Answers are grounded in this paper and cite their sources.
Discussion of investigations, performed by engineers and scientists for many years; sunspot numbers as index of solar activity which cause long term variations in hydrologic data; effects of 11 yr sunspot cycle and of assumed 100 yr cycle; data are expressed as ratios to mean in order to obtain dimensionless basis for comparison; 3 yr moving averages; long term variations from mean.
No takes yet. Share an insight, caveat, or question.
Gordon R. Williams (1961) studied this question.
Synapse has enriched 10 related papers. Consider them for comparative context: