Rain Engineering: Irrigating the Raspberries from the Shed Roof

My garden soil is quite sandy and dries out very quickly. Even some "drought tolerant" plants struggle. I'm obsessed with rain and irrigation. So I water frequently, mainly from a rainwater butt.

How much rain do I get?

28"/year or about 0.72m (Hint: Try looking up your town on Wikipedia to get your figure)
Garden area about 150m²
0.72 x 150 = 108m³
108,000 Liters
21,600 watering cans

I'm quite selective about when and where I water - annuals and new perennials get priority but this calculation explains why messing about with a watering can is not as effective as I'd like.

Some years ago, I put up a shed, attached a hosepipe direct to the gutter and irrigated a nearby Tayberry bush I'd just planted. The trouble was, it all seemed to block up and the gutter overflowed regularly. Investigating this and planning a new Raspberry bed led me to some important design elements:

Blockages

At first, I thought the most serious source of trouble was dead leaves and dirt getting in so I put a leaf guard on the gutter, a filter in the down pipe and fitted the tank outlet a little way off the bottom.

Filter materials

Then I discovered a more likely culprit, "air locks"

These air locks are different from those you get in ordinary plumbing.
There isn't enough pressure to blow them out
You can't keep air out of the system - in dry weather it drains right down
You don't want to be bleeding the system every week.

There is a solution

Make sure the pipe to the garden only goes down
Even a little up and down to clear a 150mm wall could cause a blockage

Ordinary garden hose tends to acquire little ups and downs and is unsuitable. I used the more rigid 20mm PVC conduit system intended for electrical work.

It's cheap
Easy to assemble
You don't need to glue it until you are sure about the layout
Slightly leaky until glued
May discolour after a few years exposed to the sun

Intermediate storage to cope with heavy rain

This is worth doing but deciding how much storage to provide can lead to complex calculations - or, you could just suck it and see. Some of the more geeky information out there assumes any overflow would be a disaster (EG: A museum with no overflow, disasters to average 500 years apart!).

Fit the tank with an overflow
Choose what seems like a reasonable sized tank
If rain is rare or heavy, go large

The worst that can happen is that you'll waste some of the rain.

For those of a geekier disposition, here's a few calculations (Skip)

My original calculation for the shed roof was simple. Since it had roughly the same area as the raspberry patch, piping the water across should double the perceived rainfall and make the raspberries very happy. (Raspberry plants are serious drinkers!)

There's a standard for roof drainage systems, BS EN 12056-3: which recommends designing on a basis of the rain intensity for a 2 min duration storm event. Intensity is measured in l/s per m² and my local figure is 0.18. Such rainfall events happen about once a year. I decided that a tank that could absorb such a burst would be nice: 0.018L x 2m² (roof area) x 120 secs = 0.004m = 4L

On an average day, we get 2mm of rain or 0.002m x 2m² = 0.00432m³ = 4.32L

I also measured the flow rate for 15m of outlet pipe. The best I got was about 1L/min. Tests with various irrigation devices on the end of the pipe yielded 0.05-0.5 L/m, which improves things for moderate rain.

So I bought a container with a nominal capacity of 9L
Another helpful factor is the outlet pipe volume. Mine was 15m x Pi x (0.01m)² = 4.71L so I've got about 3 times as much capacity as I "need". It should cope with 6mm of rain

Intermediate storage system

Final stage - getting the water to the plants

Many of the sprinklers and devices you'd use on a mains supplied system don't work very well. That's because of the low pressures. For example:

I used perforated hose for my raspberry bed project. This hose is rated at 6 Bar maximum but I'm only giving it about 0.2 Bar! The result is that it takes over an hour to drain. For a larger system, the answer is to use a lot more hose than seems natural.
Another possibility is to plug in one of the many "mini" irrigation systems with their tiny pipes and droppers and leave out the pressure reduction device.
As the flow is so gentle, consider just using an open hose end. This would be good for helping to establish a new tree.


Perforated hose running behind.a raspberry cane (Normally, the hose would be buried under the bark-chip mulch)

General tips

Be paranoid about keeping your pipes headed down only
Taps in the pipes are very useful
Fit an overflow!
Use water conservation measures such as mulches as well.
Beware of over-watering! That's impossible on my soil but if you are on clay...
Your rainfall pattern may be different. EG: In the East of England, they have less total rain but more heavy thunderstorms. You might want bigger tanks there.
When testing your system, be patient. It may have drained and take a long time to fill before irrigation starts
In some places in the US (and maybe elsewhere?) "rainwater harvesting" has legal restrictions due to prior-appropriation water rights.