Droughts and the quasi-20-year rainfall cycle at Grootfontein, South Africa, 1889-2016

J.C.O. du Toit

Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900



Rain is a key driver of rangeland functioning and productivity.  Droughts (periods when rainfall is well below average) are a feature of semi-arid rangelands owing to the relatively high variability of rainfall in these areas.  The effects of drought on biotic and abiotic components of a landscape are numerous, and influences can extend across wide temporal and spatial gradients.  Droughts reduce forage and water availability to animals, reduce crop yields, kill plants, and have numerous other socio-political, agricultural, and ecological impacts (Blench & Marriage, 1999; Schwabe et al., 2015).

Patterns of rainfall (and, by proxy, drought) have been described at many spatial and temporal scales in Africa (Vines, 1980; Nicholson, 2001; Res & Nicholson, 2001).  In South Africa, numerous studies have identified seasonal cyclicity in rainfall with a period (i.e. wet-period to wet-period, or dry-period to dry-period) of approximately 20 years (Tyson & Dyer, 1978; Mason & Jury, 1997; Tyson et al., 2002; Kane, 2009; Jury, 2013), including at Grootfontein in the eastern Karoo (du Toit & O’Connor, 2014). 

Droughts may be defined in various ways.  Arguably, the best way is to classify an area as experiencing a drought if the effects of drought (crop failure, no drinking water, fodder shortage, mortality, etc) are manifest in the area any particular time.  In real-time, this parameter is seldom formally recorded, and is difficult or impossible to extract from a rainfall record, which serves as the only indicator of whether a drought may have occurred. 

Identifying a drought from a rainfall record is not as simple as may be intuitively expected, because drought is dependent on my variables including rainfall amount, timing of rainfall, temperature and humidity, season, and vegetation legacy effects.  Accordingly, a wide range of statistical models have been developed to identify and predict droughts (see Mishra & Singh (2010, 2011) for reviews) which range significantly in approach and complexity. 

In this paper, the well-known quasi-20-year cycle is fitted to long-term seasonal (July-June) data, and the occurrence of notably-dry periods are superimposed in order to describe patterns of droughts, and also to attempt to provide future predictions on the likelihood of dry periods in the future.   



A data set of monthly rainfall records from Grootfontein from July 1889 to December 2016 was used (see du Toit & O’Connor (2014) for details).  A 4-parameter sinewave regression was fitted to the seasonal data (Formula 1); all parameters were significant, and the period was 21.3 years.  Monthly data were then aggregated into a range of temporal divisions based 1) on the seasons described for Grootfontein by du Toit & O’Connor (2014), 2) on portions of the calendar year, which are commonly used in long-term weather forecasting, and 3) on multi-year aggregations.

Cumulative rainfall was calculated for each temporal division, and the lowest 13 values (corresponding to approximately the lowest 10% of values) were plotted against the cyclic regression that had been calculated earlier.    


 ………………………………………………… Equation 1


Table 1.  Temporal divisions described for Grootfontein rainfall

 Seasonal divisions

 Calendar year divisions

 Multi-year divisions

 Season (July to June)


 2-year average

 Warm season               

 1st half (January – June)

 3-year average

 Cool season

 2nd half (July – December)


 Spring (September – November)

 1st quarter (January – March)



 2nd quarter (April – June)



 3rd quarter (July – September)



 4th quarter (October – December)




(See Appendix 1).  Seasonal rain was strongly associated with the 20-year cycle, with all but 1 drought season occurring outside a trough, a finding consistent with du Toit (2014).  Interestingly, the current trough (around 2016) is the first in which there has not been a 1 in 10 year drought.  The early 1900s had the densest grouping of very dry seasons.  Warm season, Spring and Winter droughts showed some clumping early in the century and some association with troughs, while Cool season droughts were dispersed approximately evenly.  Summer droughts were strongly clumped in the 1910s and 1960s, while Autumn droughts appear to have been a feature of the mid-1970s to the mid-1990s. All dry Years occurred before 1980, and most before 1930.  Dry 1st-half seasons approximately mimicked patterns for summer rainfall, while dry 2nd-half seasons were well distributed over time, with possible clumping in troughs.  1st, 2nd, 3rd, and 4th-quarter seasons all indicated some degree of clumping in troughs.  All 2-season and 3-season droughts, which would be associated with very dry conditions over significant periods of time, occurred before the mid-1970s. 



Dry seasons appear to follow the quasi 20-year drought cycle, although this has not been the case for the current predicted dry spell.  Other seasons show various types of clumping, but a general feature of most droughts is that they occurred more often before about 1980, and long-term droughts were a feature before the mid-1970s.



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Appendix 1.  Rainfall cyclicity at Grootfontein superimposed with the driest 10% of all occurrences of various temporal divisions.  See Table 1 for descriptions.







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