droughts. The current reduction in the yield of the Western Cape Water Supply
Scheme due to invading alien plants is estimated to be 38 million m
3 per year
(Le Maitre et al. 2019). If Cape Town’s daily water consumption was 550 Ml per
day, then the yield reduction is equivalent to nearly 60 days of supply. In other
words, the infamous “Day Zero”, the day that dams would essentially run dry, and
domestic users would have to collect their own water from supply points, could have
been delayed by 60 days. This estimate ignores the cumulative impact of the water
used by invasions from the beginning of the drought in 2015 until a date for “Day
Zero” was set in 2017.
The one aspect that has been neglected in most of these studies (but see Cullis
et al. 2007, Table 15.2), is the potential impact on users who are not supplied by
large water supply schemes and dams, but who depend on small water supply
systems, abstraction via weirs or diversions, or pumping directly from a river. A
similar impact would hold for groundwater users who depend on water pumped from
aquifers where the recharge areas have been invaded or where the root systems of
invaders can reach the water table. These users have little or no ability to buffer the
impacts of invasions on water availability, reducing their water security.
15.7 Impacts of Potential Invasion Scenarios and Climate
Change
15.7.1 Invasion Scenarios
The most recent estimate of the national impacts of invasive alien plants on river
flows is that they reduce the MAR by about 1.44 billion m
3 /year, or 2.9% of the
naturalised mean annual runoff (less than 50% of the 3.30 billion m
3 /year estimated
in 2000) (Le Maitre et al. 2016) (Fig. 15.2). The difference is mainly due to two
things: (a) the estimated unit area reduction was 970 m
3 /ha/year versus the 1998
estimate of 1900 m
3 /ha/year, mainly because of updates in the models; and (b) the
2007 mapping resulted in a total condensed invaded area of 1 million ha (Kotzé et al.
2010) compared with 1.76 million ha mapped in 1997. However, the 2007 survey
excluded South Africa’s arid interior, so the latest results under-estimate the full
impacts. In addition, the extent of the high-water-use riparian invasions was underestimated by the 2007 mapping. If the proportion riparian invasions was increased to
match those from 1998, this would increase the total reduction by about 1 billion m
3
per year (Le Maitre et al. 2016).
The greatest estimated impact is due to wattles (Acacia mearnsii, A. dealbata and
A. decurrens) which account for 34% of the reductions, followed by Pinus species
(19.3%) and Eucalyptus species (15.8%) (Le Maitre et al. 2016). Most of the wattle
(70%), eucalypt (60%), and pine (40%) invasions, and the majority of poplar
(Populus) and willow (Salix) invasions occur in the Grassland Biome, which
explains why the impacts in this biome are relatively high. Data from 2007 for
444
D. C. Le Maitre et al.
Scheme due to invading alien plants is estimated to be 38 million m
3 per year
(Le Maitre et al. 2019). If Cape Town’s daily water consumption was 550 Ml per
day, then the yield reduction is equivalent to nearly 60 days of supply. In other
words, the infamous “Day Zero”, the day that dams would essentially run dry, and
domestic users would have to collect their own water from supply points, could have
been delayed by 60 days. This estimate ignores the cumulative impact of the water
used by invasions from the beginning of the drought in 2015 until a date for “Day
Zero” was set in 2017.
The one aspect that has been neglected in most of these studies (but see Cullis
et al. 2007, Table 15.2), is the potential impact on users who are not supplied by
large water supply schemes and dams, but who depend on small water supply
systems, abstraction via weirs or diversions, or pumping directly from a river. A
similar impact would hold for groundwater users who depend on water pumped from
aquifers where the recharge areas have been invaded or where the root systems of
invaders can reach the water table. These users have little or no ability to buffer the
impacts of invasions on water availability, reducing their water security.
15.7 Impacts of Potential Invasion Scenarios and Climate
Change
15.7.1 Invasion Scenarios
The most recent estimate of the national impacts of invasive alien plants on river
flows is that they reduce the MAR by about 1.44 billion m
3 /year, or 2.9% of the
naturalised mean annual runoff (less than 50% of the 3.30 billion m
3 /year estimated
in 2000) (Le Maitre et al. 2016) (Fig. 15.2). The difference is mainly due to two
things: (a) the estimated unit area reduction was 970 m
3 /ha/year versus the 1998
estimate of 1900 m
3 /ha/year, mainly because of updates in the models; and (b) the
2007 mapping resulted in a total condensed invaded area of 1 million ha (Kotzé et al.
2010) compared with 1.76 million ha mapped in 1997. However, the 2007 survey
excluded South Africa’s arid interior, so the latest results under-estimate the full
impacts. In addition, the extent of the high-water-use riparian invasions was underestimated by the 2007 mapping. If the proportion riparian invasions was increased to
match those from 1998, this would increase the total reduction by about 1 billion m
3
per year (Le Maitre et al. 2016).
The greatest estimated impact is due to wattles (Acacia mearnsii, A. dealbata and
A. decurrens) which account for 34% of the reductions, followed by Pinus species
(19.3%) and Eucalyptus species (15.8%) (Le Maitre et al. 2016). Most of the wattle
(70%), eucalypt (60%), and pine (40%) invasions, and the majority of poplar
(Populus) and willow (Salix) invasions occur in the Grassland Biome, which
explains why the impacts in this biome are relatively high. Data from 2007 for
444
D. C. Le Maitre et al.