Assessing the potential impact of rainfall change on southern African river flows
Date
2010-08-03
Authors
Lakhraj-Govender, Rakhee
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Abstract
The primary aims of this study are firstly to determine long-term rainfall and river flow trends
for a variety of South African catchments over the period 1905 to 2008 and secondly to compare
and correlate the rainfall and river flow data for these catchments. Historical rainfall trends were
analysed using Mann Kendall statistic and Sen’s method was used for the determination of the
magnitude of the trend. The Kendall tau method was used to determine the correlation between
rainfall and river flow. To this end, long-term annual, seasonal and 5-year moving average
annual rainfall and river flow data for the Tugela, Mgeni, Orange, Vaal and Breede River
catchments in southern Africa were analysed. The Western Cape region displayed an increase in
annual rainfall over the period 1918 to 2008, whilst the eastern-central, western-central and
eastern regions show decreasing annual rainfall trends over the period 1905 to 2008. Eighty
percent of the rivers significantly declined in annual river flow over the period of record.
The majority of the rainfall stations experienced insignificant changes for the seasonal analysis
during early, mid and late rainfall seasons, with the early season defined as October and
November, the mid season defined as December to February, and the late season defined as
March and April for the summer rainfall regions. For the winter rainfall region the early rainfall
season was defined as April and May, the mid rainfall season as June and July and the late
rainfall season as August and September. Only the Villiers rainfall station exhibited significant
decreasing trends and the rainfall at the Mistley station in the Mgeni River catchment displayed
a significant decreasing trend during the late rainfall season. Of the catchments studied, the
Tugela River indicated a 73% decline in flow during the early rainfall season, followed by the
Vaal River (69 % decline during the early rainfall season), both of which were significant for all
seasons. The Mgeni and Tugela Rivers displayed significant decreasing trends in seasonal flow
at the 95% significance level, with the exception of the early rainfall season for the Mgeni at
Table mountain gauge. The Orange River flow displayed an insignificant increasing trend
during all 3 seasons. A study of the Orange River catchment rainfall data indicated that the
record length has an important role in the outcome of the percentage change over time.
The 5-year moving average examined the frequency, duration and amplitudes of each rainfall
and river flow station in order to determine long-term trends. The frequency of wet and dry periods within the data sets was similar across the majority of stations. The Breede, Mgeni,
Tugela and Vaal Rivers displayed significant decreasing trends for the 5-year moving average
annual flow, whilst the Orange River 5-year moving average flow exhibited an increasing trend
due to the duration of flow. The average rainfall over the Breede River catchment displayed a
significant increase in rainfall with time, indicating wetter conditions in the western region of
southern Africa. However, the Breede River flow significantly decreased over time. The
average annual rainfall over the Vaal River catchment characterized by a long wet period
followed by an extended dry period, as well as the insignificant increase in the amplitudes over
time and the decrease in amplitudes of the wet periods, indicate drier conditions over the Vaal
River catchment. The 5-year moving average rainfall over the Orange River catchment indicates
an insignificant increasing trend, whilst the Orange River flow significantly increased over the
period 1910 to 2008. The significant decline in rainfall over both the Mgeni and Tugela River
catchments, together with significant declines in the Mgeni and Tugela River flows, indicate
drier conditions with time over the eastern region of southern Africa.