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 Background Objective Methodology Results & Discussion Summary II Objective III Methodology IV Results & Discussion V Summary VI Key Lessons

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 2016-Feb 2017 Fig.1. 90-day satellite estimate of percent of normal rainfall (%) (Dec-12 – Mar-11, 2017) (Source: NOAA/CPC, Red circle indicates Gaborone research area)

The impact of floods on water resources– 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 recharge in arid regions – focused recharge (Leduc et al. 2001; Scanlon et al. 2006; 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?

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

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

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

Project Post-flood Monitoring- Sampling Network

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

Groundwater levels 2015-17 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

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

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

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

Reservoirs Dilute Surface Water Gaborone Reservoir Mogobane Reservoir Nnywane Reservoir

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

MORE VARIATION IN SPACE THAN IN TIME Surface Water Ground Water

Trends in Time Since Flood: Chromium Increase Decrease No change

Summary of Trace Metals - Gaborone Dam Area SW Decreases in : River upstream River downstream Reservoir water GW Decreases upstream Increases downstream

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

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

During Flood Shortly after Flood Longer after Flood

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

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

Ministry of Land Management, Water & Sanitation Services

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