25 Mar

Building a siphon irrigation system

We planted about 30 fruit and nut trees in the fall of 2013. These are young trees from a local nursery. In this part of Portugal, summers get very hot and dry. It may not rain for 4-5 months and temperatures can exceed 40 Celsius on a regular basis. Young fruit and nut trees or berry bushes with shallow root systems probably won’t survive without irrigation. Fortunately there are a number of old stone-lined wells on the land. Most of them are still usable, and full of water. The others will need to be cleaned. This particular irrigation system takes advantage of the varied topography of the land. What we devised is a large siphon. The main garden/orchard is about 15 -18 vertical meters below one of the larger wells on the land, an ideal setup for a siphon. So in March 2014 my friend Alexander and I drove across Europe to bring tools and building material to build this system.

Objectives

To install an irrigation system on the land that would operate autonomously and deliver water to the already planted orchard of young trees over the coming summers, requiring no power to operate, and which will save a lot of time and water over the years of it’s operation.

Construction

The benefit of a siphon is that once it’s been primed, it will require almost no power to operate, as we allow gravity to do all of the work. Such a system is very much topography-dependent. The outlet of the system has to be below the water-level in which the intake is located. The more the height difference between the two, the higher the pressure, at a rate of about 1 bar per 10 meters of vertical difference. Longer pipe-lengths do not increase pressure, if anything, they reduce pressure due to friction.

siphon irrigation

The main challenge here however is to lay a pipe in the ground that spans the distance of almost 300 meters between well and orchard. Needless to say, this meant we would be doing a lot of digging, which is exactly what we did. Every morning while it was still cool, and we were still somewhat rested, shovels and pickaxe in hand we go to work digging a ditch about 25cm deep, and 15cm wide. In softer ground devoid of rock, we would sometimes dig about a meter a minute. This pace was offset, however, by the tougher stretches full of rock and root.

Alex happy to be digging trenches for siphon irrigationWe buried a 50mm PE pipe in 3 sections. It’s sold in 100 meter rolls, which we connected to each other. It is absolutely essential that there are no leaks anywhere in the system. (We actually had to fix 3 leaks in one of the pipes. Even a small leak could drain the entire well per time.) Alex devised a great way to build the water intake, which sits near the bottom of the well. Initially, I wanted to tinker up something that floats at the top of the well, and thus is always just below the surface, no matter what the water level of the well is. But Alex came up with something better. The 50mm pipe is not very flexible, so to keep the intake below the waterline, which is essential for the continuous operation of a siphon, we used a rock to weigh the intake down to near the bottom, and also a float to pull the intake up, thus keeping it a fixed distance above the bottom. We don’t want to draw in sediment off of the bottom of the well, nor do we want to risk letting air in. So we settled on about 50cm off the bottom. The intake is nothing more than the end of the PE pipe drilled full of holes, and covered with synthetic window screen. It’s all held together by plastic wire-ties, no metal was used in order to prevent any corrosion issues.

Using a large diameter pipe to span the distance is also important. The larger the pipe, the less friction in the system, and so the more pressure we have at the bottom. This is not crucial for a gravity-feed system, which would probably work well with a smaller diameter pipe, but over that distance, 50mm seemed reasonable, balancing cost, functionality and practicality of installation.

We built a housing for the system with bricks and cement. A local metal smith custom-made the door for the housing, which contains the filter, valves, water-counter, and distribution to smaller diameter irrigation pipes that lead to the orchard. The 50mm pipe feeds into a filter to prevent sand and organic debris from entering the irrigation system. We want to keep the valves and smaller irrigation pipes as clean as possible, mainly to prevent the individual drippers from clogging up. They will probably do that anyway over time, as the most fine-grained particulate matter will slip through, and it’s probably good to replace these drippers once a year.

siphon irrigation mechanics

A manual valve before the filter allows water-shut-off for servicing without having to re-prime the whole system or incur water-loss. We then have two redundant valves by Gardena. These are servo-operated electric valves with integrated timers that work at 0-12 bars of pressure. Solenoid valves require some back-pressure to function properly, which would not be a problem here, since we have almost 2 bars of pressure at the bottom of the hill, but the Gardena valves seemed like the ideal solution. Battery-powered, timer-integrated and pressure-independent, and at a reasonable cost of €55 each, using them was a no-brainer. In the future, as more of our irrigation systems will be integrated with solar power, I’d like to also operate the valves with solar, and make battery-use obsolete.

siphon irrigation valve assembly

The irrigation system feeding the orchard then consists of 20mm PE pipe and thin, flexible drip line, with a lead to each tree or berry bush. At the end of each dripline is a dripper that delivers either 4 or 8 liters an hour to the trees. By timing the irrigation, we can deliver fairly precice amounts of water to each tree, and also tailor water delivery to each plant, while still having a single watering cycle. It’s highly flexible and runs completely autonomous. By installing this system, we will save countless hours of time we don’t need to spend watering, and incalculable amount of water as well. Drip-irrigation minimizes evaporation, and allows delivery of water exactly to where it is needed,  right into the root system of the tree.

Materials & Costs

Manual valves, fittings and volume counter€80
2 Gardena valves€110
20mm irrigation pipes, drippers€120
Housing material + door€130
50mm siphon pipe and fittings€330
Total cost of system€770

Time to install: 10 days (2 persons)

Considerations & Learnings

Trees further down-slope invariably get more water out of the system as the water that fills the pipes drains out at the end of each watering cycle.

We do not have to contend with freezing winter temperatures in this part of Portugal. If you build such a system in an area where the ground freezes, this will obviously need to be considered. It’s probably easier to drain the system for the winter months than to bury pipes so deep that the freeze doesn’t reach them.

Drippers must be kept clean. They have thin membranes that regulate water-flow. These membranes are susceptible to collecting micro-particles that get past the filters. Since we draw from old agricultural wells that in some cases have not been cleaned, are exposed to the air, inhabited by critters, it’s important to check the flow of the drippers on a regular basis, multiple times per season, and to clean or replace as needed.

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