442
M. C. Moseki
In contrast, Fig. 8 shows that overall across South Africa an increase of up to
10% in short duration rainfalls may be expected in the intermediate future, with
the exception of patches south of 32 °S and north of 27 °S where models show no
discernible change from the present.
Spatially within the basin, relative increases in mean annual rainfall are greatest
in the eastern runoff-producing regions of the basin and tend to decrease westwards.
Studies also show (Knoesen et al. 2009) that rainfall across the basin is likely to
become more variable in both the intermediate and more distant futures, which is of
particular concern to water managers because such variability may result in drought
and flood events. The number of days of rainfall above a certain amount is expressed
as the ratio of the number of rain days for both intermediate and more distant futures
to those experienced presently; the higher this ratio, the more rain days are likely to
occur. In the Orange-Senqu River Basin, projections show that the increase in the
number of rain days will be greatest in the eastern regions.
Koetze et al. (2009) also assert that in the intermediate future about 40% of the
basin is likely to experience an increase in the amount of rainfall per short duration
event, while 38% of the basin is projected to experience a decrease. In the more distant
future, however, about 72% of the basin is projected to show an increase in the amount
of rainfall per short duration event. Regarding long duration rainfall, extreme events
are likely to increase, with about 83% of the Orange-Senqu River Basin projected to
experience larger extreme rainfall events with a recurrence interval of 2 years than
at present. These events have implications for the safety of water infrastructures.
3.4.2 Temperature
Indications are that mean annual temperatures will increase over the entire OrangeSenqu River Basin. The projected increase in mean annual temperatures in the Orange
River Catchment is between 3 and 3.5 °C in the intermediate term; this is expected to
grow to more than 7 °C in the more distant future (Knoesen 2012). It is worth noting
that for much of southern Africa, high increase in temperature is projected for the
future compared to the present (Fig. 9).
The highest ratios indicating an increase from the present mean annual temperatures on the order of 20–40% occur in the medium term over Lesotho (Knoesen
2012), the source of the Orange River and the Caledon, and by 40–60% in the distant
future. Increased temperature impacts a number of water-related processes, including
soil moisture, irrigation water demands, heat wave episodes, and meteorological as
well as hydrological droughts (Knoesen et al. 2009).
3.4.3 Evaporation
The projected increases in temperature will have a profound impact on evaporation, affecting runoff, recharge, and water storage. Figure 10 shows that potential
evaporation in future (2016–2045) will increase compared to present (1975–2006).
M. C. Moseki
In contrast, Fig. 8 shows that overall across South Africa an increase of up to
10% in short duration rainfalls may be expected in the intermediate future, with
the exception of patches south of 32 °S and north of 27 °S where models show no
discernible change from the present.
Spatially within the basin, relative increases in mean annual rainfall are greatest
in the eastern runoff-producing regions of the basin and tend to decrease westwards.
Studies also show (Knoesen et al. 2009) that rainfall across the basin is likely to
become more variable in both the intermediate and more distant futures, which is of
particular concern to water managers because such variability may result in drought
and flood events. The number of days of rainfall above a certain amount is expressed
as the ratio of the number of rain days for both intermediate and more distant futures
to those experienced presently; the higher this ratio, the more rain days are likely to
occur. In the Orange-Senqu River Basin, projections show that the increase in the
number of rain days will be greatest in the eastern regions.
Koetze et al. (2009) also assert that in the intermediate future about 40% of the
basin is likely to experience an increase in the amount of rainfall per short duration
event, while 38% of the basin is projected to experience a decrease. In the more distant
future, however, about 72% of the basin is projected to show an increase in the amount
of rainfall per short duration event. Regarding long duration rainfall, extreme events
are likely to increase, with about 83% of the Orange-Senqu River Basin projected to
experience larger extreme rainfall events with a recurrence interval of 2 years than
at present. These events have implications for the safety of water infrastructures.
3.4.2 Temperature
Indications are that mean annual temperatures will increase over the entire OrangeSenqu River Basin. The projected increase in mean annual temperatures in the Orange
River Catchment is between 3 and 3.5 °C in the intermediate term; this is expected to
grow to more than 7 °C in the more distant future (Knoesen 2012). It is worth noting
that for much of southern Africa, high increase in temperature is projected for the
future compared to the present (Fig. 9).
The highest ratios indicating an increase from the present mean annual temperatures on the order of 20–40% occur in the medium term over Lesotho (Knoesen
2012), the source of the Orange River and the Caledon, and by 40–60% in the distant
future. Increased temperature impacts a number of water-related processes, including
soil moisture, irrigation water demands, heat wave episodes, and meteorological as
well as hydrological droughts (Knoesen et al. 2009).
3.4.3 Evaporation
The projected increases in temperature will have a profound impact on evaporation, affecting runoff, recharge, and water storage. Figure 10 shows that potential
evaporation in future (2016–2045) will increase compared to present (1975–2006).