higher than current reductions. This review illustrates the importance of measuring
water-use over as wide a range of species as possible, and combining this with
information from remote sensing to extrapolate the results to landscapes and catchments. These methods will soon provide much more robust estimates of water use by
alien plants at appropriate spatial and temporal scales. The results of these studies
can be used in water supply system studies to estimate the impacts on the assured
yields. This information can also be used by catchment water resource managers to
guide decision-makers when prioritising areas for clearing and rehabilitation, and for
targeting species for control measures.
15.1 Introduction
South Africa is an arid country with a mean annual rainfall of about 490 mm, only
9% of which ends up as water in rivers or aquifers (Bailey and Pitman 2015). This
situation is exacerbated by the concentration of high rainfall areas in the south and
east and low rainfall areas in the western and northern parts (van Wilgen et al. 2020,
Chap. 1). Just 8% of South Africa, Lesotho and Swaziland generates 50% of the river
flows (Nel et al. 2017). The most recent national assessment of the water situation
reported that essentially all the reliably available water resources are already in use,
so no additional demands can be accommodated (DWAF 2013). The recent droughts
across South Africa have highlighted the deteriorating water security situation, and
the need to protect water source areas from land cover changes that would decrease
usable runoff. One of the key changes is the invasion and replacement of the natural
vegetation by invasive alien plant species which alter the vegetation structure and
water-use characteristics in ways which can reduce the runoff, or decrease the
groundwater recharge.
15.1.1 Brief History of Concern About Hydrological Impacts
Among the major invasive alien plants are a wide variety of tree species which were
introduced to South Africa over the past few centuries to address timber shortages
caused by: (a) the very limited extent of natural forests (Rutherford et al. 2006), and
(b) the lack of suitable fast-growing native tree species that are good for timber
(Richardson et al. 2003; Le Maitre et al. 2004). The first plantations were in the
Western Cape, but soon extended to areas of the Eastern Cape, KwaZulu-Natal,
Mpumalanga and Limpopo. Land owners downstream of the plantations on both
432
D. C. Le Maitre et al.
water-use over as wide a range of species as possible, and combining this with
information from remote sensing to extrapolate the results to landscapes and catchments. These methods will soon provide much more robust estimates of water use by
alien plants at appropriate spatial and temporal scales. The results of these studies
can be used in water supply system studies to estimate the impacts on the assured
yields. This information can also be used by catchment water resource managers to
guide decision-makers when prioritising areas for clearing and rehabilitation, and for
targeting species for control measures.
15.1 Introduction
South Africa is an arid country with a mean annual rainfall of about 490 mm, only
9% of which ends up as water in rivers or aquifers (Bailey and Pitman 2015). This
situation is exacerbated by the concentration of high rainfall areas in the south and
east and low rainfall areas in the western and northern parts (van Wilgen et al. 2020,
Chap. 1). Just 8% of South Africa, Lesotho and Swaziland generates 50% of the river
flows (Nel et al. 2017). The most recent national assessment of the water situation
reported that essentially all the reliably available water resources are already in use,
so no additional demands can be accommodated (DWAF 2013). The recent droughts
across South Africa have highlighted the deteriorating water security situation, and
the need to protect water source areas from land cover changes that would decrease
usable runoff. One of the key changes is the invasion and replacement of the natural
vegetation by invasive alien plant species which alter the vegetation structure and
water-use characteristics in ways which can reduce the runoff, or decrease the
groundwater recharge.
15.1.1 Brief History of Concern About Hydrological Impacts
Among the major invasive alien plants are a wide variety of tree species which were
introduced to South Africa over the past few centuries to address timber shortages
caused by: (a) the very limited extent of natural forests (Rutherford et al. 2006), and
(b) the lack of suitable fast-growing native tree species that are good for timber
(Richardson et al. 2003; Le Maitre et al. 2004). The first plantations were in the
Western Cape, but soon extended to areas of the Eastern Cape, KwaZulu-Natal,
Mpumalanga and Limpopo. Land owners downstream of the plantations on both
432
D. C. Le Maitre et al.