The three most affordable ways for Cape Town to boost its future water supply is to build new dams, to decommission commercial forestry plantations within the main rainfall catchment areas that feed the city’s dams, and boost current efforts to clear invasive alien plants from within these key water catchments.

This is according to a recent modelling process which the Environmental Policy Research Unit (EPRU) at the University of Cape Town’s School of Economics did, to test what the best return on investment would be of the various solutions which the City of Cape Town is considering, in order to respond to water shortages experienced by the municipality following the unexpectedly severe drought.

But the Western Cape province has reached its capacity in terms of what dam infrastructure it can install, leaving little option here to pursue this as an avenue. The next technological solutions to the problem are to look at developing groundwater harvesting, recycling used water, and desalination, as mooted by the City of Cape Town’s 2017 Water Resilience Plan.

The plan floats a three-phase desalination infrastructure installation programme that, if completed, will be able to deliver 350-million litres of water per day into the municipal water system.

The resilience plan alludes to the need to restore the water catchments that feed into the city’s dams. However, it doesn’t compare the cost of clearing invasive alien plants or commercial forestry from within its catchments with the cost of hard-engineered solutions like dam building or installing desalination plants.

Our modelling exercise shows that an investment of almost R650-million to clear and restore the ecological infrastructure of the water catchments that feed into the city’s four main dams over the next 30 years will yield a better return on investment than if we spent the same on installing desalination technology.

The catchments which feed our city’s four main dams – the Theewaterskloof, Berg River, Steenbras, and Wemmershoek Dams – are heavily overgrown in places with tree species that don’t occur naturally in the fynbos and renosterveld-type vegetation. The biggest culprits are the pine and eucalyptus species which have escaped commercial agro-forestry practices, and now grow “wild” in the mountains.

These trees are much thirstier than the locally-occurring vegetation, and disrupt the natural water flow in three ways: when rain hits the ground, they intercept the water runoff as it flows along the surface and trickles down into the soil, preventing it from reaching nearby rivers, and later our dams; they draw water directly from river areas which reduces stream flow and dries out the surrounding ground; and if the water table is close enough to their roots, they draw this from the ground, too, lowering the water table.

South Africa loses nearly 3% of its mean annual rainwater runoff to these non-indigenous plants, according to the latest estimates by the Council for Scientific and Industrial Research (CSIR). In the Western Cape, one estimate suggests that we lose the equivalent of the capacity of the Wemmershoek Dam every year to these invasive plants (with the Berg and Breede catchments losing about 6% of their runoff to invasives annually).

Commercial forestry, on the other hand, is contentious in this context: it employs about 158,000 people nationwide, and brings about R34-million into the economy each year. But these farmed non-indigenous trees have a significant impact on our country’s water, and scientists have been worrying about this since the 1960s: in high-flow areas, we now estimate that farmed timber plantations reduce stream flow by 3.2%, and in low-flow areas by 7.8%.

Here in the province, we estimate that the commercial forests in the Steenbras Dam catchment are using twice as much water as the invading trees are in the same catchment.

There is ongoing work in some of these catchments to restore the ‘ecological infrastructure’ that supplies our city with water, through clearing invasive plants, but this needs to be scaled up dramatically, and become an ongoing effort. Clearing alien trees needs continual follow-up for years after an initial patch is clear-felled.

We recently conducted a desktop modelling process on behalf of the international conservation organisation The Nature Conservancy (TNC) to see what the return on investment would be of clearing the most important water catchments that feed into our city’s four dams. We isolated the 12 most significant catchments and, on the basis of the current cost estimates to clear and maintain them over a 30 year period, calculate that this will cost R647.2-million. This could free up 50.8 million cubic meters of water each year – significantly more than the capacity of the Wemmershoek Dam (38-million m³).

Clearing the “wild” growing invaders needs careful coordination and planning between the different government departments in whose jurisdiction the invaded areas fall, such as the City of Cape Town, CapeNature and SANParks, as well as private landowners. We recommend looking at how to integrate alien plant control with the use of fire and biological control agents, that clearing efforts should focus on lightly infested areas ahead of densely invaded areas, and that we focus on priority species first.

In terms of decommissioning the commercial forests, many of plantations in the catchments are already nearing maturity. For this modelling exercise, we looked exclusively at the cost implications of not replanting commercial forests in the catchments of the Steenbras Dam. The financial investment would go to towards restoring the catchments through keeping them clear of regrowth of alien trees after the plantations are clear-felled, and to maintain this work over the next 30 years.

There are opportunity costs involved in this. The value of the commercial industry across the province, which is mostly made up of raw softwood timber, is about R224-million annually. Once processed into planks, this timber has a further end-product value worth over R526-million per year. But once the negative impacts of these plantations are factored into our calculations, such as the implications of clearing the tree invasions that are likely to occur in future as a result of these plantations having been in these catchments, the commercial viability of these operations are less impressive.

Once we factored these “externalities” into the numbers, decommissioning commercial forests is an even more cost effective means of boosting water supply to the city than clearing alien invasives. It will yield 1.2-million m³ into Greater Cape Town’s water supply each year; although that may not look huge, compared with other initiatives, it’s a very low-cost intervention. As for the jobs lost due to cession of forestry, this can be offset by the jobs create through catchment restoration efforts, including ensuring new plant invasions don’t occur after the harvesting is done.

In August this year, The Nature Conservancy will launch a fund that is geared towards raising some of the R650-million needed to clear, restore, and maintain those 12 key water catchments. DM

Dr Jane Turpie is an environmental economist with the Environmental Policy Research Unit (EPRU) at the University of Cape Town’s School of Economics. Turpie is also the director of Anchor Environmental Consultants, and was the lead researcher on these findings which were produced for The Nature Conservancy.