AGRICULTURE NEWS - “Due to very strong and consistent winds, long daylight hours and high daytime temperatures, evaporation rates in Western Cape dams are extremely high, and account for millions of cubic litres of water lost to evaporation annually,” says Cobus Meiring, spokesperson for the Southern Cape Landowners Initiative (SCLI).

According to Meiring, the loss of available fresh water is exacerbated by high densities of invasive alien plants in primary catchments, and given the daunting short, medium and long-term projections of the growing demand for water from the City of Cape Town and surrounds, alternatives must be explored to either increase availability of run-off water or better protect what we have in storage dams.

Continues Meiring, “As far as water evaporation rates are concerned, and as a mere example (and conditions vary depending on the location and local conditions such as wind patterns affecting any particular dam), a fairly shallow dam such as the Voëlvlei Dam near Gouda has a storing capacity of 168 000 cubic meters of water, but is likely to annually lose the equivalent of all that water and more to evaporation, and explains to some degree why the City of Cape Town is vulnerable during periods of prolonged drought as the 2017 Day Zero scenario proved.

“As an even more relevant example, the capacity of the Steenbras Dam near Cape Town is approximately 123 million cubic meters, which is not only a vital source of fresh water to the City of Cape Town, but just as importantly, it provides vital hydro-electric generated power to the City of Cape Town, and so protecting the water from evaporating fulfils a double function.”

Although it is important to note that each dam is unique and the most effective methods for reducing evaporation will depend on specific site conditions, there are several methods that can be used to reduce evaporation from dams including:

• Limiting surface area: Minimising the surface area of the dam's water body by reducing the height of the dam or constructing smaller reservoirs can reduce evaporation.
• Covering the water surface: Covering the surface of the dam with floating covers can significantly reduce evaporation.
• Planting vegetation: Planting vegetation around the dam can provide shade and reduce wind speed, thereby reducing evaporation.
• Limiting water temperature: Maintaining water temperature below the atmospheric temperature can reduce evaporation. This can be achieved through methods such as mixing deep and shallow water layers.
• Limiting wind speed: Reducing wind speed by constructing windbreaks around the dam can reduce evaporation.

According to Meiring a South African product to prevent large-scale evaporation is a first of its kind in the world.

“Following years of development, floating rubber covers designed and manufactured by a Pretoria-based company and conceptualised by SCLI, now provides an effective, lasting and durable solution to protect critical water resources from evaporation,” says Meiring.

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