DSD-INT 2017 Use of RIBASIM in Lesotho - Passchier
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The document discusses the water resources situation in Lesotho, highlighting its surplus of quality water in the highlands, which faces increasing demand and shortages in the lowlands. It emphasizes the need for infrastructure upgrades, including the Ribasim model, to assess impacts of climate change and improve water transfer systems, particularly between the highlands and lowlands. Recommendations include exploring tunnel transfers and reconsidering the design of the Polihali dam to ensure sustainable water supply for lowland cities like Maseru.
DSD-INT 2017 Use of RIBASIM in Lesotho - Passchier
- 1. Use of RIBASIM in Lesotho Ron Passchier Deltares October 2017
- 2. Lesotho – the Mountain Kingdom
- 3. Lesotho – main characteristics •Landlocked country in Africa •Independent since 1966 •99.7% Basotho •Population about 2.2 million •Density 68 / km2 •Capital: Maseru •Area: 30,355 km2 •Upper basin of the Orange river •Major rivers: Senqu & Caledon •Elevation 1400 – 3482 m (Drakensberg) •Rainfall 500 – 1200 mm •Mainly surface water, no groundwater Octobre 2017
- 4. Topography – Lowlands versus Highlands October 2017 Lowlands Highlands
- 5. Lesotho – Part of Orange river basin Orange river basin has two main tributaries in Lesotho: • Senqu • Mohokare Lesotho Mohokare Senqu
- 6. Lesotho Highlands Water Project (LHWP) •Bi-national Project (Lesotho – Republic South Africa (RSA)) •Provide water to RSA •Various phases •Phase 1A & 1B finalized (2003): • 2 major dams (Katse & Mohale) • Transfer tunnel to RSA (18 m3/s) •Phase 2: new dam Polihali (ready to be started) •Later phases to be decided •Max. 70 m3/s
- 7. Treaty with RSA – 18 m3/s Hydro power for Lesotho Lesotho Highlands Water Project – Phase 1
- 8. Lesotho Highlands Water Project – Phase 1 Two dams: Katse and Mohale
- 9. 1 januari 2008 Later phases: Polihali dam Polihali damMetolong dam
- 10. Lowlands
- 11. Highlands
- 12. LHWP Dams (Mohale and Katse)
- 13. Water-resources situation in Lesotho 1. Surplus of water of excellent quality, but concentrated in the Highlands 2. Highlands ‘empty’, Lowlands concentration of population and thus demands 3. Lowlands suffer shortages in PWS (2–3 m3/s) 4. Shortages likely to increase in near future 5. Temporary solution by small reservoirs in Lowlands region (Metolong dam) 6. Main problem: enormous sediment load in the rivers reservoirs are short-lived Octobre 2017
- 14. Possible solutions 1. Reduce water demands? 2. Highlands as a source! 3. Technically feasible: • Existing links, or even better: • New connection to Lowlands (Mohale – Metolong – Maseru capital) 4. Technically, yes, but in practice problematic: • Existing treaty • Make sure sufficient water available for RSA
- 15. Project program – continuation of earlier work 1. Upgrading of the RIBASIM model 2. Addition of climate change impact 3. Analysis of water resources situation with new data and infrastructure 4. Implementation of RIBASIM at Department of Water Affairs of Lesotho Octobre 2017 • Frederiek Sperna Weiland – Climate change assessment • Laurène Bouaziz – Hydrological modelling • Ron Passchier – Team leader and water resources modelling
- 16. RIBASIM application • Whole of Lesotho, including upper Mohokare basin in South Africa • Based on earlier schematization and input data • Upgrading of infrastructure • Updating of model data, including climate change (CC) • Implementation at office of Department of Water Affairs • Application with various scenarios (e.g. CC) Octobre 2017
- 17. Climate change assessment • First two GCM scenarios are chosen from a set of 22 future climate change scenarios: average (‘central’) and dry conditions. • The daily gridded scenario time-series of precipitation, temperature and potential evaporation are used in hydrological model of the Senqu and Mohokare rivers. • With the hydrological model we calculate changes in natural river discharges as a result of climate change. • The percentage monthly changes are used to modify the original inflow series of the RIBASIM model. Important result: in general over Lesotho there will be an increase in water availability in the order of 3 – 8%, with the notable exception of the Lowlands, which show an overall decrease Octobre 2017
- 18. Application of RIBASIM in Lesotho Octobre 2017
- 19. New RIBASIM schematization Katse Polihali Mohale Metolong
- 20. Introduction of Phase 3 – 4 loop in model
- 22. Mohale to Metolong – RIBASIM
- 24. Setup RIBASIM Scenarios • Combination of: • Hydrological scenarios • Demand scenarios • Three Hydrological scenarios: • Present climate (“CC 0”) • Average (or ‘central’) climate projection (“CC 1”) • Worst case (or ‘dry’) climate projection (“CC 2”) • Two demand scenarios: • Present • Future (2050) Octobre 2017
- 25. Model scenarios Combination of hydrological and water demand scenarios Nr CC Description 1 Present demand situation A Present climate (“CC 0”) B CC Average (“CC 1”) C CC Dry (“CC 2”) 2 Future (2050) demand situation A Present climate (“CC 0”) B CC Average (“CC 1”) C CC Dry (“CC 2”) 3 Future (2050) demand situation with Mohale to Metolong transfer B CC Average (“CC 1”) C CC Dry (“CC 2”) 4 Future (2050) demand situation with Polihali scheme B CC Average (“CC 1”) C CC Dry (“CC 2”) Present Future
- 26. Total water balance with synthetic series Long series of monthly average values with one extreme stress period
- 27. Example of impact of stress period – reservoirs
- 28. Example for future demand situation
- 29. Supply – demand ratio PWS Maseru (future)
- 30. Result of scenario 3 with Highlands - Lowlands
- 31. Flow through the tunnels between reservoirs Katse Mohale To Lowlands 18 m3/s
- 32. Supply – demand ratio Maseru (with transfer) 100% 90%
- 33. Result for Scenario 4B – Phase 2 LHWP
- 34. Polihali hydrology (daily flows)
- 35. Polihali hydrology (monthly flows)
- 36. Conclusions and recommendations • Water supply to Maseru and other cities in Lowlands is not sustainable in the future by local sources • Climate change may form crucial issue (Lowlands less rainfall) • New Metolong dam will have short life time due to sedimentation • Solution can be found by looking at Highland system (LHWP) • LHWP will not suffer noticeable by taking water from Mohale to Lowlands (“demand in margin of error”) • Therefore it is advised to explore the possibility to implement a tunnel transfer between Highlands (Mohale dam) and Lowlands (Metolong dam) • Design of Polihali dam (Phase 2 of LHWP) should be reconsidered • General on RIBASIM: necessary tool to understand complex water systems
- 37. Thank you!