by Bill and Mary Weaver
Subsurface Drip Irrigation (SDI) can be pricey to install- about $1,000 to $1500 per acre on average in northern California, according to Dr. Timothy Hartz of UC Davis, although installation prices vary widely with the type of system.
Still, SDI has appeared to be the perfect irrigation system for water-starved California growers and farmers, including growers of processing tomatoes in northern California. “SDI is used on more than 50 percent of processing tomato acreage, and with anything else grown in rotation. If the SDI system has good filtration, periodic flushing, chlorination and is a properly designed and installed system with buried drip tape, it can be used for many years,” continued Hartz. “This, of course, lowers the per-crop cost of the system installation.”
SDI preserves moisture by preventing surface evaporation. It places the water right in the root zone where it is used. Leaves stay dry, helping to prevent foliar diseases. Underground, rather than overhead irrigation also helps to eliminate herbicide washout. Also, workers can enter fields immediately after irrigation, because the ground surface will be dry.
There are also very clear water savings compared with other types of irrigation systems, but the savings will depend on the efficiency of the system that SDI replaces. “If you think in terms of water use efficiency (tons of fruit per inch of water applied), the combination of higher yield and water savings can increase water use efficiency by 50 percent or more,” stated Hartz. Again, the exact amount depends on the efficiency of your previous system. “Experience with processing tomatoes suggests that yields rise 15 to 25 percent on average, when a field is converted to SDI.”
SDI use is compatible with no-till, minimum till, and conservation tillage practices, which won’t disturb the buried drip lines. Systems are frequently installed using GPS systems, so the growers will know the precise location of each buried line.
However, this seemingly ideal irrigation system for northern California during the recent drought has its nemesis: the pocket gopher. “Subsurface drip irrigation creates an ideal habitat for these gophers,” said Dan Putnam, UC Davis. This is particularly true in western alfalfa fields, where the soil is not disturbed from year to year, and where the critters are surrounded by one of their favorite foods.
Although the biggest pocket gopher problems with SDI are in alfalfa fields, there is the potential for damage to the buried irrigation systems in any field that has a population of pocket gophers, according to UC Davis wildlife specialist Roger Baldwin. “The higher the population of pocket gophers in your area, the bigger the potential problem for buried systems, whatever the crop.” These gophers will gnaw everything in their path, including buried drip lines.
Pocket gophers can actually be attracted to fields recently converted to SDI. Digging up the soil to run water lines and drip tape leaves the soil less compacted, which makes for easier movement by the constantly burrowing and gnawing pocket gophers. It has also been found that they reproduce more rapidly in the ideal environment of irrigated fields.
In some areas, professional on-farm gopher monitoring and control is available. Growers can use commercially available baits, traps and repellants, as well as owl boxes to house predators.
The possible use of SDI irrigation in potato fields in eastern Oregon, an area of low rainfall sometimes referred to as “high desert,” has been studied. Research has shown that use of SDI significantly reduces water use in growing potatoes. Nutrients can be more effectively applied through fertigation in subsurface drip, and late blight and other fungal diseases have been reduced because potato leaves stay dry.
Researchers have also found that using SDI produces higher quality potatoes, with slightly higher yields. The differences in tonnage, however, have not been significant enough to cover the high cost of installing SDI — most growers believe — and potato growers have been slow to adopt the system.
A hybrid irrigation system, a cross between center pivot and drip, has shown promise in some areas of California where it has been tried. The initial cost is not prohibitive. Growers simply use their already installed center pivot systems. Drip lines, long enough to reach and drag along the ground, are attached to the center pivot sprinkler drops.
In addition to releasing the water right at the soil surface, minimizing evaporation, the water is spread out over the soil surface as the center pivot turns, so it is quickly absorbed.
This system keeps both the leaves and the wheel tracks dry. Since water applied to the wheel tracks is wasted, water consumption by this hybrid system is cut compared to the usual center pivot system.
These hybrid systems are still relatively new, but the manufacturer has tested them with many field and vegetable crops, and has found it to be particularly valuable on uneven terrain.
In other west coast irrigation research, David Bryla, USDA-ARS, who presented his findings at the Great Lakes Expo, stated that during the third and fourth year after planting, marketable yield and berry weight were higher with drip irrigation than with sprinkler or microsprinkler use. He emphasized that the blueberry is an extremely shallow rooted crop that needs to be irrigated frequently to avoid water stress, particularly when the fruit are sizing.
UC Davis personnel will be part of a different type of water conservation study, for which funds were recently granted. Other states, including Michigan and a number of eastern states, have done work establishing that adding organic matter to the soil, particularly with sandy soil, can tremendously increase its water-holding capacity. UC Davis and other California locations working will be taking that a step further by studying the role of carbon (which feeds microscopic beneficial soil dwellers which help to form soil aggregates) in the water-holding capacity of soils.
Researchers will study the microbiological and other soil factors that affect the desired soil aggregate formation and stability, using a number of row crops including tomatoes, and different farming practices including carbon inputs, rotations and irrigation methods. They plan to design better water management strategies based on their findings.
Irrigation, northern California and north
by Bill and Mary Weaver