1. Alfred Kabo Petros
Understanding the Impacts of Climate Change on Urban and Peri-Urban Groundwater Resources- Lessons learnt from Botswana
Ministry of Land Management,
Water & Sanitation Services

2. Contents Background Objective Methodology Results & Discussion SummaryII
Objective
III
Methodology IV
Results & Discussion V
Summary VI
Key Lessons

3. Between 18 and 23 February 2017, Botswana was hit by the tropical depression
Dineo reached Botswana downgraded from cyclone status to a tropical storm, the storm's remnants triggered destructive floods in Botswana.
Botswana received between 150 to 400% of normal rainfall over Dec Feb 2017
Fig day satellite estimate of percent of normal rainfall (%) (Dec-12 – Mar-11, 2017) (Source: NOAA/CPC, Red circle indicates Gaborone research area)

5. The impact of floods on water resources– many open questions
The exceptional rainy season in Botswana provides an opportunity to monitor the effects of heavy rainfall on water resources quantity and quality in arid regions
Increasingly recognized that floods are key to groundwater recharge in arid regions – focused recharge (Leduc et al. 2001; Scanlon et al ; Taylor et al. 2013)
Predicted increase in frequency and magnitude of floods, as well as land use change, is expected to have a positive impact on replenishment of groundwater resources
Research question: Do extreme rainfall events in peri-urban environments support either or both the replenishment of water resources and their contamination?

7. Objective
Investigating the bearings of the 2017 extreme floods on groundwater resources in the Gaborone catchment area

8. Land use-Catchment and Potential Risk Areas
Gaborone dam
Nnywane dam
Notwane dam
Mogobane Dam
Dairy plant/farming activities
Mining/quarrying activities Major landfills
Major dams
Nnywane Dam

9. Project Post-flood Monitoring- Sampling Network
ERT
Compilation of
rainfall records
Gaborone dam level/capacity
DWA borehole monitoring network
Design of project specific hydrological monitoring network
Dams and rivers
Boreholes
Time-lapse geophysics

10. Project Post-flood Monitoring- Sampling Network

11. Gaborone Reservoir –Before and During the 2017 Flood
2016 floods
4y drought
2017 floods

12. Groundwater levels
Flood 2017
Flood 2016
North catchment
(dolomite)
South catchment
(basement)
4y drought
2017 floods caused water table rise of up to 15m in Lobatse area and up to 10m in Ramotswa area
2016 floods caused rise of up to 5m in Ramotswa, negligible in Lobatse
Gaborone dam

13. South catchment (Lobatse- Ramotswa)
high levels
= recharge
Borehole overtopping
(flood)
Decay
South catchment (Lobatse- Ramotswa)
North catchment (Gaborone)

14. Groundwater Levels Post - Dineo (PULA monitoring)
Upstream Gaborone Dam

15. Spatial Variability in Averages
Surface Water
Groundwater
Major Ions
Organic Carbon
Trace Metals
Low
High

16. Reservoirs Dilute Surface Water
Gaborone Reservoir
Mogobane Reservoir
Nnywane Reservoir

17. Spatial Variability in Averages
D/stream
Upstream
Surface Water
Groundwater
Major Ions
Organic Carbon
Trace Metals
Low
High

18. MORE VARIATION IN SPACE THAN IN TIME
Surface Water Ground Water

19. Trends in Time Since Flood: Chromium
Increase Decrease No change

20. Summary of Trace Metals - Gaborone Dam AreaSW
Decreases in :
River upstream
River downstream
Reservoir water GW
Decreases upstream
Increases downstream

21. High-res Groundwater time-series May-Nov 2017 (5 min step)
Rural area upstream dam
Gaborone dam
Suburban area downstream dam

22. Flood plain time-lapse resistivity surveys
Landfill downstream dam (length 480m; depth 100m)
Gaborone dam
Landfill upstream dam (length 720m; depth 100m)
High conductivity plume immediately downstream landfill
Shrinks and moves downstream

23. During Flood
Shortly after Flood
Longer after Flood

24. 2017 event in Botswana triggered both a major, rapid, ubiquitous recharge event over the catchment
Delayed, slower recharge downstream major dams (continues 1 year after the flood)
Possible contaminant migration in peri-urban areas, yet concurrently diluted by recharge water volumes

25. Do extreme rainfall events support either or both the replenishment of water resources and their contamination in the Peri urban environment?
Probably both! But spatially variable
– Need to understand better these spatial controls: Geology? GW/SW interactions? Antecedent conditions?
Managing floods can also help in managing subsequent droughts
Urgent need for management of ‘pollution hotspots’ during and between floods

26. Ministry of Land Management,
Water & Sanitation Services

27. Thank you!