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WEATHER UNIVERSITY: Runoff versus absorption - how much rain is enough?

You can almost hear them!  A recent rainfall had topped up the levels at the Webber Lake Falls near Middle Sackville, N.S., when Warren Hoeg captured the beauty of the falls in this photo.
You can almost hear them! A recent rainfall had topped up the levels at the Webber Lake Falls near Middle Sackville, N.S., when Warren Hoeg captured the beauty of the falls in this photo. - Contributed

One of the driest summers on record is winding down. We’ve had showers here and there, but nothing widespread or significant. Lawns are crispy, lake levels are low and wells are going dry.

While the area hardest hit with drought-like conditions is southwest Nova Scotia, rainfall totals for June, July and August were – without exception – well below average across Atlantic Canada this summer.

Last Tuesday, Sept. 18, during the evening, moisture from the remnants of tropical storm Florence brushed the south-western tip of Nova Scotia. Tusket received almost 100 mm of rain, Yarmouth over 80 and Shelburne got 50mm. The numbers seem impressive, but did it help?

The other day I got an email from Bill. He was wondering what percentage of the rain that falls actually seeps into the ground? Great question!

Bill, as is so often the case in real estate, it comes down to location. What happens to the water after it reaches the ground depends on topography, soil condition, density of vegetation, temperature, and the extent of urbanization; the rate of rainfall is also an important factor.

In Canada, about 70 per cent of the annual precipitation returns to the atmosphere by evaporation from land and water surfaces and by transpiration from vegetation. It’s estimated that 20 to 27 per cent of the precipitation eventually reaches a stream, lake, or ocean, by overland runoff during and immediately after a rain. So that means only three to 10 per cent of our rainfall goes back into the system by taking the slower route – moving though the ground.

Cindy Day is the chief meteorologist for SaltWire Network.

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