Ahead of predicted heavy rain this weekend that as already seen a river watch alert on the Moruya River concern is often expressed that global warming might be changing the rainfall pattern experience in Moruya. The Bureau of Meteorology ( BOM) provides downloadable rainfall statistics for the Moruya Pilot Station and I decided to investigate this data to try to see what is happening.
a) Annual Rainfall. Yearly averages since 1910.
Obviously the rainfall varies considerably from year to year. The trendline (dotted line) shows a slight increase from about 900mm – 1000mm per year over the 100 year period – but this is not immediately clear from the data. Note also that most years since 2000 have been below the trendline.
b) 20 Year Average Rainfall.
From this table we can see that the rainfall increased until 1990 but has decreased over the past 20 years. c) 20 Year rolling averages.
Rolling averages result in a smoother graph. The figure for 1913 is 822mm of rain. This means that the average for the period 1894 – 1913 is 822mm. This graph indicates that the rainfall seemed to be greater for the period 1940 – 1995. Since the start of the 21st century it has dropped back to what it was at the start of the 20th century. Again the trendline shows a slight increase over the whole period. d) 50 Year Rolling Averages
This graph shows the average for the previous 50 years. Thus in 1925 we show that the average rainfall for the period 1876 – 1925 is 888mm. In 2016 the average for the previous 50 years was 987mm. Once again the rainfall average rises slowly and then drops slightly as we enter the new century. The trendline shows a steady increase in rainfall. e) Average Rainfall: Summer and Winter.
The rainfall during the warmer months (October – March) is slightly greater than the rainfall during the cooler months (April – September). Once again in this table we see that the rainfall during the middle of our period was higher than the rainfall at the beginning and the end of the period. 2. Variability in Rainfall and Extreme Rainfall Events. Some have expressed the opinion that Global Warming will lead to more extreme weather events. a) Extreme Rainfall Months and Days.
In this table we firstly look at months of high rainfall and months of low rainfall. Once again we see that the middle period (1945 – 1979) experienced more months with high rainfall.It is more difficult to discover a pattern in the dry months. The final column shows days with extremely high rainfall (floods). Again there does not seem to be a real pattern and we cannot say that the number of extreme weather events is on the increase. b) Variability in Rainfall.
How much does our total rainfall vary from year to year? From our first graph it is obvious that it varies a lot. In 1934 the total rainfall was 1822mm and in 1940 it was only 546mm. Is this variability on the increase? To try to answer this question I measured the standard deviation of the annual rainfalls in the 35 year period preceding the given year. Thus in 1910 I show a value of 274mm which means that the standard deviation for the period 1876 – 1910 is 274mm. Once again we see an increase in values up to about 1980 and then a drop back to what it was in the early 1900s. The trendline shows that there is a slight increase in variability over the whole period. 3. CONCLUSION. Based on the rainfall data downloaded from the Bureau of Meteorology there does not appear to be a significant change in rainfall patterns associated with the warming experienced in Moruya. Perhaps more work needs to be done on studying clusters of rainfall data. In the period 1945 – 1960 Moruya seemed to experience very high annual rainfalls but in the period 2000 – 2016 most annual rainfalls seem to have been below average (below the trendline). REGARDING both rainfall and temperature it will be critical to watch the next decade. Temperatures appear to have been rising since the year 2000 and rainfall appears to have been decreasing. Is this just part of the normal fluctuation and the worm will turn any year now or has there a significant change over the past 20 years?