80 % 60 % 40 % 20 % 10 %

  34. 4. Restricting water users

  35. Basic principles • Water users can be restricted • Based on allowable risk of non-supply • Objective to avoid failure (zero storage volume in reservoirs) • Water users prioritised into classes • Balanced allocation within a single class • Restrictions in low priority class first, to protect higher classes • Restrictions are part of operating rules and should always be tested and checked

  36. Min0.5%1.0 %2.0 %5.0 %25 %50 %75 %95 %98 %99 %99.5 %Max Restrictions vs. failureMidmar Dam (with restrictions) Lowest draw-down DSV = 31

  37. Min0.5%1.0 %2.0 %5.0 %25 %50 %75 %95 %98 %99 %99.5 %Max Restrictions vs. failureMidmar Dam (without restrictions) Supply failure DSV = 31

  38. Example: Allocation decision(1 of 3) Demand distribution Demand Distribution into classes (million m3/a) Million m3/a Low Medium High Irrigation Domestic Total 50 70 120 25 14 39 15 21 36 10 35 45

  39. 5.8 3.2 15 21 10 35 Example: Allocation decision(2 of 3) Curtailed demands (Case 1) Demand Million m3/a Distribution of restricted demand into classes (million m3/a) Low Medium High Total allocated Prop. supplied Allocation to individual users Irrigation Domestic 90 75 % 9 23 % 36 100 % 45 100 % 30.8 59.3

  40. 0 0 % 15 42 % 45 100 % 0 0 6.3 8.7 10 35 Example: Allocation decision(3 of 3) Curtailed demands (Case 2) Demand Million m3/a Distribution of restricted demand into classes (million m3/a) Low Medium High Total allocated Prop. supplied Allocation to individual users Irrigation Domestic 60 50 % 16.3 43.7

  41. Practical • Given the following priority classification table, set of short term yield curves and demands, what is the required restrictions to be Gazetted for the various user sectors if the dam is at 40% storage in the decision month? Urban: 4 million m3/a Industrial: 3 million m3/a Irrigation: 15 million m3/a

  42. 1 in 50: 5.6 Total available: 20 million m3 1 in 200: 3.4 1 in 10: 8.6 1 in 100: 4.4

  43. 1 in 50: 5.6 Total available: 17 million m3 1 in 200: 3.4 1 in 10: 8.6 1 in 100: 4.4

  44. End of module

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