It would be easy to think California may not have a lot to learn from farmers in places like the Great Plains. After all, the Golden State is a leader in so many things: computer technology, environmental policy, social justice issues, lifestyle and culinary trends.
But farmers in the Great Plains and other parts of North America have mastered something that is only beginning to creep into California: overhead irrigation. This is the class of crop irrigation tools that includes those giant, crawling center-pivot sprinklers we see from the airplane window as bright green crop circles far below. Pretty much everywhere but California, overhead irrigation is a common and relatively affordable technology that can distribute water frugally and produce just as much crop yield as other forms of irrigation, including drip systems.
In California, only about 2 percent of the farmland is watered by overhead irrigation systems. Around 40 percent is still flood-irrigated, a much less water-efficient practice. But those numbers are changing. Advances in technology – pioneered in other states – have brought down cost and improved efficiency in overhead irrigation. This is beginning to catch the attention of California farmers. Installing overhead irrigation is now about half as expensive as drip irrigation, it’s cheaper to manage on an ongoing basis and it’s nearly as frugal with water as drip systems.
Jeffrey Mitchell, a cropping systems specialist with the University of California Cooperative Extension, recently published the results of a long-term experiment with overhead irrigation in California’s San Joaquin Valley. The results suggest it can be very effective with most crops – saving water and money and producing equal or better crop yield (the exception was tomatoes, which do particularly well on drip irrigation).
Water Deeply recently spoke with Mitchell to learn more about his results and the potential of overhead irrigation in California. He emphasized the importance of integrating other farming practices also common elsewhere in the United States but relatively rare in California, such as conservation tillage, in which crop waste is left behind after harvest and the field is not plowed between crops.
Water Deeply: Why isn’t overhead irrigation used more in California?
Jeffrey Mitchell: The invention of the first overhead system was back in the mid-1960s in Nebraska. I remember hearing a little bit of local lore that the third center pivot ever sold globally was actually sold in California. The subsequent sales and use of overhead systems worldwide increased significantly. However, in California it didn’t.
In talking with a lot of people, there are a number of reasons that are given. There were stories about the wheels getting stuck in mud – and that was nothing less than a train wreck. Another thing pointed out historically was the fact that in the old days – more than 30 years ago – there was an idea that they couldn’t keep up with the (water) demand of the crop, the transpiration of the crop. So you’d get behind on the irrigation.
A third factor was the high rates of evaporation. In the old days, the pivots actually were much higher pressure. They were like impact sprinkler systems, running at 60 psi, and the spray sprinklers were actually mounted on the top, 16 or 17ft (around 5m) high, and they would spray out all over the place. So you had random rain patterns falling, but in reality there was a great susceptibility to evaporation.
The other thing that’s probably also a factor is the great diversity of California’s cropping systems. It’s not like people are only growing wheat or corn or beans. Many farmers, in fact most annual crop farmers in California, have a greatly varied kind of rotation they use. So there wasn’t the familiarity, and the experience base never got too far with the great diversity of California’s crops.
Each of those things has really been overcome over the years with new advances in technology, advances in learning.
Water Deeply: Why are you and others looking at it more closely now?
Mitchell: What we are trying to do is anticipate future constraints that are going to be weighing on farming systems way out into the future – including water shortages, labor shortages – by merging different sorts of technologies. There are potential advantages that might be derived from adding other kinds of farming practices that are not currently used very much in California, but that might have value and benefit, particularly in terms of water, for overall production efficiency.
Actually, these are not new practices. It is new right now because it hasn’t been used in California. But all of these things have been used with great advantage in many other areas of the world.
I’ll just cut to the chase here. It may be some of these practices I’m talking about – these measures such as reduced-disturbance tillage, preserving crop residues – might be underappreciated as opportunities for people to use in California.
If you ask many farmers who have recently bought overhead systems and who use them now, they would probably, first of all, cite the ability to reduce labor, and that’s something that is extremely important. The maintenance, the installation of subsurface drip systems – while they are very, very efficient – they do require considerable labor.
Water Deeply: What were your results in terms of water efficiency?
Mitchell: There are different ways of determining the efficiency of an irrigation system. One is to calculate what’s called the coefficient of uniformity. Probably drip irrigation has very high uniformity. That means it’s close to 95 percent or so. There’s only a very little bit (of the crop) that’s not receiving the intended amount of water. We measured 93 percent uniformity with overhead irrigation.
Whereas surface irrigation – using flood or gravity – they tend to not have as high uniformity values because as you open a pipe or you open a siphon tube at one end of the field, by the time that water gets all the way down to the other end of the field, it’s had much more time to infiltrate on the top end of the field. So you end up with much more water applied there. Surface irrigation can be very uniform and efficient as well. But as a general rule, they don’t tend to have as high efficiency as other forms of irrigation.
Water Deeply: It is hard to believe overhead irrigation could be nearly as efficient as drip. When most people think of it, they see water spraying everywhere from these roving sprinklers high off the ground.
Mitchell: You have to remember now, we’re talking about the water application devices of today. And in recent years, they use pressure regulators so the amount of water that’s coming through the nozzle is well controlled, and they’re also very close to the ground. So they’re not spraying water. These are low to the ground, and there are various delivery nozzle practices that can be used.
Another advantage could be in salinity management, which is harder to do with a drip system. It may not have relevance or applicability in every crop setting, but in some environments people might be banking on the ability to uniformly leach salts out of the root zone.
Water Deeply: Were crop yields just as good with overhead irrigation?
Mitchell: Overall, with the exception of tomatoes, yes, they were on a par with the conventional irrigation systems.
Water Deeply: So is overhead irrigation primarily something that will interest a farmer who doesn’t want to invest in drip irrigation?
Mitchell: No. Depending on what the scheme or goals of a given farmer are, it could be an option for precision irrigation. It’s a complex set of determinations that come into play there. Labor factors in there, learning curve issues, applicability to different crops.
The very early 1980s is when drip started coming to California, and there were learning curves that had to be overcome, and there’s now 35 years of experience with drip irrigation. So all these things have to be taken into account when deciding.
Water Deeply: Does it require precisely pressurized water, and the vast network of valves and regulators that drip systems require?
Mitchell: It doesn’t have the same pressure requirements as the high-pressure impact sprinklers that you see some irrigators move from field to field. It doesn’t require that much pressure, but generally something on the order of 30 psi is what’s required.
What you need for center pivot, for instance, what happens in the installation, is you have a choice of water that is piped underground to the center pivot point in the middle of the field, then it is somehow pressurized and it goes out of the pipe above ground.
Another form is called the linear-move system, and that can also have a buried pipe, but it can have different kinds of canals or ditches that it draws water from and it pumps or pressurizes water there.
Water Deeply: What are the drawbacks?
Mitchell: The biggest thing that probably needs to be understood and recognized is you have to be able to match the water application rate to the crop and the soils. That’s how fast the water is coming out of the system into the soil. That’s very important. If you apply water faster than the soil can take it in, you’re going to have ponding of water, and it’ll start creating moving rivers of water on the surface, and that will cause nonuniformity.
In places where there’s a merging of these technologies, they don’t have these kinds of problems. But where you have bare fields where the soils haven’t been managed, the risk of having nonuniformity due to ponding of water, lack of infiltration and the risk of having evaporation is higher. So if you’re going into this with a clay soil that is very slow to take in water and don’t have some of these other kinds of additional benefits, you’re going to have problems. That’s going to result in nonunifomrity, and you’ll be back to square one.
The other thing would be, it can be very precise and automated, and you can manage it with your computer and your phone and be done with it. But you have to really understand the water-holding ability of your soil and how much water you’re putting on. Learning the day-to-day seasonal management for a given crop, and how to maintain high productivity yields, that is important as well.
Water Deeply: So, do you think overhead irrigation can help reduce California’s overall water demand?
Mitchell: Coupled with these practices I’m talking about, absolutely, yes. It may not be an absolute wave of conversion, but I think it’s inevitable the use of these systems will increase in certain sectors in California. I think that’s without question. But it’s not going to be irrigating every acre of land either, no.
What I have in mind here is, it’s truly the overall system. That’s where you see the benefits of addressing water shortage conditions. You need to have a lot of different tools or technologies brought to the table.
A big part of it is to reduce soil water evaporation by reducing tillage disturbance and preserving water residency to cool the surface down. That’s another kind of management practice that leads to the benefits we’re talking about. And that’s not new either. That’s been done in lots of other places.
In certain regions of the Great Plains – South Dakota and Nebraska and parts of Kansas – at one point in time they would have to fallow the ground every other year. In the early 1990s, they were realizing that was not economically sustainable. So what they did there was they developed alternatives for tillage management, using no-till systems where they would not till the soil between plantings. This reduced some of the water loss, and they ended up being able to crop with greater diversity and greater intensity, and it’s really transformed those systems there. That’s a whole new frontier for many areas of California to bring these practices together.
It’s not going to be an easy thing. I don’t want to give you the wrong impression. There’s a lot more work and a lot more innovation that has to occur in order for these things to be much wider spread here.