Crystalline Bedrock aquifers underlie about 40% of Sub-Saharan Africa and can generally sustain low-intensity abstraction. However, pumping rates and dependency is increasing in many areas, particularly for cities like Addis Ababa, Dakar, Nairobi and Dodoma. Projected growth in population and water demand for agriculture, plus the effects of climate change, mean that it is essential to develop a better understanding of the sustainable yields from these types of aquifers.
The study focuses on five groundwater abstraction boreholes, 3 in Uganda, 2 in Tanzania.
Long term groundwater records are only available for one of the boreholes and it shows that recharge happens more when the rainfall is more intense, which is often associated with periodic El Niño Southern Oscillation (ENSO) events.
Chemical analysis of the water was used to determine the residence times of the groundwater (how long the water has been in the aquifer since it fell as rain). Overall, that most pumped water comes from modern recharge (within the last 10-60 years), so while abstractions are not mining pre-modern groundwater, there may be a component of older water that is coming out.
Groundwater abstraction appears to be supported by recharge from across multiple years, rather than just the most recent wet season.
The investigation of the five sites shows that long term, high intensity groundwater abstraction is possible from East African weathered crystalline basement aquifers, but the sustainability is constrained, in part, by the high inter-annual variability in recharge. Therefore operation of such pumping stations needs to include sustained monitoring of groundwater levels, pumping rates and rainfall as a minimum.
Groundwater Recharge – the set of processes that govern how rainwater seeps through soils and rocks to replenish aquifers – is not well understood across much of Africa. It is important to understand because it is central to determine the sustainable use of groundwater resources;
The authors analyse three rare sets of long-term (19-25 years) groundwater-level observations from three different, but common, geological settings in Benin;
The year-to-year changes in groundwater storage correlate well with rainfall patterns, but there were big differences the relate to the type of geology:
In the shallow, sand aquifer as much as 40% of the rainfall becomes groundwater
In the deeper sandstone and weathered crystalline rocks, a much lower proportion of rainfall becomes groundwater recharge (13% and 4% respectively)
Recharge was found to occur on a seasonal basis; however on a daily basis the groundwater fluctuations are best explained with a threshold of 5-15 mm per day – meaning that only more intense rainfall events lead to recharge.
These results are consistent with the growing body of evidence that, in Sub-Saharan Africa, intensification of rainfall associated with climate change may increase groundwater recharge.
Because the groundwater recharge is so strongly influenced by geology, it is essential for water resource planning that good geological maps are available and used, and that investment is made into long-term groundwater monitoring of strategic aquifers.
GRACE (Gravity Recovery and Climate Experiment) satellite can be used to estimate changes in water storage on time resolution of 1 month and a spatial resolution of about 450 x 450 km.
GRACE can be used to estimate groundwater storage changes where it is the dominant water mass. It is therefore useful in many areas of Sub-Saharan Africa where there are relatively few direct groundwater level measurements.
The paper focuses on the major sedimentary aquifers basins, where the majority of Africa’s groundwater resources are to be found. Away from these basins, groundwater storage is 1-2 orders of magnitude less.
There is no evidence of continuous long-term declining trends of Total Water Storage (mostly groundwater) in any of the major sedimentary aquifers, which indicates that none are stressed by current abstraction rates – howeverit is important to stress that local scale depletion may be occurring but is beyond the resolution of GRACE to detect.
There are also some interesting findings in regard to the combination of GRACE and Land Surface Modelling and how well (or not) they represent groundwater recharge processes in the different basins.
Read the full paper here:
Bonsor, H.C.; Shamsudduha, M.; Marchant, B.P.; MacDonald, A.M.; Taylor, R.G. Seasonal and Decadal Groundwater Changes in African Sedimentary Aquifers Estimated Using GRACE Products and LSMs. Remote Sens.2018, 10, 904. http://www.mdpi.com/2072-4292/10/6/904
Following an UPGro Catalyst Grant, over the last three years much work has gone into making use of roads for water management. Roads have in many areas an enormous impact on hydrology. Now often negative with roads causing erosion and sedimentation, or creating floods and water logging, this can be turned around to making roads instruments for water harvesting.
Under the RoadsforWater initiative see also www.roadsforwater.org this approach is introduced in ten countries already contributing to improved water security for more than 2 Million people – hoping to get much higher still. With a global investment in roads amounting to more than 1 Trillion dollar, ‘adding’ water management to road development and maintenance can have an enormous impact.
We now have very good news and a request to make:
RoadsforWater is among the 11 finalists of the 2017 – Resilience Award! We invite you to vote for this powerful initiative before Monday (15th Jan) Midnight (US Eastern Standard Time)?
Floods and droughts, feasts and famines: the challenge of living with an African climate has always been its variability, from the lush rainforests of the Congo to the extreme dry of the Sahara and Namib deserts. In north western Europe, drizzle and rain is generally spread quite evenly across the year, as anyone who has gone camping in British summer will tell you. But when annual rainfall happens within just a few months or weeks of the year then it is a massive challenge for farmers, towns and industry to access enough water through long dry seasons and to protect themselves and their land from flooding and mudslides when the rains come.
New research suggests that Africa’s aquifers could be the key to managing water better. Professor Richard Taylor at UCL explains: “What we found is that groundwater in tropical regions – and Sub-Saharan Africa in particular – is primarily replenished from intense rainfall events – heavy downpours. This means that aquifers are an essential way of storing the heavy rain from the rainy season for use during the dry season, and for keeping rivers flowing.”
Tropical groundwater may prove to be a climate-resilient source of freshwater in the tropics as intense rainfall favours the replenishment of these resources, according to a new study published in Environmental Research Letters.
Roads can devastate a landscape – scarring it, creating barriers for wildlife and accelerating stormwater so that valuable farmland, habitats and homes get washed away or polluted. What if didn’t have to be that way? What if roads would work with the grain of nature rather than against it?
One of the UPGro teams, lead by Frank van Steenbergen, at Meta Meta Research, has being doing just that. Over the last year, their UPGro Catalyst project has been researching how roads can be used for rainwater harvesting on a landscape scale to recharge aquifers and ponds for later use in the dry seasons.
Working closely with the Mekelle University and the Government of Ethiopia, Frank and his team (including the Institute for Development Studies) has not only been testing the theory but they have been putting into practice. In the region of Tigray, the methods of road design have captured imaginations as well as water and now the government is keen to roll these ideas out further around the country.
The Catalyst project is now complete and a number of resources are now available online: