Rethinking On-Farm Storage

While many irrigation practices are adapting to meet the demands and regulations of the modern day, other practices have proven their value over time. Some practices, such as on-farm storage, fall into both categories. While it has had a place in farming for many years, on-farm storage is getting a fresh look in some parts of the country.

“There are many, many reasons for on-farm storage if you go back in history,” says Charles Burt, PhD, PE, CID, CAIS, the founder and retired director of California Polytechnic University’s Irrigation Training and Research Center (ITRC). “Historically, irrigation districts had canals, and they couldn’t deliver a constant flow rate. It would bounce up and down, and they couldn’t deliver water to match the labor schedule. If you only wanted your workers to irrigate during the daytime, you had to store what came at night in a reservoir, and then you used that plus the daytime flow to irrigate.”

While on-farm storage at the head of a field has historically been used in places such as California’s San Joaquin and Coachella valleys, tailwater storage is a current topic of conversation in other parts of California.

“Currently, on-farm storage is a topic that is being discussed in California by some of the regional water quality control boards to regulate growers so they don’t spill irrigation water off their fields,” says Stuart Styles, PhD, CID, CAIS, a Cal Poly professor who most recently retired as the director of the Cal Poly ITRC after 25 years. “In Central California, there is a proposed rule that is going to go into effect in the next five years or so where all growers will be required to build on-farm storage to capture runoff water.”

At the head of the field

While some larger farms engage an engineer to design and build on-farm reservoirs at the head of a field, most reservoirs aren’t designed by an engineer at all.

“Most on-farm reservoirs are built by a local construction company,” Burt says. “If you want to store incoming flow all night long, and you have a flow rate of 2 cubic feet per second coming in, that’s about 900 gallons a minute. That tells you how much capacity you need.”

According to Burt, the reservoir’s location will dictate its design. “You want to find the right spot that isn’t too far from anything and is easy to get in and get out of, and figure out the dimensions,” he says. “Whether the location is broad or high and deep influences the configuration of the reservoir.”

“You don’t put it in because it’s nice, you put it in because you have to. If you want manageable water, sometimes you have to have a reservoir.”
— Charles Burt, PhD, PE, CID, CAIS, founder and retired director, Irrigation Training and Research Center, California Polytechnic University

One key consideration in choosing a location is that the water surface only accounts for half of the area a reservoir requires. “You can’t let the water go up to the top of banks, so there needs to be a safe distance between the top of the banks and the highest water surface, called freeboard,” Burt says. “The inside and outside banks are sloped, and the top has a certain width. When you start drawing it out for a small reservoir, if you need an area of half an acre, it’s going to take you an acre, ballpark. You can make it broader or deeper and higher, but it really depends on where the water is coming from, if it has a little pressure on it and how much land is available.”

Burt says the majority of farms do not have reservoirs, and the presence of a reservoir is heavily influenced by area of the country and the system of water delivery. “It’s going to cost you more to build a reservoir,” he says. “You don’t put it in because it’s nice, you put it in because you have to. If you want manageable water, sometimes you have to have a reservoir.”

Ultimately, the decision to build a reservoir comes down to a cost-benefit analysis, as land used for a reservoir leaves less land for growing. “If you’re a farmer and you have to take one acre out of 100 acres for a reservoir, that’s 1% of your land,” Burt says. “But if the reservoir enables you to do a better job of irrigation and water management, there is no question in my mind that you get more than a 1% increase in yield, more efficient labor, and better quality or tonnage. That’s why the guys do it.”

In some locations, wells and reservoirs go hand in hand, which serves two primary purposes, Burt says. First, pumping well water into a reservoir allows air bubbles to disperse. “You don’t want to put that in the pipeline,” he says. “The air blows pipelines up.”

Additionally, well water that contains iron can cause bacteria to grow in drip systems, which then has the potential to plug emitters. “If you shoot water out from the well to the reservoir, it oxidizes the iron and you don’t have that problem,” Burt says. “Also, well flow rates vary over time and may not match the more constant flow rates that are needed for on-farm irrigation systems. Reservoirs provide the buffer between the supply and usage.”

At the tail of the field

A reservoir is also useful at the tail end of a field, where it captures water that would otherwise run off, allowing that water to be put back on to the same field or another field.

“Tailwater storage is a straightforward way to improve efficiency dramatically,” Styles says. “Growers put in a large enough reservoir to capture any tailwater that is running off the field and put in a pump and a return pipeline to the head of field so the same water can be returned to the field.”

While the payback period depends on the cost of both water and electricity, Styles says that the savings can be significant. “Typically, tailwater storage will save between 10% to 20% of the water being applied to the field,” he says. “We’ve got growers paying $30 per acre-foot and others paying $1,000 per acre-foot for water, so the potential payback is even more if the cost of your water is more. There’s a huge range.”

In regions like the desert Southwest in California, tailwater reservoirs can be the difference between having an adequate source of water or not. “In a lot of cases, it is vital that growers don’t let water run off their field,” Styles says. “There are close to 1 million or 1.5 million acres in that category that would benefit from having tailwater recovery pumps and on-farm reservoirs. If they don’t have water, they can’t farm. It doesn’t pencil out to a good cost recovery, but it provides a source of water that they didn’t have access to in the past.”

“Tailwater storage is a straightforward way to improve efficiency dramatically.”
— Stuart Styles, PhD, CID, CAIS, professor and retired director, Irrigation Training and Research Center, California Polytechnic University

Reservoir maintenance

While reservoirs provide a buffer and increase a grower’s flexibility with irrigation, they can also pose some challenges. Over time, algae and plant growth can clog irrigation systems. “Cattails, bullrush and tules will grow in shallow reservoirs,” Styles says. “These can be maintained with spraying or mechanical removal. Reservoirs greater than 10 feet deep will minimize plant growth. Outlet screens must be accessible and cleaned regularly to prevent blockage.”

Just as not all growers service their tractors or other equipment, not all growers are proactive about servicing their reservoirs. However, there are some simple solutions to remedy common issues. “There are problems with reservoirs, but it turns out that there are solutions,” Burt says. “With a reservoir, you can [get] rid of air, iron, algae and bicarbonate at the same time. If you inject sulfur dioxide into the reservoir, it kills the algae and gets rid of the bicarbonates.”

Burt also says that much of the water from California canals and wells has sand, and if connected directly to an irrigation system, the sand has the potential to overload filters and clog irrigation systems. “The reservoirs drop the sand out, which is an additional benefit,” he says.

Lauren Sable Freiman is a freelance writer based in Cleveland and can be reached at laurensable@gmail.com.