A Deep Dive into Subsurface Irrigation

Text and Images by John Henry Nelson

Civilization, at least on the scale that we know it, would not exist without irrigation. The rise of all culture is intimately linked with good farming practices, and irrigation has always been the most important dynamic in increasing food production.  On average, irrigated crop yields are 2.3 times higher than those from rain-fed soil and proactive intervention will be increasingly needed to feed the growing populations of the world

We are rapidly heading into a world in which water scarcity is a new normal and where agriculture consumes an astounding 70% of all fresh water on the planet. This means that as farmers and gardeners we have a responsibility to use water wisely and develop irrigation methods that reduce our environmental impact on the water sources we use.  Since the 1960’s, global population has doubled but water use for farming has tripled. This is creating an immense burden on aquifers, wetlands, and rivers around the globe. Legal battles have been fought and it is expected in the near future wars will be waged over water – our most valuable resource.

Farmers are becoming more dependent on irrigation water extracted from aquifers that are getting depleted more quickly than they are able to recharge naturally with rainwater. At this point, farmers are having to dig deeper wells and install stronger pumps that require more energy to get the needed water to the surface.

Relying on and depleting precious water from our aquifers is not sustainable, and it is critical that we develop irrigation methods that use water wisely and at the same time harmonize with natural ecosystems. Rainwater harvesting is the most underutilized water resource we have and good practices in that field will be key for irrigation in the future.  Rainwater harvest for irrigation has been proven to work even in arid climates with as little as 6 inches of annual rainfall. In Mills River, North Carolina where Living Web Farms is located, the farms receive an average of 48 inches of rainfall a year and we can easily harvest enough water for all our irrigation needs.

The Virtues of Digging In

There are four types of irrigation: flood, drip, overhead, and subsurface. Though each has its advantages and disadvantages, we’ll only be exploring subsurface, or, sub-irrigation in this post.

Sub-irrigation is an irrigation system deliberately and purposefully buried below the surface. One advantage it has over other types of irrigation is that it keeps the infrastructure out of the way of all gardening activities such as cultivating, seeding, mowing, and harvesting.  A second benefit is that it serves as a permanent irrigation improvement and does not need to be replaced or repaired every season like plastic drip tape irrigation does.

Essentially, when you sub-irrigate you are creating raised garden beds in which you lay 4-inch pipe in the pathways between the beds, and cover it with wood chips. When you turn on the irrigation water, it fills the pipe under the path and the water soaks into the soil of the garden bed. The 4-inch perforated high-density polyethylene black pipe we chose is available at hardware and home improvement stores and is widely used for gutter and foundation drains.

This type of irrigation allows water to soak deep into the soil and stimulates more soil life by hydrating the entire soil profile – not just a small circle of water directly under the plants as is the case with drip irrigation.  In farming we are essentially working with soil and soil biology, and it is the soil’s bacteria and fungal life that feed the plants.

Additionally, with deeper soaking of the soil there is less surface water evaporation. Evaporation concentrates salts at the surface, which is a common predicament in areas that do not experience sufficient flushing of these salts from natural rainfall. This often happens to soils in arid climates, and is also a common problem for greenhouses because they are covered and do not see atmospheric precipitation. Sub-irrigation does not concentrate the salts like surface applications do, and can actually be used to flush the salts out of the soil.

Though subsurface irrigation can be applied to all types of growing situations, its up-front labor cost and higher material costs compared to drip tape make it most feasible to use in a permanent high-production growing area. We choose the greenhouse at our Grandview Farm because of its abundant harvested water supply, and its permanent high-production crop growing setup.

Planning for the Future

If you are considering subsurface irrigation, the first thing to do is to establish your water source. You’ll need to have an ample water supply to make sub-irrigation saturate the soil deeply in a short amount of time- usually 30 minutes or less. At our Grandview Farm I designed and built a permaculture water-harvesting system to catch storm water runoff on the farm with swales and berms and rooftop catchment. The water is stored in ponds that gravity-feed the greenhouse with irrigation water.

No pond to supply water? It doesn’t take much to simply collect and store rainwater directly from the greenhouse roof. With a gutter and downspout system,rainwater can be collected in a cistern placed above or below ground, depending on your strategies for antifreeze protection in the winter months. The problem with cisterns are their relatively small size and high cost-per-gallon as compared to pond storage. Another option would be to directly pipe rainwater into the greenhouse and into a distribution box that flows into the 4-inch pipes. This could be a good way of utilizing rainfall and store water directly into the soil.

A concrete distribution box fills with harvested rainwater and distributes it to the 4-inch perforated lines

The Grandview greenhouse sits on a slight slope and the beds are level on contour, but each bed steps down a few inches similar to a level terrace but barely noticeable. With the greenhouse sitting on a slope the water has to be distributed evenly.

To do this I used a small concrete septic tank distribution box also known as a D-box to even out the flow of water to all the 4-inch drain pipes used for the irrigation. These pre-cast boxes are available at your local concrete yard for about $50. You could also buy a plastic D-box at a plumbing store but it will likely cost about $20 more. I like the concrete version as it provides a solid piece of hardware to connect to and is a little less expensive to boot.

Best practice is to locate the D-box at one end of your raised beds. It will need to be slightly higher than the irrigation pipes in the bottom of the paths so the water will flow from the box to the 4-inch pipes. After locating the D-box you are ready to plumb in the water source. Our source is a 2-inch line that is gravity fed from a pond on the highest spot on the farm.

You can use a transit to establish a level bed for the trenches, or work with a simple water level

The next step is to dig out the path trenches, making sure the bottom of the trenches are at least 7 to 8 inches below the top of the raised beds, and as mentioned before, slightly lower than the outlets on the distribution box so gravity can do its work. I used a laser transit to level the bottom of the trenches for the 4-inch pipes to lay in. There are alternative tools for making sure this bed is level, including a homemade water level, but the end result needs to be a flat and level bed any way you accomplish it.

The pipes are covered with wood chips to a point 2 inches below the tops of the bed mounds

After digging the trenches simply lay the pipe out in the bottom of each one and connect the ends to the D-box. Then cover the pipes with wood chips. I keep the wood chips approximately 2 inches lower than the top of the garden beds to prevent the chips from getting in the beds. This creates a nice firm and level walk path.

You can get woodchips from your local arborist for free most of the time, since chips are a waste material from their tree removal or cleanup. The branches are run through an industrial chipper that makes small 1’’x1’’ pieces, which are the perfect size and also happen to be a great source of carbon and food for soil life. Woodchips and fiber act like a sponge and can help drought- proof a garden as they soak up and hold water.

Chips work best either on the surface of the garden bed or in the paths, but should not be mixed in directly with the garden soil because in their slow decay they tie up elements such as nitrogen in the soil.

The chips provide a firm and level
pathway and do not contaminate
the soil beds.

Now you’re ready to irrigate the beds. Limit deep-soak irrigation to 30 minutes or less if possible. Be careful not to over-irrigate, because this will cause the soil to waterlog, a condition that favors anaerobic bacteria which make it nearly impossible to grow healthy food crops.

With a subsurface system like this, you can irrigate once or twice a week, as opposed to every other day like you would with drip or overhead irrigation. Sub-irrigation does use more water than drip irrigation, but you end up irrigating less often. And if you are using rainwater for irrigating, you can afford to use more water and still end up with a more sustainable and resilient operation that is better for the environment and does not deplete the aquifer.