Grahamstown may be nearing the end of a 100-year weather cycle. Roger Rowswell, who has been monitoring rainfall in this corner of the Eastern Cape for 36 years, offers a glimpse into our possible climatological future.
Grahamstown may be nearing the end of a 100-year weather cycle. Roger Rowswell, who has been monitoring rainfall in this corner of the Eastern Cape for 36 years, offers a glimpse into our possible climatological future.
Rainfall plays an important role in our everyday lives. Floods cause chaos and misery; drought causes water shortages and crop failure. And if there's one rule about the weather in Grahamstown, it's that the weather is never consistent. We have wet spells, floods, dry spells and droughts – seldom of equal duration. It is during the wet spells and floods that sufficient storage must be accommodated in dams for the dry spells ahead.
Grahamstown has had its fair share of droughts and due to the steady growth of the town and its population, many dams have been constructed over the years to keep the town supplied with domestic water.
Shortly after arriving in Grahamstown in 1985 I installed a rain gauge at home and have kept records from January 1986 onwards. I couldn’t accept that Grahamstown’s average annual rainfall was a mere 550mm – the figure being bandied around at the time.
Rainfall varies across town, with the southern areas usually receiving more than those to the north, as a result of orographic rain. However, in the event of thunderstorms, the northern areas often receive a bit more (see Graph 1 – Grahamstown rain 1986 to 2011).
For a historic look at rainfall in Grahamstown over the past 100 years or so, see Graph 2. This gives one an idea of the variations in annual rainfall and you can clearly see drought and wet periods.
Because there isn’t a record from a single station spanning this period, four stations have been used with good overlap periods to ensure similar trends. They are the Old Gaol in Somerset Street (1880–1974), Oatlands or Gowie (1891–1935), Strowan farm (1915–2011) and Grant Street (1975–2011). Although Strowan is out of town, it helps link both Old Gaol and Grant Street data, as it follows similar trends.
Previous graphs and rainfall data-sheets, in which the Old Gaol and Grahamstown TNK have been presented as a single record, are incorrect because Old Gaol was in Somerset Street until 1974 and Grahamstown TNK from 1975 onwards is up near Waainek. These two locations have very different rainfall.
From the graph it appears Grahamstown may be at the end of a roughly 100-year cycle, with a wet spell and a drier spell to follow. One would need several hundred more years of data to verify this, but rainfall is about the same as it was a hundred years ago, with a long, drier period in between, with lows in 1927 and 1949.
For interest, there are two lists – one of extreme weather events such as droughts, heavy rain and snow, for the Grahamstown area. The other is a record of when Settler’s Dam has overflowed – drawn up with the help of P Ford, of Begelly, on the Kariega River.
Comparing the years that Settlers' Dam has overflowed with the historic rainfall graph, it appears that Grahamstown needs to exceed 700mm annual rainfall for this event to happen. It is in those years when there has been a month with in excess of 200mm, or two successive months with rainfall in excess of 150mm each.
There was one exception, namely 1997, when 654mm was recorded in Grant Street (737mm was recorded in Darling Street.
There is a hypothesis that rainfall is linked to solar or sunspot activity and its cycles of about 21 years. The theory involves complicated mathematical calculations, but what I have read seems very convincing.
For more information about Grahamstown's rainfall, read Grahamstown’s Water Supply – A brief history from 1812 to 2008 by RLG Mullins.
* Roger Rowswell attributes his strong interest in geography and meteorology to his excellent geography teacher, JW (Klippies) van der Spuy in [grades 11 and 12]who wrote the matric geography book for the Cape Province in the 60s. The same teacher inspired his brother to become a meteorologist, spending year-long stints in the Antarctic, Marion Island and Gough Island.
