Kensington North Watersheds Association

County Line Courier Articles

How Much Water is in 10 mm. of Rain?

(2013-05-08) For many, the extremely dry summer of 2012 was the watershed story of the year. How many conversations did we each have with neighbors and acquaintances about the dry weather, its impact on crops, lawns, gardens, tourism, water tables, fish habitat, forests, wildlife, and our own lives in general? The reality is in the Kensington North area it did not really rain from May until late August. There were a few localized showers, that definitely helped, but they did not soak into the soil and recharge the water table. They kept crops alive, but did not promote good growth, especially in our valuable late potato crop.

A very common unit used to measure water on land is the acre-inch. An acre-inch is the volume of water that would cover one acre to a depth of one inch, which amounts to 22610.7 Imperial gallons or 102,790 litres. It sounds like a lot. But it also sounds very vague. In metric terms, which are much easier to calculate, a similar question would be how much water is required to cover 1 hectare to a depth of 10 mm. The answer is 100,000 litres. Again, it sounds vague.

The tanker milk truck that picks up at Nobra Farms in Irishtown has a full-to-the-brim capacity of 33,000 litres. A Canadian Football Field, such as the field at Kensington Intermediate Senior High School, is 101 by 59 metres, which is 5959 square metres, about 0.6 hectares. So, it would take almost 2 very full trucks to cover the K.I.S.H. playing field to a depth of 10 mm. An acre-inch would be a little over 3 very full milk trucks to the acre. A little easier to imagine, in a sense, but what does 10 mm. of rain or irrigation mean to a crop in a field?

Only a portion of water that falls on a field is available to plants. Some of the water may soak into the ground down to the water table, some may be lost as surface runoff, and some evaporates from the surface back into the atmosphere. The remaining water remains in the root zone, and a portion of that available water will be used by plants. As well, there are many other variables. If the rainfall occurred very quickly, in a big downpour, more may be lost to surface runoff, in comparison to an equal amount of rain that falls over several hours that can gently soak into the soil. Hot weather increases the amount that is lost to evaporation. Different soil types and soil conditions have a wide range in the soil's ability to retain water, sandy soils low in organic matter being the worst, and a loam soil high in organic matter being the best. Also, the topography, the lay of the land, has great influence on water retention. A higher sloped field will tend to have greater amounts of runoff. Large late season plants can uptake and transpire more water than young, early season plants.

Lucky for us, for the most part ample amounts of clean water fall from the sky, and we don't have to bring in over the bridge!

The article can be found online in the County Line Courier, Vol. 21, No. 9, Pg. 8.

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