4 . 3 T r e n d s i n s e a s o n a l r a i n f a l l a n d r i v e r f l o w The Vaal, Tugela, Mgeni and Orange River catchments receive rainfall during summer while the Breede River catchment receives winter rainfall. Southern African rainfall exhibits distinct seasonality within each year (Tyson, 1978), with the larger quantity of rainfall received between October and March (Mason, 1996), with the exception of the Western Cape. The December, January, February season is an important season because the atmospheric circulation over southern Africa is dominated by tropical circulation features (Landman et al. , 2008) e.g. tropical easterlies and easterly waves (Tyson, 1986). The following section investigates the seasonal rainfall trends in different regions across southern Africa and correlates these with seasonal river flow. 4 . 3 . 1 V a a l R i v e r The early (October and November), mid (December, January and February) and late (March and April) rainfall seasons at the three stations in the Vaal River catchment all indicate decreasing trends over the period 1909 to 2008 (Fig. 16 to Fig. 18), with only the early rainfall, mid rainfall and late rainfall seasons at the Villiers station displaying highly significant decreasing trends at the 95% significance level (Fig. 16). The early rainfall season displayed a 17% decline at both the Bloemhof and Klerksdorp stations and a 12% decline was observed at the Villiers station over the period 1940 to 2008. The mid season rainfall declined between 3 to 6% decline (Table 6) at all three stations over the period 1940 to 2008, but the percentage change in rainfall during the late season differed greatly between stations, indicating variability in late season rainfall. The late rainfall season at the Villiers station experienced the greatest decline in average rainfall (37%), followed by a 25% decline at the Klerksdorp station over the period 1940 to 2008. 5 0 y = - 0 . 7 x + 2 0 2 . 2 Q = - 0 . 7 p = 1 . 0 y = - 0 . 5 8 x + 3 1 5 . 4 5 Q = - 0 . 5 p = 0 . 9 5 y = - 0 . 7 x + 1 3 6 . 4 1 Q = - 0 . 7 p = 0 . 9 9 Figure 16: Early, mid and late rainfall season trends for the Villiers rainfall station in the Vaal River catchment. 5 1 y = - 0 . 1 3 x + 9 9 . 3 6 Q = - 0 . 1 p = 0 . 6 6 y = - 0 . 1 4 x + 2 3 4 . 3 4 Q = - 0 . 1 p = 0 . 6 1 y = - 0 . 0 9 x + 1 1 2 . 2 9 Q = - 0 . 0 9 p = 0 . 6 3 Figure 17: Early, mid and late rainfall season trends for the Bloemhof rainfall station in the Vaal River catchment. 5 2 y = - 0 . 0 0 5 x + 1 0 7 Q = - 0 . 0 0 5 p = 0 . 5 2 y = - 0 . 1 6 x + 2 8 9 . 4 3 Q = - 0 . 1 6 p = 0 . 6 7 y = - 0 . 4 x + 1 4 6 . 4 1 Q = - 0 . 4 p = 0 . 9 3 Figure 18: Early, mid and late rainfall season trends for the Klerksdorp rainfall station in the Vaal River catchment. 5 3 Table 7: Percentage changes of rainfall and river flow (grey shade) observed over selected time periods for the early, mid and late rainfall seasons at southern African stations. P e r c e n t a g e c h a n g e f r o m t h e f i r s t h a l f t o t h e s e c o n d h a l f S t a t i o n s P e r i o d o f t h e r e c o r d i n g p e r i o d M i d L a t e a r l y E V a a l R i v e r 1 9 4 0 - 1 9 7 4 , 1 9 7 5 - 2 0 0 8 6 9 5 1 1 B l o e m h o f 1 7 3 1 5 K l e r k s d o r p 1 7 5 2 5 V i l l i e r s 1 2 6 3 7 M g e n i R i v e r 1 9 5 1 - 1 9 8 0 , 1 9 8 1 - 2 0 0 8 4 4 3 2 5 M i s t l e y E s t a t e 0 . 3 4 7 N e w H a n o v e r 6 5 2 T u g e l a R i v e r 1 9 3 2 - 1 9 7 0 , 1 9 7 1 - 2 0 0 8 7 3 2 0 1 5 S 8 2 8 w a r t w a t e r M o o r s i d e 7 6 1 4 T u g e l a F e r r y 1 4 8 6 B r e e d e R i v e r 1 9 4 4 - 1 9 7 6 , 1 9 7 7 - 2 0 0 8 3 5 2 2 2 7 M a l a b a r 3 8 2 4 8 T o u w s r i v i e r 7 2 1 6 O r a n g e R i v e r 1 9 3 2 - 1 9 7 0 , 1 9 7 1 - 2 0 0 8 1 3 7 1 5 L i l l e 3 2 0 2 M i d d e l p l a a t s 0 . 6 8 9 Z a s t r o n 0 . 8 8 1 3 O r a n g e R i v e r 1 9 6 7 - 1 9 8 7 , 1 9 8 8 - 2 0 0 6 1 0 2 1 2 3 L i l l e 5 1 7 1 0 M i d d e l p l a a t s 2 0 3 1 8 Z a s t r o n 6 1 2 3 T h a b a T s e k a 2 0 7 1 6 S 1 1 . 8 1 7 e m o n k o n g The Vaal River flow experienced the greatest decline in flow (69%) during the early rainfall season over the period 1940 to 2008, which is possibly due to the large rainfall changes observed for the late rainfall season. The large rainfall changes during the late rainfall season will impact flow during the following early rainfall season. As previously discussed, the abstraction of water toward the latter period of record may be higher than the earlier period of record. The average early season rainfall over the Vaal River catchment for the period 1905 to 1960 was 198.93mm, which decreased to 157.02mm for the period 1961 to 2006 at Villiers, indicating a 5 4 21% decline in rainfall. However, only a 9.5% decrease in average rainfall was observed during the mid rainfall season and a 33% decline in the late rainfall season. The Vaal River flow exhibits a highly significant decreasing trend for all three rainfall seasons (Fig. 19), which is consistent with the trend observed for the annual flow (section 4.2). The flow during the early rainfall season declined by 69% over the period 1940 to 2008, while the flow only declined by 5% and 11% during the mid and late rainfall seasons respectively. The 9.5% decrease in the average rainfall experienced during the mid rainfall season compares well with the 5% decrease in flow, since the smallest changes in flow and rainfall were experienced during the mid rainfall season. An examination of the Vaal River flow data for the months of the year (Appendix, Table 4) indicate a low flow period from May to October. The Vaal River exhibited particularly high flow during 1943, where even the months of traditional low flow, (May and July) displayed flow in the order of 300 to 400m 3/s, whilst the highest flow of the year recorded 796 m 3/s in November 1943 (Fig. 16). The correlation coefficients for the Vaal River flow with the rainfall at the three stations were weak for all seasons (Table 9). The correlation coefficient for the Vaal River flow and the Bloemhof station during the mid rainfall season was 0.3, which was the same correlation coefficient obtained for the annual relationship (section 4.1), indicating that the mid rainfall season is the main contributor to the annual correlation between rainfall at the Bloemhof station and the Vaal River flow. 4 . 3 . 2 O r a n g e R i v e r The early rainfall season at the Lille station shows a 3% decreasing rainfall trend whilst the mid and late seasons indicate 17% and 10% increasing rainfall trends respectively over the period 1932 to 2008 (Fig. 20), suggesting that the mid and late rainfall seasons are the major contributors to the overall annual increasing trend observed for the annual rainfall at the Lille station (section 4.2). In contrast, the early and mid rainfall season at Middelplaats display a 0.6% and 8% respective increase in rainfall, whilst the late rainfall season experienced a 9% decreasing trend over the period 1911 to 2008 (Fig.18).