Changes in rainfall pattern in the eastern Karoo, South Africa, over the past 123 years
Rainfall is a key driver of ecosystem processes, especially vegetation dynamics, in semi-arid regions. Rainfall amount, including droughts and extended wet periods, seasonality, and, possibly, concentration, influence vegetation composition in the eastern Karoo. A monthly rainfall record of 123 years from Grootfontein was analysed to search for evidence of cyclicity in rainfall amount, seasonality, and concentration. Rainfall was substantially higher during the late 1800s and after 1990 than it was during the rest of the 20th century. Extended sequential below-average runs of years characterised the drought periods of the early 20th century and the 1960s. There was strong evidence of an approximately 20-year rainfall cycle, except for spring rain. Additionally, annual and seasonal rainfall showed evidence of a longer cycle, between 44 and 77 years, which may be related to the southern oscillation index. The additive effects of the two cycles described annual and seasonal rainfall with R2 values typically > 0.5. Rainfall seasonality was also related to the longer-term cycle, while rainfall concentration showed some evidence of having entered a new, more concentrated state since 1988. The analysis reveals that rainfall at Grootfontein is not a random process, but rather appears driven by cyclical processes. Rainfall at the site is predicted to decline over the next approximately 20 years, and the high levels of variation and complex causal factors will make it difficult to discriminate between natural variation and possible effects of climate change on rainfall.
Keywords: semi-arid, rainfall concentration, rainfall seasonality, periodicity, cyclicity