Presentation by Ron Passchier (Deltares) at the River Basin Planning and Modelling symposium, during Delft Software Days - Edition 2017. Wednesday, 25 October 2017, Delft.

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

21. Mohale to Metolong
Mohale
Metolong
Maseru

22. Mohale to Metolong – RIBASIM

23. MOHALE
KATSE
MUELA
MASERU
To RSA
METOLONG

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!