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25 August 2011

Difference in rainfall figures for Mossel Bay

The SAWDIS received the following question via email:

"Why is there such a big difference between the SAWDIS rainfall figures and those of the SAWS for Mossel Bay?  You indicated that Mossel Bay received 105mm while the SAWS indicated that 65mm was measured."

I noticed the big difference when RSG read the daily rainfall figures a day or two ago and was waiting for a question in this regard to appear.  I cannot talk on behalf of the SAWS and do not know how and where the SAWS measure rainfall in Mossel Bay.

The SAWDIS use the following methods to correlate rainfall figures before it is being released. The SAWDIS weather station is situated in Heiderand, Mossel Bay.  This is about 7 km from the Mossel Bay CBD.  The automated weather station was calibrated and installed according to the manufacturers specifications.  To ensure further accuracy a manual rain gauge was installed on the same premises.  Both measurements are then compared with each other to ensure further accuracy.  A third accuracy test is then conducted by comparing the two measurements from the Heiderand weather station with a third measurement from a manual meter installed on the premises of the SAWDIS. The distance between the two stations is aprox. 400m. This method is accurate and far better then just relying on one instrument to measure rainfall.

The SAWDIS have no doubt that the reflected rainfall of 105mm is correct as measured in Heiderand, Mossel Bay.  Please also bear in mind that rainfall differs quite considerably from one area to the other in Mossel Bay.

Finally rest assure that the figures provided by the SAWDIS is correct and reflect the actual rainfall measured in Heiderand, Mossel Bay.