Abstract
Statistically significant 21-year periodicity is present concurrently in South African annual rainfall, river flow, flood peak maxima, groundwater levels, lake levels and the Southern Oscillation Index. This is directly related to the double sunspot cycle. The first years of the periodic sequences are characterised by sudden, regular and therefore predictable, reversals from sequences of well below average rainfall and river flow (droughts) that are suddenly broken by sequences of well above average events (floods). These reversals are directly related to corresponding six-fold increases in sunspot activity at this time. The two sunspot cycles that comprise the double sunspot cycle also have fundamentally different effects on the hydrometeorological responses. These observations are solidly based and will require a re-assessment of the nature of the solar activity that gives rise to them.