How Windhoek banks on water security underground

If you’ve been to Namibia, the country’s strained relationship with water is obvious. The parched landscape makes for an impressive sight. Dunes that undulate from glaringly white to copper red attract visitors from afar. At places, a lone aloe might break the otherwise bare horizon. The bitterly cold Atlantic that runs along its coast jealously guards moisture, rarely letting enough go to form clouds heavy enough for rain. It is the most arid country south of the Sahara Desert.

A dry capital in a dry country

The capital, Windhoek, is placed at the centre of the country, and far from water. The closest perennial river to Windhoek, the Okavango River, lies along the northern border 700 km away. Windhoek itself receives only about 360 mm rain per year, with annual evaporation rates as much as 2 170 mm.

Water to the city is supplied by NamWater, the national supplier of bulk water in the country. Windhoek’s water supply is mostly from the so-called “three dam system” through which water is transferred to the Von Bach Water Treatment Plant near the town of Okahandja. This is supplemented by the Windhoek reclamation scheme (I’ve written about how Windhoek taps water from sewage before) and groundwater in the Windhoek and northern aquifers.

Large-scale abstraction from the complex Windhoek Aquifer started in the 1950s and is a vital part of the supply to keep the largest urban and industrial development in the country going. However, the aquifer was being tapped dry to feed growing numbers of people and industry.

The Windhoek aquifer is tapped dry

In 2018, the population of the city was 326 000, projected to grow to 790 000 by 2050. It’s of little surprise that water demand is set to almost double in the same time, from 27 million m³ per year to over 50 million m³.

Already by the late 1990s water levels in the Windhoek aquifer had dropped by about 40 m in the main wellfield areas, and were steadily declining in other areas too. The resources from the aquifer were being mined beyond what it could sustain.

Experts said the aquifer would need up to a decade of rest to recover fully, and authorities started investigating different options for water supply to the city. They settled on managed aquifer recharge (MAR), among other reasons because it was the most cost-effective alternative. The idea was to use the Windhoek Aquifer as a water bank. Treated surface water would be transferred to the aquifer for storage, to be used when water was scarce.

Banking water underground

Starting in 1997, the project became the first major MAR scheme in the world to be constructed in a complex, fractured, hard-rock aquifer. The aquifer is injected with treated, drinking-quality water. The water that is put in the aquifer has to adhere to strict quality guidelines in order to prevent it from affecting the quality if the groundwater quality, and for clogging of the boreholes to take place.

The water is a blend of three-parts dam water, and one-part treated, reclaimed water. After seven years of recharge, parts of the aquifer were filled to their highest levels since the 1950s when large scale abstraction began.

The overall aim of the ongoing project is to use as much of the aquifer’s storage as possible. After the initial success, the scheme was expanded in 2011 with ten more injection and abstraction boreholes each.

A safety net during drought

When Namibia experienced extreme drought in 2016, a major crisis loomed for the bigger towns in the central area of the country, including Windhoek. Part of the emergency response was the installation of 12 more large, deep-well boreholes in the Windhoek area. There were linked to the Windhoek water supply network.

This project allowed for three times the previously available capacity from the Windhoek aquifer to be abstracted. This would cater for about two-thirds of the 2016 suppressed water demand of the city. As a result, not a single tap in Windhoek was closed during the crisis, and the benefits remain available to the city’s residents for similar emergency situations in future.

Keeping taps open into the future

Water supply in arid Namibia and Windhoek remains a major challenge. Inland water sources are already tapped to capacity, and the city has repeatedly stated that they are very mindful of the challenges of the fragile environment that they make a living in.

The Windhoek aquifer is key to this process. When fully developed, it is expected that the city can bank enough water there to keep them for three years during drought. This will allow the city the opportunity to continue, and even flourish, long beyond what other cities in a similar situation would have been able to.

Sources:

  • Transformational Strategic Pan for the City of Windhoek 2017 – 2022
  • Water Management Plan for the City of Windhoek (version 2/2017)
  • Windhoek, Namibia: From conceptualising to operating and expanding a MAR scheme in a fractured quartzite aquifer for the city’s water security by Ricky (EC) Murray, Don Louw , Ben van der Merwe and Immo Peters

Notes

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