AFRICA
5. DETECTION OF TRENDS OVER TIME IN HYDROLOGICAL DRIVERS AND RESPONSES OVER
5.8 Ranges of Streamflows between High and Low Flow Years
accumulated winter months' streamflows in the later period. The Gouritz Primary Catchment has ~cluster of QCs at its outlet showing a decrease inthe highest flows in 10 years of accumulated winter streamflows in the later period.
Ratio _ <0.3 _ 0.3-0.5 . '" 0.5-0.7
~0.7-1.0
o
No change~1.0-1.3 1~if(!I'~mj1.3 - 1.5 _1.5-2.0 _ 20-3.0 _ >3.0
Model: ACRU Schulze (1995 & updates) Baseline Land Cover:
Aoocks (1988)
Figure 5.11 Ratio oflater (1980 - 1999) to earlier (1950 - 1969) highest accumulated winter months' streamflows in 10 years
1969. This analysis gives an indication of whether the flow extremes are changing. An increase in the range is of concern for storage structures in particular.
The Primary Catchments which have most of their QCs showing an increase in the range between the lowest and highest flows in 10 years in the later period are the Vaal, Mvoti, Thukela, Mfolozi, Gouritz, Gamtoos, Coastal Rivers, Great Kei and Fish Catchments. The Mvoti and Mfolozi Primary Catchments, in particular, display an increase in the later period for most of its QCs, and the increases are in the order of 1.5 times and greater.
Ratio
_ <0.3
_ 0.3-0.5
~0.5-0.7
0 0.7 - 1.0
o
No change0 1.0 - 1.3 h<'.\l1.3 - 1.5 1.5 - 2.0 _2.0.3.0
_ >3.0
Model: ACRU Schulze (1995 & updates) Baseline LandCover:
Acocks (1988)
Figure 5.12 Ratio of later (1980 - 1999) to earlier (1950 - 1969) range of the lowest and highest accumulated annual streamflowsin 10 years
The Olifants/Doorn, Buffels and Keiskamma Primary Catchments have QCs which show a decrease in the range between the lowest and highest flows in 10 years in the later period. The Orange Primary Catchment has QCs showing both an increase and decrease, and notable is the decrease in range in the area of the Caledon tributary which feeds the Orange Catchment. This area shows decreases in the lowest flows in 10 years in the later period as well as a decrease in the highest flows in 10 years in the later period; thus the 'dry' periods are increasing in severity and the 'wet' periods are decreasing in severity. However, the range between the lowest and highest flows in 10
years is decreasing. The QCs in the lower Orange, Olifants/Doom and Buffels Primary Catchments which show decreases in the range in the later period are in semi-arid areas, and thus again it is unlikely that the changes there will have significant impacts on the water resources.
Figure 5.13 depicts ratio of the range between the lowest and highest flows in 10 years of accumulated summer streamflows between the periods 1980 - 1999 and 1950 - 1969. The Mfolozi, Thukela, Mvoti, Mzimvubu, Great Kei, Sondags, Fish and Gouritz Primary Catchments have a majority of their QCs displaying increases in the range in the later period.
The Orange Primary Catchment shows a mixture of increases or decreases in the later period. Again the Caledon River catchment in the Orange Catchment shows a marked decrease in the later period. However, in the lower Orange Catchment QCs are showing an increase in the range between the lowest and highest flows in 10 years of accumulated summer streamflows in the later period. The Olifants/Doom Primary Catchment shows increases in the range between the lowest and highest flows in 10 years in the lower reaches, while in the upper reaches decreases in the range in the later period are evident. The Buffels Primary Catchment shows decreases in the range between the lowest and highest flows in 10 years of accumulated summer streamflows in the later period for the majority of its QCs.
Figure 5.14 depicts chang~s in the ratio of the range between the lowest and highest flows in 10 years of accumulated winter months' streamflows for the periods 1980 - 1999 vs 1950 - 1969. The patterns shown in Figure 5.14 are similar to those shown in Figure 5.11 in which changes in the highest flows in 10 years of accumulated winter streamflows are illustrated. The Limpopo, Buffels, Olifants/Doorn, Breede, Sondags, Boesmans, Keiskamma and Mzimvubu Primary Catchments all have QCs showing a decrease in range in the later period. The Gouritz Primary Catchment also has a cluster of QCs towards the outlet of the catchment which shows a decrease in range in the later period.
Ratio
_ <0.3
_ 0.3-0.5
~0.5-0.7
CJ0.7-1.0
D No change
1.0 - 1.3 1.3 -1.5 1.5 - 2.0 2.0 - 3.0
_ >3.0
Model: ACRU Schulze (1995& updates) Baseline LandC~er:
Aoocks (1988)
Figure 5.13 Ratio of later (1980 - 1999) to earlier (1950 - 1969) ranges between lowest and highest accumulated summer months' streamflows in 10 years
25"$
U'"E 30"£
Ratio
Model: ACRU Schulze (1995 & updates) Baseline Land Cover:
Aoocks (1988)
Figure 5.14 Ratio of later (1980 - 1999) to earlier (1950 - 1969) ranges between lowest and highest accumulated winter months' streamflows in 10 years
In the upper reaches of the Orange Primary Catchment, the QCs show an increase in range in the later period, while in the lower reaches the QCs tend to show a decrease in the later period in the range. The Vaal Primary Catchment has QCs in its upper reaches which show a decrease in the range between the lowest and highest flows in 10 years of accumulated streamflows in the later period, while in the lower reaches the QCs show an increase in the later period. The Thukela and Mvoti Primary Catchment show increases in the range between the lowest and highest flows in 10 years of accumulated winter streamflow in the later period.