WET SPELLS – Langkloof

Wet spells are the inverse of dry spells and are a temporary aberration that can occur in low as well as high rainfall areas.

Three durations of wet spells were considered for each Quinary catchment, viz. periods of either 2, 3 or 6 consecutive months of above 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 wet spell is unique to that Quinary.

For a wet spell of a defined duration to be identified as “wet” when analysing a monthly sequence of rainfalls over a 30- or 50-year period, its rainfall had to be 10% or more above the median. The number of wet 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 wet 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 wet spells, wet spell frequencies were first computed for a given duration using the 50-year historical record to obtain a baseline. Thereafter wet 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 wet spells, be they higher (implying more wet spells in future) or lower (signifying fewer wet spells in future), could then be computed on a Quinary basis, and mapped.