3 April, 2019
Alien plants threaten to use up half of the inflows into two important dams in SA
|Berg River Dam - Aerial View - 31 October 2007
to 50% of the annual inflows into the Western Cape's Berg River Dam catchment
and Limpopo's De Hoop Dam catchment could be used up by alien plants over a
period of 45 years, if they are left uncleared.
is the warning issued by water experts Dr David Le Maitre of the CSIR; Dr James
Blignaut of Stellenbosch University; Prof. Lynette Louw, Prof. Tally Palmer and
Mr Ian Preston of Rhodes University in a recent paper published by the
Water Research Commission in the Water SA
journal, titled Impact of invasive
alien plants on water provision in selected catchments. the authors analyse
the impacts of failing to control invasive alien plants on the future water
supply in the catchments of two important dams. Alien plants are not indigenous
to South Africa and have been brought into the country, either intentionally or
unintentionally, and have invaded natural areas by themselves. The alien trees
typically use more water than our indigenous trees and plants.
their greater use of water and South Africa's limited water resources, it is critical
to understand the impacts that not controlling these plants will have on our
water supplies. This study assumed that no alien plant management would occur in
the two dam catchments over a 45-year period - the average lifespan of dams
built in South Africa. In each catchment, the authors base current invasion
levels on data from the National Invasive Alien Plant Survey, which was conducted
in 2007 and 2008. Using 2008 as the base year, the estimated costs of clearing are
based on the cost of clearing, including the costs of follow-up clearing.
dynamics of alien invasive plants are such that repeated follow-up clearing is
required to counter the regeneration of the plants," explains Le Maitre, adding
that a follow-up fire within two years of the initial clearing can kill all the
seedlings and minimise the need for further treatments. It is important to note
that the results show the estimated effects of the uncontrolled growth of
invasive alien plants on river flows into the dam, rather than on yield.
Berg River Catchment
in Franschhoek, the dam is part of a scheme that provides water to Cape Town.
The government's Working for Water programme has been tasked with clearing
invasive alien plants and, at the end of 2013, spent more than R90 million
clearing the equivalent of 3 600 hectares of dense stands of pines and
acacias in the Berg River Dam catchment.
the worst case scenario, where the average tree age is 20 years, the invasion
would increase from 3% to between 49 and 99% of the invadable area in the
catchment. The result would be that river flows into the dam would be reduced
by up to 50%, with significant implications for Cape Town and the irrigation
schemes that depend on water from this dam. To clear the invasion at this
point, would cost over 3000% more.
De Hoop Catchment
De Hoop Dam is located on the Steelpoort River, and is predominantly invaded by
Eucalyptus and wattle species. At the end of 2013, the Working for Water
programme had spent close to R3 million clearing the equivalent of 180 hectares
of dense alien invasive plants in the catchment.
the worst case scenario, as per the Berg River Catchment, invasion in the De
Hoop catchment would increase from 7% to between 53 and 55% of the invadable
area of the catchment. River flows into the dam would be reduced by between 42 and
44% and the cost to clear the plants at this point would be over 700% more.
results, intended to be conservative and illustrative rather than precise, demonstrate
the inevitable outcome of the rapid spread and densification of invasions in
areas of natural and semi-natural land cover when control is not effective or
not done at all.
are built to supply a certain volume of water at a 98% level of assurance, i.e.
they are designed in such a way that water restrictions will be required in two
out of every 100 years. Allowing invasions to proceed unchecked would mean far
more frequent water restrictions. However, these are only two of the dams whose
inflows are at risk from unmanaged alien plants invasions. "If these problems
are not tackled, we will see a major water security problem develop over time across
the country," says Le Maitre. Given these findings, the authors state that
building a dam without a plan to clear its catchments of invasive alien plants,
and to maintain them in that state, is tantamount to fruitless expenditure.
Dr David Le Maitre
Dr James Blignaut
Prof Lynette Louw
Prof Telly Palmer
Mr Ian Preston profile:
David Mandaha, CSIR Media Manager
012 841 3654
072 126 8910
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