Tuesday, January 28, 2014

Gravity-Fed Drip Irrigation

In response to the most severe drought in 40 years, The California State Water Project just shut off its spigots to dozens of communities and farmers, in order to maintain the little water to use "as wisely as possible".

What could be wiser than reducing your dependence on California to produce cheap organic food, given their reliance on top-down irrigation projects that are susceptible to collapse as their snowpacks begin to fail?

What could be wiser than utilizing the water that already runs off your home or garage to water your garden, and avoid paying hundreds of dollars a summer to water mitigation company for a public resource?

At Freedom Gardens at the Fairgrounds, we hope to educate the public about ways in which they can reduce their dependence on a susceptible food-system. We hope to free ourselves from the need to use public water systems to water our gardens, by capturing rainwater. We will free ourselves from the need to use fossil fuels to pump water, by employing small solar powered pumps to fill tanks for a gravity-fed drip irrigation system for our gardens.  We hope to further reduce water use by employing cultural practices that encourage healthy deep root systems, and utilizing soils high in water-holding organic matter.

We need to move forward on rainwater harvesting, but last summer we solved one part of the equation: we built a gravity-fed drip irrigation system to water our gardens.

Our garden is located at the Missoula County Fairgrounds, next to two large horse barns on the far south end of the property.  Between the 1920's and 1941, the area served as the Missoula Airport.  The water pipes serving this part of the property are original wood pipes, and are so leaky that the Fair can only afford to turn them on twice per year.  As such, we did not have a reliable water source at our garden.  The success of any agricultural project relies on water.

We designed a gravity-fed drip irrigation system.

The fairgrounds uses a 3000 gallons water truck to keep the dust down.  The Fair Caretaker, Glen, agreed to use this truck to fill our reservoirs if we made it convenient for him.

We were able to procure a load of industrial bulk containers, or IBCs for short.  Each IBC holds approximately 280 gallons - a string of 8 would allow us to get >2000 gallons of water in only one trip.  When considering a water reservoir consider your chosen container's previous use.  Our IBCs previously housed a soy-based glue, which we were able to wash out with a pressure washer easily.

The beginnings of the bulk head extension. 
With all our supplies, we were ready to begin piecing the irrigation puzzle together.The first piece of the puzzle was to link all of the IBCs together  using a header that all tanks could be filled and drained as one unit rather than eight individual units.  Our first attempt included attaching the bottom four IBCs in parallel, then branching upwards into the tanks above with 2 inch PVC.  We quickly realized that the air inside the tanks needed to go somewhere prior to being replaced with water.  The solution became apparent as soon as the problem - we needed to add bulkheads to each tank.

Bulkheads allow us to add a second hole to each IBC, which in turn allows an exit for air as water fills the void.  The key to remember here is that the air will eventually be replaced by water as the bottom tanks fill and the top tanks begin taking on water.  To prevent water loss and to allow the top tanks to fill, we simply built the bulkhead exits higher than the maximum fill point on the tanks.  We could now move on to the next stage of problem solving - filling the tanks quickly and efficiently.
A fire hose connects the reservoir
the tanker truck
After inspecting the tanker truck, we purchased a simple fire hose would allow us to attach to both tanker truck and reservoir. Our first attempt was to connect to only one of the top IBCs in order to fill the reservoir, but we quickly noted that the tanker was able to pump water faster than the reservoir was able to receive it.  We solved this problem by creating a manifold that allows water to enter through each of the top tanks at once.  By spreading the tanker flow from one to four tanks at once, we were able to minimize the time necessary for a complete and efficient fill.  We were now ready to pursue the irrigation lines.

As we mentioned in previous blog entries, the fairgrounds staff conveniently created to large berms to the west of our plot.  The berm closest to our coldframes stands roughly 12 feet tall.  Given that pounds per square inch [psi] increases by .4 for each foot, we calculated that our lowest system psi was 12ft * .4 = 4.8 psi. Given that each IBC is roughly 4 feet tall, and we've stacked two on top of each other, we calculated our maximum system psi to be (12ft + 4ft + 4ft) * .4 = 8 psi.  Once our calculations were complete, we knew that we needed to identify an irrigation system capable of functioning within 4 and 8 psi.  A perusal of available irrigation components yielded the John Deere T-Tape series - perfect not only for its function at low flows, but also in consideration of cost, equal water distribution, and conservative water discharge.

Chris tacking up the irrigation supply line
With drip tape ordered and in the mail, we laid a main line of 3/4 inch polyethylene pipe from the irrigation reservoir to a manifold box where we diverted the main stream into three separate polyethylene lines, each containing its own shutoff valve leading to a row of coldframes.  If you look back at the coldframes blog entry, you'll note that we have three rows of coldframes with the middle row having two separate sets of boxes.  We trenched the irrigation main lines under and into the center back of both the front and back row of boxes.  In the middle section, we split the line in the area between the boxes and then ran a line into the side of each box.  Another note of wisdom here - tape the ends of your irrigation lines shut before moving them through the dirt.  This will keep dirt from entering and inevitably clogging your lines.

End caps in two different configurations
Once the irrigation lines were in the boxes, we used a tee to split the line once more (in the front and back rows) and ran an equal length of line from tee to each end of the box.  The idea here was to keep the lines as equal as possible so as to ensure equal irrigation distribution.  Our last step in laying the irrigation mains was to secure them to the back of the boxes and cap the ends. Interestingly enough, the hardware store had a limited supply of caps, so we needed to gather two separate configurations.

The last step to completing the irrigation system was to install the newly arrived John Deere T-Tape and of course, testing.  We were able to purchase the necessary connectors and termination components for the t-tape upon ordering, so all we really needed to do was pierce the irrigation main, insert the connector, slip on a length of t-tape, and place a terminator on the end.  It doesn't just sound easy, it really was!

With the irrigation system installed, we eagerly tried one line alone.  Upon seeing it successfully work, we curiously attempted to run two lines at one - success.  Could we run all three boxes at once?  You betcha!  Our irrigation system was complete and we were ready to get growing!

One of our nearly finished fifteen coldframes