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Water For Thought!

Updated: Mar 12

We can calculate how much water we need to keep our vegetable gardens watered through the high season (May 1 to October 15 in Vancouver). For a baseline, we can use a 9’ x 10’ roof area with a downspout to one fifty gallon water barrel. This collection system will be able to deliver enough water to keep a 4’ x 4’ vegetable garden watered for the duration of the high season.

I was wondering how much water I would need to catch and collect to keep my vegetable garden watered through the summer drought without needing to use expensive (and likely scarce) water from the municipal water supply.

I googled my question, ""how much water do I need to keep my vegetable garden healthy?". 

The answer came back "one inch of water per week per square foot of garden area, which is .62 gallons per square foot. This is a rough calculation that varies depending on soil type, average temperature, sun exposure, plant type, and wind." 

Mitchell, K. (2023). What is an Inch of Water? The Purdue Landscape Report, 23(9).

I am going to use a simple garden plot size to run these calculations. If my garden area is 4' x 4' = 16 sq ft; 16 sq ft x .62 gallons = 9.92 gallons per week. It is preferable to keep the garden soil moist by applying three waterings over the course of the week. My 4' x 4' garden plot would need 10 gallons of water applied in 3 waterings per week (ie. 3.3 gallons per watering).

For these calculations I am assuming our drought for the summer of 2024 is going to last four months, from Jun 1 to Sep 30.

In Vancouver, Stage 1 water restrictions are automatically activated from May 1 to Oct 15. Stage 2 water use restrictions are activated due to unseasonably warm and dry conditions, above average drinking water use, and accelerated reservoir drawdown.

Stage 1 water restrictions limit lawn watering to one morning per week. Stage 2 water restrictions include banning the application of drinking water to lawns or boulevards at any time and prohibit top up or fililng of aesthetic water features.

During the summer of 2023, as of July 28th, the Lower Mainland was at Drought Level 4 on the 0 to 6 Drought Classification Scale used by the Province of British Columbia.

To understand rainfall levels in Vancouver last year we have monthly data of total precipitation per month.

Here is the month to month breakdown of precipitation in Vancouver in 2023. We will use these numbers to predict precipitation for Jun to Sep 2024.

I am converting mm to inch for these calculations to match the watering data provided in the Purdue Landscape Report article. If we tally up the rainfall for Jun, Jul, Aug and Sep, and then average the total across four months, we received an average of 1" of rain per month in Vancouver from Jun through Sep. This 1" of water can be used to account for 1 week per month of rainfall on the garden. The other three weeks I will have to use the water in my rainfall catchment system.

My four month timeline is comprised of 18 weeks. If I did not rely on rainfall on the garden, my 4' x 4' garden will need 9.92 gallons (10 gallons) of rainwater per week, or 40 gallons per month. Or, put another way, 10 gallons x 18 weeks = 180 gallons of water to keep my garden growing through the drought.

If I subtract the one week of rainfall from our rainwater catchment requirements, I will need 14 weeks of rainwater in our containers to make up the difference, or 140 gallons of rainwater.

Now I need to answer the question - how much water can I collect in my rain containers?

We know that 1 sq ft of water x 1" deep = .62 gal.

In Vancouver it can rain 1" in 1 hour without too much trouble. I'm going to use that for my calculation.

I'm going to say that 1 hour of heavy rain will result in .62 gal of water per sq ft. If I want to fill a 55 gal rain barrel, I need heavy rain to fall on a 89 sq ft surface (eg a 9 x 10 shed roof) for one full hour.

Last fall we were surprised at how little time it took for our 50 gallon water barrel to get filled from our 10 x 10 shed roof. We had to install an overflow back into the storm drain system because we quickly exceeded the capacity of that single rain barrel.

I can now figure out how much garden I can water from my 50 gallon water barrel. This one barrel could easily provide all the water I need for a 4' x 4' vegetable garden. When I add in the fact that 1/4 of my rainwater requirement will be falling directly from the sky, and that that same rainfall will fill my rain barrel to cover the rest of the month, it looks like I will have plenty of water to weather a four month drought.

The other way to look at this calculation, is that I have five downspouts coming off of the main roofs of the house, those roofs comprise approximately 1,000 sq ft of catchment surface (rain shed). If I have a rain barrel under each of these five downspouts, I will easily be able to collect another 250 gallons of rainwater. This means I could maintain 5 more 4' x 4' gardens on my property, or 24' x 4' of garden in total, at a minimum.

I suggest, as a rule of thumb, we can calculate 1 50 gal rain barrel catching water off of a minimum 9 x 10 roof will provide water for a 4' x 4' vegetable garden through 4 months of drought / low rainfall Jun to Sep.

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