By Pierre van Rensburg
Windhoek—the capital city of Namibia, the most arid country in Sub-Saharan Africa—has long met its severe water supply challenges through innovation, including the world’s first treatment plant for reusing potable water. Today, the city’s growing population is increasing its demand for water at the same time as climate change exacerbates its dangerously scarce supplies.
Ambitious long-distance water transfer projects that could provide a long-term solution are many years—and many millions of dollars—from coming online. .
The history of Windhoek’s water supply challenge
Windhoek sits at an altitude of 1650 meters above mean sea level on the country’s central plateau, wedged between the Namib desert on the west coast and the Kalahari desert on the eastern border with Botswana. The city receives only 358 mm of rainfall per year, and evaporation is extremely high: of the total precipitation, 83 percent evaporates, 14 percent is soaked up by vegetation, 1 percent recharges groundwater, and only 2 percent generates run-off which can be stored. Namibia is the only sub-Saharan country that has no perennial rivers or permanent natural water bodies within its borders; the nearest perennial rivers are more than 700km away from Windhoek. At the same time, the population of Windhoek is growing at one of the highest rates in the country—3.1 percent per year—as people migrate to the city seeking employment.
Windhoek’s early German settlers sunk wells along the Klein Windhoek river to remove groundwater, thus drying up the area’s numerous hot springs. They depended on groundwater until the Avis dam was built in the early 1930s, but the dam’s reservoir was completely empty 4 out of every 10 years, and in 1959 the dam’s yield (with a 95% reliability) was reported as effectively zero. By the time the Goreangab dam was commissioned, the country was over-withdrawing water from boreholes at a rate of more than double the calculated sustainable yield. In 1971, the government launched the first large-scale National Supply Scheme, which met the city’s water supply challenge with a new surface water reservoir and water treatment facility 70km north of Windhoek, as well as two subsequent dams within 200km of the city.
Windhoek’s water supply in a climate-constrained future
According to vulnerability assessments for Namibia, climate change will increase the temperatures by 1-4 degrees; rainfall will decrease by about 20 percent, but increase in intensity. The recent past demonstrates this pattern: unusually high intensity rain storms were recorded in 2004, 2006, 2008, and 2011, while abnormally dry years were recorded in 2013, 2015, and 2016—and in 2016, for the first time ever, the dam serving as the main source of water for Windhoek received no inflows.
Windhoek’s water supply is primarily climate dependent; surface run-off captured in surface water dams is highly susceptible to evaporation, with estimated losses of approximately 50 percent over a 10 year period. Under the climate scenarios, evaporation is predicted to increase 10-15 percent, further straining water supplies.
Long-term water supply options
In 2003, water planning studies for the central areas of Namibia and Windhoek predicted that demand would use all the available water by 2013—which came true at the end of that year. But from 2003 to 2013, the area experienced unusually high rainfall, frequently far exceeding the average annual amounts—and delaying implementation of proposed schemes to augment water supplies. But the 2014-16 drought clearly exposed the system’s shortcomings, and several emergency schemes just barely managed to save the capital city from disaster.
There are no feasible long-term supply options within a 300km radius of Windhoek. Today, two long-distance supply options are undergoing feasibility studies:
1. Linking of the current supply system for the central areas and Windhoek to the Okavango river, which is located to the north of Windhoek on the country’s border with Angola. This long-distance water transfer scheme, which would cover approximately 700km and has high infrastructure and upgrading costs.
2. Desalinate sea water at the coast and transfer it over 365km and up to 1650m above sea level. The exceptional altitude challenge complicates this long-distance water transfer scheme in terms of operational cost.
The proposed schemes—which are both plagued by inherent weaknesses and uncertainties—are considered the only viable long-term options. Regardless, the primary concern is that they are still in preliminary study phase and are unlikely to be implemented before 2024—which would be more than a decade after the fast-growing city first exceeded the limits of its water supply.
Water supply augmentation strategies
a) Direct potable reuse
Direct potable reuse is arguably Windhoek’s most successful strategy for augmenting its water supply. Following acute shortages in the late 1950s (which included water rationing in 1957), the city administration commissioned the world’s first potable reuse plant in October 1968. The scheme has proven invaluable over the years and never more so than during the recent drought, when the city depended on it for almost one third of its potable water supply. In Windhoek DPR has evolved over the years and today’s facility comprises a technologically advanced and fully automated water reclamation plant.
b) Non-potable Reuse
Building on the success of its pioneering potable reuse scheme, Windhoek installed a dual pipe system in the early 1990s to irrigate sports fields and public facilities with lower-quality treated water instead of using potable water for this purpose. This non-potable reuse process has enabled the city ever since to maintain reasonable recreational facilities despite numerous water supply challenges. The irrigation water, which costs 50 percent less than potable water, is hugely popular with public institutions seeking to maintain their facilities in periods of limited supply. During the 2014-16 drought, the scheme was expanded and provided 8 percent of the city’s total water supply.
c) Water demand management
Following the large-scale supply schemes of the 1970s, Windhoek residents felt that their water supplies were fairly secure. Even after the drought in the early 1980s, per capita usage crept up to above what could be considered reasonable for a semi-arid city in a water-scarce location. And despite the effective application of demand management practices during the drought of 1996-97, consumption again increased until fully serviced customers on average consumed more than 250 litres per day and high-income groups more than 350 litres per day.
At the start of 2016, the Windhoek City Council declared a water crisis, imposing severe water restrictions. By the end of the year, average consumption had fallen to 122 litres per day, with high-income groups using less than 200 litres per day. This success led to the city launching a continuous water management strategy aimed at maintaining reasonable consumption levels despite the supply status. .
d) Windhoek Managed Aquifer Recharge Scheme
In the early 2000s, the city launched extensive studies on using the Windhoek Aquifer to more effectively store surface water underground, thus reducing evaporation losses. Despite its storage capacity the aquifer has a low rate of natural recharge with limited supply capabilities, but the studies found it could be successfully recharged artificially, and so the city established the Windhoek Managed Aquifer Recharge Scheme. In years of abundant rainfall, surface water is injected into the available underground storage, where losses are estimated to be as low as three percent. The stored water is then withdrawn during a drought, when surface water sources diminish or fail. At current demand, the calculated available storage could supply Windhoek for up to three years.
The pre-implementation studies raised concerns regarding the potential for polluting the aquifer, especially in the recharge zone to the south of the city, where the aquifer is poorly protected and numerous fault zones provide direct access. A rather progressive and rather controversial 2007 decision by the city council established an Aquifer Protection Zone prohibiting any development from taking place in the area—which encompasses most of the remaining flat, developable land in and around the city.
Caption: During the 2014-16 drought, conventional surface water supplies all but failed. In December 2016, these supplies were effectively replaced by withdrawals from the aquifer under the Windhoek Managed Aquifer Scheme, which met 60 percent of demand. Direct Potable Reclamation yielded most of the remaining demand, reducing surface water supply to only 3 percent of the total.
An innovative approach to water supply has enabled Windhoek to grow and survive in an inherent water scarce environment. Rare water augmentation schemes such as DPR and the WMARS yet again saved the day during the recent 2014 to 2016 drought, but the future of water supply to the city remains uncertain and the security thereof will require more of the same in the coming decade.