Fluctuations in river flows are important as they affect irrigation and therefore the livelihood of people. The Vaal river in South Africa is and important water source in the region and was studied by Alexander et al (Journal of the South African Institute of Civil Engineering Vol 49 No 2, June 2007, Pages 32–44, Paper 659) who give this record of its flow which demonstrates the huge fluctuations in water supply over time:
The spectrum of the Vaal river flow shows a number of prospective cycles, several of which are significant (in bold):
Alexander claims a 22 year cycle based on sunspot cycles with alternate cycles reversed. However the spectrum shows a cycle of 18.3 years which is different enough from 22 years to be confident that it is not the correct period. I think that Alexander gets this value because he uses only the lows of the cycles and not the highs:
It seems more likely that the detected period of 18.3 years is related to a lunar cycle than a solar one. The lunar cycle of 18.6 years strongly affects patterns in tides in the sea and there are reports of it affecting rainfall through affecting atmospheric tides. Sometimes it also appears as a 9.3 year cycle.
The 10.35 year cycle might possibly be related to solar activity, although the measured period is rather shorter than the expected 11.1 years. Along with the 18.3 and 10.35 year cycles, other cycles of 6.49, 4.46 and 2.37 years were compared to the fluctuations:
When these five cycles are combined the fluctuations can be compared to the actual variations in river flow and a projection can be made. Because the latest data in the study are some 14 years ago, the projections could be tested against real data if it is available.
Whether this projection is useful or not is not known at this time. However cycle projections might be a useful tool in planning in the many regions where water flows sometimes lead to disasters.