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Alfred Kabo Petros

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. Contents. I. Background. II. Objective. III. Methodology. IV. Results & Discussion.

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Alfred Kabo Petros

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  1. Alfred KaboPetros 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 I Background II 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 between150 to 400% of normal rainfall over Dec 2016-Feb2017 Fig.1. 90-day satellite estimate of percentof normal rainfall (%) (Dec-12 – Mar-11, 2017) (Source: NOAA/CPC, Red circle indicates Gaborone researcharea)

  4. The impactoffloodsonwaterresources–many open questions • The 2016-17 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 rechargeinaridregions–focusedrecharge(Leducetal.2001; Scanlon et al. 2006; Taylor et al.2013) • Predictedincreaseinfrequencyandmagnitudeoffloods,aswell as land use change, is expected to have a positive impact on replenishment of groundwaterresources Research question: Do extremerainfall events in peri-urban environments support either or both the replenishment of water resources and theircontamination?

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

  6. Land use-Catchment andPotentialRiskAreas Dairy plant/farmingactivities Mining/quarryingactivities Majorlandfills Majordams Gaboronedam Notwanedam Mogobane Dam Nnywane Dam Nnywanedam

  7. ERT Project Post-floodMonitoring- SamplingNetwork • 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 ERT

  8. Project Post-floodMonitoring- SamplingNetwork

  9. Gaborone Reservoir –BeforeandDuringthe2017Flood 2016 floods 4y drought 2017 floods

  10. Flood 2017 Flood 2016 Groundwater levels 2015-17 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 South catchment (basement) Gaborone dam North catchment (dolomite)

  11. highlevels =recharge Borehole overtopping Decay (flood) South catchment (Lobatse- Ramotswa) North catchment (Gaborone)

  12. Groundwater LevelsPost-Dineo(PULAmonitoring) Upstream GaboroneDam

  13. Spatial Variability inAverages Groundwater SurfaceWater MajorIons OrganicCarbon TraceMetals Low High

  14. Reservoirs Dilute SurfaceWater SurfaceWater GaboroneReservoir MogobaneReservoir NnywaneReservoir

  15. Spatial Variability inAverages D/stream SurfaceWater Upstream Groundwater MajorIons OrganicCarbon TraceMetals Low High

  16. MOREVARIATIONINSPACETHANINTIME SurfaceWater GroundWater

  17. Trends in Time SinceFlood: Chromium Increase Decrease Nochange

  18. Summary of Trace Metals- Gaborone DamArea • SW • Decreases in: • Riverupstream • Riverdownstream • Reservoirwater • GW • Decreasesupstream • Increasesdownstream

  19. High-res Groundwatertime-series May-Nov 2017(5 min step) Rural area upstreamdam Gaboronedam Suburban area downstreamdam

  20. 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 downstreamlandfill • Shrinks and movesdownstream

  21. DuringFlood Shortly afterFlood Longer afterFlood

  22. 2017 event in Botswana triggered both a major, rapid, ubiquitous recharge eventover thecatchment • Delayed, slower recharge downstream major dams (continues 1year after the flood) • Possible contaminant migration in peri-urban areas, yet concurrently diluted byrecharge watervolumes

  23. 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

  24. Ministry of Land Management, Water & Sanitation Services

  25. Thankyou!

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