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Planning a Rainwater Irrigation System

Our vegetable production has five annual costs: seeds, potting mix, electricity, compost, and water. The greatest of these is water. This will not be the case for every gardener. Our water costs are especially high because we pay full city water costs which also include fees for sewer use because they assume that the water we use will be flushed down the sewer. The costs have got me dreaming about building our own backyard well, but I'm not sure about the legalities there, so let's start by examining the potential for rainwater collection and integration with our current drip irrigation system. Here's a look at the key questions I will be asking.

  • How much water do we need for our drip irrigation system each season?

  • How much rainwater could we collect?

  • What volume of rainwater would we need to store?

  • How much money could I save with a rainwater irrigation system?

  • How could the rainwater be incorporated into the existing drip irrigation system?

  • Could our rainwater irrigation be gravity fed?

Read on to find out the answers to each question, and maybe even pull out your own calculator to figure out how feasible rainwater irrigation would be on your property.

How much water do we need for our drip irrigation system each season?

We can answer this one by calculating the total area we would like to irrigate and then figuring out how much water we will need for each square foot of bed space.

The garden plot we are starting with is our home plot shown below, which includes seven 50 ft² field beds, eight 15 ft² raised beds and eight 50 ft² beds inside our high tunnel. All of that growing space amounts to a total of 870 ft².

photo of our home garden plot
Our home plot includes standard field beds, smaller raised beds, and a high tunnel for total of 870 ft² of bed space.

To calculate the amount of water we'll need for irrigation, we can either use some data from gardens we have metered in the past or make a rough estimate. Let's start by referring to some meter readings from past seasons. At another plot with 1200 ft² of bed space, we used 3541 ft³ of water for the full 2024 season. That amounts to 35 inches of water applied to those beds throughout the season. The length of our frost free growing period is 18 weeks so those 35 inches of water average out to about 2 inches per week during the height of our growing season. If we assume that we'll need to apply that same depth of water at our home plot with 870 ft² of bed space, then we will need 870 ft²x 35in/(12in/ft) = 2537.5 ft³ of water.

This demand for water is assuming that we won't get much supplementary irrigation from natural rainfall, which is never a sure thing here in Saskatchewan. Below is a chart showing our 2024 rainfall to give you an idea of what we're working with. Last year, we only had one week between May and September which had more than an inch of rainfall, and that week was unusual. Among those four same months, there are 10 weeks during which we received less than 3 mm of rainfall. This data helps reinforce the need for supplementary irrigation in our region, but at first glance, it doesn't look like we'll have much rainwater to capture.

rainfall in Saskatoon
Weekly Rainfall in Saskatoon During the Summer of 2023

How much rainwater could we collect?

To get a better sense of the average rainfall we could expect between May and September, I looked at the precipitation data for the last three years. During this time, we received an average of 150mm or 6 inches of rainfall per year between May and September.

Next, I opened up Google Earth and used the measurement tool to quickly approximate the area of the roof surfaces we could potentially use for rainwater collection. If we add these areas together, we get a total of 2080 ft² and if would could somehow manage to gather all of the rainwater from the many different roof surfaces contributing to this total, then we could collect 2080 ft² x 6 in / (12 in/ft) = 1040 ft³ of rainwater throughout the the growing season. To give you some perspective, that's 141 standard 55 gallon rain barrels full of water.

rainwater harvesting potential on our property
Rainwater could be harvested from the roof surfaces highlighted in yellow.

What volume of rainwater would we need to store?

It would be silly to build a rainwater irrigation system and then have our rain barrels overflow every time it rained, so we really want to make sure that when rain does occasionally fall from the sky here, we are ready to collect and store all the water we get. If we refer again to the record of rainfall from last season, we can see that the weekly precipitation only surpassed 1 inch on one occasion.

rainfall in Saskatoon
Weekly Rainfall in Saskatoon During the Summer of 2023

If we plan for a rain barrel capacity that could contain up to 1 inch of rainfall over our total roof collection area of 2080 ft² then we would need to store 2080 ft² x 1 in / (12 in/ft) = 173 ft³ which is the equivalent of 1294 gallons. Oooh, that's a lot . The standard rain barrel can hold 55 gallons, but it would take 24 barrels to contain all 1294 gallons from that 1 inch of rainfall. Maybe I should rethink this...

What if I just collected rainwater from our high tunnel and house for a total collection area of 1490 ft²? And what if I started with just 6 rain barrels with a total capacity of 330 gallons?

How much rainfall would it take to overflow my 6 barrels and how often would that actually happen?

Well, 330 gallons equates to 44 ft³ and if we spread that volume of water over 1490 ft², we would have a depth of 0.35 inches or 9 mm. Next, I looked through the daily rainfall data from last season to see how often our 6 rain barrels would overflow and by how much. It turns out that they would overflow 8 times and we would have missed the opportunity to collect 237 ft³ or $27.60 worth of water.

Okay, let's imagine I had 8 rain barrels instead of 6 for a capacity of 440 gallons or 59 ft³. If we spread that volume of water over 1490 ft², we would have a depth of 0.48 inches or 12 mm. There were only 4 occasions when we received more than 12 mm of rain in one day last year, and with this larger capacity, we would have missed the opportunity to collect 180 ft³ or $20.99 worth of water.

That's more than enough math on this subject, but hopefully you get the idea. There is a need to have a sufficient capacity to store the rainwater when it falls, but that capacity also comes with a cost. It's just a matter deciding how serious you are about collecting every last drop of rainwater, and maybe doing a bit of math to figure out the storage capacity that would give you the best return for your investment.

How much money could we save by using rainwater for irrigation?

At this point, we know we can use 2537.5 ft³ of water in our home garden plot throughout one growing season and we could potentially collect an average of 1040 ft³ from all of the roof surfaces highlighted in the image above. Our 2024 water costs were $0.1166/ft³ so without any rainwater supplementation, it is costing us about 2537.5ft³ x $0.1166/ft³ = $296 to irrigate our home garden plot. If we started using the rainwater for irrigation, we could save 1040 ft³ x $0.1166/ft³ = $121 on our garden water bill each season.

Now that I have presented you with the numbers, it's probably becoming more obvious why we haven't set up a rainwater irrigation system yet. We don't have much rainfall to work with, our small roof surfaces are scattered about our property, and the amount of money we could save with even our best rainfall collection efforts is still not that significant. After we account for the cost of the equipment needed for rainwater collection and distribution, we may be looking at no money savings at all.

How could the rainwater be used with the existing drip irrigation system?

Since our rainwater supply will definitely not cover all of our irrigation needs, our foundational drip irrigation system still needs to be able to draw from our household water lines. Somehow, we just want to be able to inject rainwater into the system when it is available. Hmmm? I think I'm going to need a diagram.

supplementing irrigation system with rain water
This is my proposed method for supplementing our existing irrigation system with rain water when possible.

The simplified diagram above shows how a drip irrigation system could be fed by both household water supply and an intermittent rainwater supply. Most of the components are pretty self explanatory, but there are a couple of details worth adding. The water level detection sensor (B) must open the valve (C) whenever the rain barrel is empty and close the valve whenever the rain barrel contains water. This way, the use of rainwater is prioritized whenever it is available. Next, the water pump (E) must be pressure actuated (ie. It must turn on automatically when a drop in pressure is noticed.) because it has to start pumping whenever the timer in the control centre (F) opens up the irrigation water lines, and it also has to turn off automatically when the automated valve (C) is opened. Since our household water supply pressure is much higher than the pressure supplied by our pump, the pump should turn off as soon as a rush of water bursts into the system when the valve is opened. That's it really. I thought it would be more complicated than this, but the use of the water level sensor and automated valve makes everything else quite simple.

Could our rainwater irrigation be gravity fed?

I know some of you are wondering about the possibility of building a gravity fed rainwater system instead of depending on a pump, so I'll address this question quickly. It is a nice thought, but since we'll want to use the same drip irrigation lines that we've already got in place, we will need to make sure to have a sufficient amount of pressure even when using rainwater. Drip irrigation systems require the lowest water pressure of any irrigation systems so that will work in our favour, but we'll still need to supply our drip lines with at least 10 PSI. The water pressure at the bottom of a barrel is 1 PSI for every 2.31 feet of water depth, so if the water in the barrel was 2.31 feet high, the water pressure at a tap at the bottom of the barrel would be 1 PSI. If the water in the barrel was 3 feet high, the water pressure at the bottom of the barrel would be 3ft ÷ 2.31ft/PSI = 1.3 PSI. That's not great news is it.

Now you might be wondering how high you would have to raise that water barrel to get that 10 PSI you needed for the drip lines. The math tells us that you would need to raise the bottom of the barrel 10 PSI x 2.31 ft/PSI = 23.1 ft above the surface of your garden to ensure that the water coming out of the barrel was always a minimum of 10 PSI. Don't let me stop you from pursuing this gravity fed goal if it really excites you, or if you garden on the side of a mountain. You may be able to find drip lines that require a lower operating pressure, and you could build your own distribution pipes that dribble water out larger holes and therefore require much lower pressure. Just beware that drip systems with extra low pressure will struggle with uneven water distribution. I want to count on even water distribution and I don't want to build an entirely new set of drip lines for my garden that can operate at a significantly reduced pressure, so that's why I would opt to distribute our rainwater with a pump. If you feel guilty about the power usage or don't have access to power on your garden site, just pick up a solar panel for your pump.

Next Steps

The results of this investigation into the potential of rainwater harvesting on our property were interesting but less than exciting. We just don't have a huge volume of rain to gather here, or one large surface from which to collect that water simply. Despite this, I am tempted to still go ahead with the building of a partial rainwater collection and irrigation system that uses rainwater from our house and high tunnel. Most of the water from these two sources could be amalgamated without too much trouble, and I think it would be fun to build the system and share the method with other growers who could potentially benefit from rainwater harvesting a bit more than us, so stay tuned for an update in the future. If you're already a part of our Seed to Table course community and you're keen to build a rainwater irrigation system, then head over to this post in our Classroom area where I'm helping some others plan their systems. We can talk about more specifics there.


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