- Grootfontein Rainfall graph
Below is a graph showing weighted rainfall from the past 10 years. The green line is actual weighted rainfall, the red line is 50th percentile (“average”) weighted rainfall, i.e. what we’d expect in a ‘normal’ year, and the blue line is 10th percentile (i.e. driest 10% of years).
Over the past decade, 2006/7 was dry, 2015 was ‘normal’, and all of the others were much wetter than normal.
In terms of this model (Danckwerts and Barnes), a drought starts when the green line dips below the blue line, and ends when it emerges above the red line. Thus, the last time we had a drought was in 2007. Without rain, the green line will dip below the blue line on 17 January 2016, signalling the start of 1-in-10-year drought.
In this link, similar graphs are shown for the period since 1916 (pg 8-9).