From "The Weather and Climate of Southern Africa", by P D Tyson and R A Preston-White (2000), Oxford University Press Cape Town.
The cycles of 18 years, 9 dry and 9 wet, as measured during the historical recorded 20th century are:
dry - 1905/6 - 1915/6
wet - 1916/7 - 1924/5
dry - 1925/6 - 1932/3
wet - 1933/4 - 1943/4
dry - 1934/5 - 1952/3
wet - 1953/4 - 1961/2
dry - 1962/3 - 1970/1
wet - 1971/2 - 1980/1
dry - 1981/2 -