Rainfall

- courtesy of Hornby Island Groundwater Society

Rainfall is the lifeblood of the Gulf Islands - our only source of potable water. Rain falls in winter and is stored for summer when the usage peaks. This page contains statistics on rainfall on Hornby Island, as well as information on how to calculate the yield of a "rainwater harvesting" system.

Rainfall Data

The closest and most relevant Environment Canada weather station is at Comox airport. However unofficial rainfall data has been collected since 1995 by a retired hydrology engineer at his home on the west side of the island (Shingle Spit Road near Mount Road). This is presented here to show both the monthly and the annual variation. The original data was in inches; metric values are calculated.

The annual water cycle: monthly rainfall

The annual water cycle starts some time in October when the winter rains begin. The rainfall is more or less constant from then through the end of January. Sixty percent of the annual rain falls in these months.

The amount of rain falls off from February through April. A further 25% of the annual rainfall occurs in these months.

May through September is the dry season, accounting for only 15% of the annual rainfall. July is the driest month, and what rain does fall usually comes in the beginning of the month. August is slightly wetter, even though the first two weeks of August have traditionally the least chance of rain. Note that June is usually wetter than May, and September is slightly drier than August.

The winter population of the island, when the rain falls is just under 1000 persons, minus those who are in Costa Rica, Bali or Kauai at any given time. In the summer months of July and August, the number of people on the island is 4000 to 5000, most of whom, by island standards, are profilgate with the use of water. Thus the demand for water increases by a factor of ten or more when the monthly rainfall is least.

Recharge and discharge areas

Hydrologists classify land areas as "recharge" or "discharge" areas according to whether they capture or release groundwater, and a certain amount of ink and paper has been devoted to defining whether or not the Uplands area, including the 1000 acres of unallocated Crown Land, is a recharge area. As is obvious by the rainfall data, the terminology is not relevant for Hornby Island and the Gulf Islands in general because in winter the whole island is a recharge area and in summer the whole island is a discharge area.

Annual variation in rainfall

Annual data is available for the last 15 years. The rainfall years run from October to September. The average annual rainfall is 52.6', or 1335 mm. By comparison, Environment Canada lists the 30 year average for Comox airport as 1179 mm, Qualicum River (Fisheries Station) 1314, Powell River 1104, Departure Bay 938. We suspect that the rainfall is lower on the north and east sides of the island than it is at Shingle Spit, but have no data at this point.

The standard deviation is 7.6", or 192 mm. The minimum value, important for designing a collection system, occured in the first year, October 1989 to September 1990, and is 1.6 standard deviations lower than the average.

It should be noted that the last two years, which were generally thought to be very dry, were respectively at and slightly above the average. However in these two years, the rainfall numbers in June were the lowest recorded. Apparently rainfall in June is more important than the last few inches in January, proving how much we still have to learn about our water supply.

Hydraulic connection to the Beauforts

There is a rural myth circulating on Hornby Island that the pool of fresh groundwater in the rocks of Hornby Island is hydraulically connected to the water in the Beaufort Range on Vancouver Island. (A parallel myth has Saturna Island connected to Mt. Baker.) This myth suggests that we have an essentially limitless supply of water if we only pump hard enough.

No hydrologist who has examined the situation supports this idea. All we can say is do not count on it!

Rainwater Harvesting

Rainwater capture

How much rainwater can be captured on a roof? The theoretical maximum can be calculated as:

Square feet of projected roof area x rainfall in inches x 0.52 = Imperial gallons. Note that projected roof area is the area of ground under the roof, not the area of roofing.

In metric units, square metres of projected roof area x mm of rain = litres. (1 m² = 10.8 ft²; 1 inch rain = 25.4 mm; 1 IG = 4.54 litres.)

The amount of water which it is practical to reclaim is, of course, less. There will be spillage, some water is lost to cleaning out the system at the start of every rain event, and the collection may be interupted when leaves and pollen are falling. More to come...