By ROGER ROWSWELL
In 2019 Grahamstown only received 56.9% of the annual long term average (LTA), 723.2mm and 59.1% of the annual long term median (LT Med), 695.7mm rainfall.
So far rainfall has been significantly better in 2020 than in 2019. January 2020 received 129.3mm compared to 40.1mm in 2019, February 2020 172.7mm against 18.0mm in 2019. It is not often that both January and February exceed 100mm in the same year, as far as can be established it has only happened 4 times in the past; 1909, 1934, 1974 ( Old Gaol records) and 2006 (Darling St records). This does not necessarily imply that we are going to have a very wet year. My gut feeling is that 2020 rainfall will be marginally above the LTA and LT Med.
A heavy fall of rain is termed a deluge.
Rainfall in both January and February 2020 included some good deluges; 42mm on 7th January and 34mm and 32mm on 4th and 7th February respectively. This resulted in some runoff into Howison’s Poort Dam. Prior to 2020 the area has had very scattered, isolated instances of deluges, these were; March 2019 – 35mm, Sep 2018 – 33mm, Jan 2018 – 36mm, Nov 2017 – 42mm, Oct 2017 – 48mm, Jan 2016 – 41mm, Apr 2015 – 75mm, Apr 2014 – 46mm & 47mm and Oct 2013 – 50mm.
Compare the above to 2011 and 2012 when Settlers Dam last overflowed; 57mm and 38mm in May 2011, 30mm and 84mm Jun 2011, 70mm and 34mm Jul 2011, and 55mm and 57mm in Nov 2011 followed by 50mm, 90mm and 46mm in Oct 2012. The greatest deluges that have been experienced at Darling St since 1986 was in 1989 when 110mm (Oct) and 114mm (Nov) were recorded in a 24hr period.
We need something of this magnitude to fill our dams to the south. Should this happen one can expect some local flooding in low lying areas especially along water courses, drainage canals, etc.