|Rain Water Tanks|
In planning or designing your property to grow more food, your first consideration should always be water. Water is life, and if you get your water design right, everything else will fall into place, and your plants will grow up strong, happy and healthy.
Start by thinking about all the hard surfaces on your property – the surfaces that shed water, such as your house, garage and shed roofs. If you figure out the approximate area of these surfaces, you can then calculate how much water you could collect. Let’s say your house and garage roofs total 250 square meters. Let’s also say you live in an area of Melbourne that receives an average of 600 mm of rainfall each year. If you multiply these two numbers (250 x 600), you get the total litres of water hitting your roof each year, in this case 150,000 litres. Multiply this by 0.85 to account for loss due to splash and evaporation, and you have the amount you could realistically catch and store, in this case 127,500 litres. Now that is a lot of water!
How much storage?
Now you have an idea of how much water you could catch and store, it’s time to have a think about how much you need to catch and store. The ideal situation is having enough water in your tank or tanks to keep your edible garden flourishing through a three full months without a drop of rain in late summer. This is called designing for extremes. As a rule of thumb, you want to store 8-10 litres of water per day for every square meter of garden bed and 5 litres for every establishing fruit tree in the Summer period. Say you had a 20-square-meter garden and ten fruit trees. That’s about 230 litres of water per day, or 20,700 litres of water to get you through three months. That's a decent sized tank.
Types of tank
Okay, so you know about how much water you want to store. You now have to figure out what kind of tank you want. The most common options are poly (plastic) and steel, though bladder tanks, underground tanks and concrete tanks may turn out to best suit your site. Steel tanks available in zincalume, colourbond, and long life stainless, can be made to size -- good for small properties or unusual spaces. However most steel tanks require a concrete slab which is an extra expense. Larger (20,000 litre plus) steel tanks can be bottomless, and lined with a plastic lining and don't necessarily require a slab. Poly tanks can be sat on packed sand, easier and cheaper to make. They probably last at least as long as zincalume and colorbond steel, but not as long as stainless. Concrete tanks are generally used only on rural properties, as the walls are thicker. Bladder tanks can be situated under decking.
How many tanks
It often makes sense to distribute your required storage over two tanks, especially if it is hard to get water from different sides of the house to one place.
Working with gravity
If your land is flat or if your land slopes downhill from your house or garage, you have the option of gravity feeding water to your gardens without a pump. If your land slopes uphill from your house then you are going to need a pump, with two basic set-up options. The first is to catch the roof water in an initial tank or a plastic catchment pit, and then to pump water with a float-triggered pump from that tank to a header tank at the top of your property, which you can then gravity feed from. The other option is pump directly to the garden. We much prefer the first option.
Legally, you have to ensure that when your tank overflows, that this surplus water returns to the stormwater drains.
As the rainwater flows from roof to tank, there are a variety of options you or your plumber should know about when installing the system. In short, and starting at the roof, there are options of :
Okay, so you have your tank or tanks of the right material, capacity and number for your site. They are plumbed into your downpipes so they fill up when it rains. We now need an efficient system for getting water from the tank to our growing systems – our herbs, vegetables, and fruit trees (we at VEG believe that any plant needs to justify getting watered by providing us or wildlife with food or other services). This is the distribution side of the equation and our distribution systems follow a basic pattern regardless of whether the water is coming from a tank or the mains. We start with an automatic timer you can set to irrigate whenever you want. Below this is an inline filter to remove any remaining sediment in the water and if necessary a pressure reducer so the system doesn't blow apart. We then run 19mm black poly pipe along one edge of the garden or orchard to be irrigated, using 90 degree elbows to get around corners. Sometimes this pipe we run in a big circle back to the beginning, which gives us equal pressure all the way around. This pipe we call the main.
So that’s our introduction to installing rainwater tank systems to irrigate gardens and orchards. We hope it is helpful and easy enough to read. We welcome feedback that would help us improve it, including any recent tank or plumbing innovations we’re not yet aware of. Also check out other articles in our iVEG library, with articles on backyard greywater options and even backyard black water options coming soon! Yeah!