Observations on Rainfall
by Mel Hobbins of Traveston
Rainfall is probably the most important element in growing crops here in Australia and its extremes tend to form a regular topic of conversation.
With the passing of 2016 I have accumulated 10 consecutive years of detailed rainfall.
Numbers as we know never lie but of course how we select and use them can result in misleading conclusions. With Rainfall it is the extremes that make the headlines but to growers it is more often the more mundane that are more important.
The following are just some of the facts to come out of my 10 years of records:
– Average annual rainfall 1,466.4 mls
– Highest annual rainfall 1,972 mls – 2010
– Lowest annual rainfall 1,082 mls – 2016
Month 10 Year Average No of Monthly Falls Under 25mls over 10 years No of Monthly Falls Over 100mls over 10 years
Jan 239.1 0 8
Feb 248.2 0 8
Mar 189.5 2 8
Apr 148.9 2 4
May 82.9 1 3
Jun 87.5 3 4
Jul 44.3 6 1
Aug 76.7 5 3
Sep 52.9 3 2
Oct 56.5 4 2
Nov 77.5 1 3
Dec 162.4 0 7
– Average rainy days per year 118
– Average rainy days with less than 10mls 82.5
– Average rainy days with more than 10mls 34.5
– Average rainy days with more than 25mls 16
Daily Falls greater than 200mls
– April 2009 220mls
– Jan 2013 242mls
– Feb 2015 205mls
The numbers alone only tell a part of the story, the individual attributes of a property in particular soil type, degree of slopes and type of vegetation cover are crucial to interpreting and monitoring them.
On my property I reckon that it is only falls of greater than 25mls that penetrate the soil to any degree. The exception of course are periods, like early January 2017 where it was predominately cloudy with light showers, over several days. Here even light falls can accumulate usefully, especially where the soil is already moist.
So with an average of only 16 such days, shortfalls such as 2015 with 10 such days and 2016 with 11, cause problems especially when they occur in the hotter months of the year.
At the other end of the scale we end up with run off, which is OK for filling dams and creeks but often it is of such an intense nature that unless the land is flat or swales are in place the majority of the rainfall runs away without doing any good.
On my property as a rule anything above 50mls will cause run-off and above 85mls will get some flow in the creeks. As soon as we approach 150mls then we are in a potential flooding situation. This only affects a 12acre paddock in between two creeks, part of the Dingo Creek system, which runs into the Six Mile. The whole paddock can be covered in fast flowing water up to a meter deep but it tends to disappear, from the paddocks, within half an hour of rain stopping.
I also have, entering the property, a drain way off the road, which can add significantly to the volume of water passing through the property in major events. In fact I have use a series of overflows within my system of swales to prevent “wash outs” in the cultivated areas.
For me the most difficult time of year is between October and when the regular and significant rains set in. I have learned to restrict planting too many tropical veggies until this happens. While I have developed techniques to ensure success it is a lot of hard work and takes time. Crops planted/sown when the conditions are more friendly always catch up with those that had to deal with the heat and dry.
The key is observation and preparation……..there is no doubt that working with “Mother Nature” is much easier and rewarding than attempting to fight her!
I hope you found this of some interest.