DRY SPELLS – Stellenboschberg

Dry spells are a creeping, slow onset natural hazard, which can manifest themselves through a lack of rainfall, a lack of available soil moisture for crops, a reduction of streamflows below a critical threshold, a reduction in the amount of water stored in reservoirs, or reduced levels of groundwater. However, unlike aridity, which is a permanent feature of the climate in low rainfall areas, dry spells are a temporary aberration that can occur in low as well as high rainfall areas.

Three durations of dry spells were considered for each Quinary catchment, viz. periods of either 2, 3 or 6 consecutive months of below normal rainfall. Normal rainfall was defined as the sum of the median monthly rainfalls from a long rainfall record for the duration under consideration and for the Quinary being assessed. Since each Quinary has a unique median rainfall for each month of the year, the criteria for identifying a dry spell is unique to that Quinary.

For a dry spell of a defined duration to be identified as “dry” when analysing a monthly sequence of rainfalls over a 30- or 50-year period, its rainfall had to be 10% or more below the median. The number of dry spells of a defined duration (e.g. 3 consecutive months) in the years being assessed (e.g. a 30-year record from GCMs) were summed and then divided by the number of years to obtain an average of dry spells per year, which was then mapped. Note that only the frequency per annum was calculated and not, for example, the seasonality.

To assess the impact of projected climate change on dry spells, dry spell frequencies were first computed for a given duration using the 50-year historical record to obtain a baseline. Thereafter dry spell frequencies were computed from five different CMIP5 GCMs for present climatic conditions (1976-2005) and then for the immediate future (2016-2045). Changes in the means of frequencies of dry spells, be they higher (implying more dry spells in future) or lower (signifying fewer dry spells in future), could then be computed on a Quinary basis, and mapped.