15.5 Extrapolating to Larger Spatial and Longer Temporal
Scales
Modelling is a commonly applied technique to extrapolate ET or transpiration
estimates to larger spatial and longer temporal scales. The availability of observed
data to verify the estimates and assumptions involved significantly reduces uncertainty (Allen et al. 1998). Addressing the water resource impacts of invasive alien
plants in a truly integrative manner would require researchers to deal with large
number of species, typically occurring as mixtures of varying densities, across a
range of climates and with access to both groundwater and surface water. One way of
assessing this variability at large spatial scales is to use remote sensing data to
estimate evapotranspiration from invasive alien plants and contrast them with
simultaneous evapotranspiration estimates from native vegetation. This approach
does have limitations (e.g. impact of clouds, resolution of images in time and space,
requirements for verification on the ground), but also the potential to estimate the
water resource impacts of a wide range of invasive alien plants in a consistent
manner over a large area, and compare them with other land cover classes (Gibson
et al. 2013; Meijninger and Jarmain 2014; van Niekerk et al. 2018). The only study
of this type thus far found that the annual evaporation from areas with invading
species was greater than from adjacent areas of natural vegetation across both the
Western Cape and KwaZulu-Natal (Meijninger and Jarmain 2014). This is consistent
with the generally greater water-use observed in the species and stand level studies
described above and a promising development. The spatial and temporal resolution
of the images is increasing and will make these techniques more robust and useful in
the future.
15.6 Translating Impacts on Runoff to Impacts on Yields
The implications of streamflow reductions due to alien plant invasions for the
assurance of yields from large water supply reservoirs, and bulk surface water
resource systems, have been examined in a number of studies over the past two
decades (Gillham and Haynes 2001; Larsen et al. 2001; Le Maitre and Görgens
2003; Dzvukamanja et al. 2005; Cullis et al. 2007; Le Maitre et al. 2019). They
spanned a range of bioclimatic regions and landscapes across southern Africa and
used differing algorithms, hydrological and systems models, actual or hypothetical
dams, definitions of yield at a given assurance and levels of invasion, modelling
periods and “current” and “future” levels of invasion. Despite the differences in
approaches, methodologies and assumptions among these studies, they all found that
allowing invasions to continue without any control would result in a significant
reduction in system yields (Table 15.2).
The effects of the reductions in runoff into the dams are likely to be greater during
droughts, which would magnify the impacts on yields, especially during prolonged
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
441
Scales
Modelling is a commonly applied technique to extrapolate ET or transpiration
estimates to larger spatial and longer temporal scales. The availability of observed
data to verify the estimates and assumptions involved significantly reduces uncertainty (Allen et al. 1998). Addressing the water resource impacts of invasive alien
plants in a truly integrative manner would require researchers to deal with large
number of species, typically occurring as mixtures of varying densities, across a
range of climates and with access to both groundwater and surface water. One way of
assessing this variability at large spatial scales is to use remote sensing data to
estimate evapotranspiration from invasive alien plants and contrast them with
simultaneous evapotranspiration estimates from native vegetation. This approach
does have limitations (e.g. impact of clouds, resolution of images in time and space,
requirements for verification on the ground), but also the potential to estimate the
water resource impacts of a wide range of invasive alien plants in a consistent
manner over a large area, and compare them with other land cover classes (Gibson
et al. 2013; Meijninger and Jarmain 2014; van Niekerk et al. 2018). The only study
of this type thus far found that the annual evaporation from areas with invading
species was greater than from adjacent areas of natural vegetation across both the
Western Cape and KwaZulu-Natal (Meijninger and Jarmain 2014). This is consistent
with the generally greater water-use observed in the species and stand level studies
described above and a promising development. The spatial and temporal resolution
of the images is increasing and will make these techniques more robust and useful in
the future.
15.6 Translating Impacts on Runoff to Impacts on Yields
The implications of streamflow reductions due to alien plant invasions for the
assurance of yields from large water supply reservoirs, and bulk surface water
resource systems, have been examined in a number of studies over the past two
decades (Gillham and Haynes 2001; Larsen et al. 2001; Le Maitre and Görgens
2003; Dzvukamanja et al. 2005; Cullis et al. 2007; Le Maitre et al. 2019). They
spanned a range of bioclimatic regions and landscapes across southern Africa and
used differing algorithms, hydrological and systems models, actual or hypothetical
dams, definitions of yield at a given assurance and levels of invasion, modelling
periods and “current” and “future” levels of invasion. Despite the differences in
approaches, methodologies and assumptions among these studies, they all found that
allowing invasions to continue without any control would result in a significant
reduction in system yields (Table 15.2).
The effects of the reductions in runoff into the dams are likely to be greater during
droughts, which would magnify the impacts on yields, especially during prolonged
15 Impacts of Plant Invasions on Terrestrial Water Flows in South Africa
441