My brother lives on a farm in central NSW. Tuesday morning they had over 100 mm of rain in the order of 4 hours. Wednesday afternoon there was a further 63 mm in half an hour and Thursday afternoon it started raining heavily about 5. By about half an hour later there’d been just under another 50 mm.
Some of the water from the local to distant area runs past their house and into a creek.
When the last fall of rain came the water running past the house was about 20 m across and 400 mm deep and running at about 40m/min. My brother watched a moving leaf to get the rate of flow. As the runoff from the rain increased, the depth of the water increased to about 700 mm.
If calculations are made on 40 m/min X 20 m X 400 mm, you get 320 cubic metres per minute which is 19200 cubic metres per hour which is 19200000 litres per hour or 19.2 megalitres per hour.
It has been running at least this deep for the last 48 hours which calculates to be 921.6 megalitres or 921600 cubic metres of water.
At the times where the depth raised to 700 mm the rate of flow would have been faster- it was dark so we can only estimate that it was possibly at least half as fast again. This calculates to 50.4 megalitres per hour or 50400 cubic metres of water per hour.
That 3 day rainfall is half the average annual rainfall.
All in the day of being a farmer!