Efficiency of Drip and Overhead Irrigation Systems
Carol Miles, Martin Nicholson and Madhu Sonde, WSU Vancouver Research and Extension Center
Irrigation is critical for successful summer vegetable production in the Pacific Northwest. Irrigation should be efficient and effective to avoid over or under application. Over application is a wasteful use of a natural resource, may lead to erosion and surface water or ground water contamination, and costs money. Under application can result in yield depression or crop loss. Efficient irrigation systems require the selection of an appropriate method for the crop being grown, adequate monitoring of the irrigation system and of water delivery, and appropriate application rates depending on the growth stage of the crop. In western Washington, vegetable growers most commonly use a solid set overhead sprinkler irrigation system while fruit growers use drip or trickle irrigation systems. Advantages of drip irrigation systems as compared to overhead sprinkler systems include reduced water use, reduced erosion and runoff potentials, and decreased weed growth. The installation of an efficient irrigation system begins with good design and choosing appropriate pipe and emitter sizes to assure adequate water delivery. Through drip irrigation, also called trickle or micro-irrigation, water is applied slowly and directly to the roots of plants through small, flexible 3/8 to 3/4-inch diameter flexible plastic pipes and flow-control emitters or by perforated or porous pipes.
In this study, we evaluated and demonstrated the effects of drip and overhead sprinkler irrigation systems on water usage, weed growth and yield of winter squash.
This unreplicated study was conducted over three years (2001 – 2003) at the WSU Vancouver Research and Extension Unit.Overhead sprinkler and drip irrigation systems were set up in adjacent blocks within the same field, but far enough apart to avoid cross irrigation. Winter squash was planted in rows spaced 6 feet apart, and plants were spaced 3 feet apart in the row (Figure 1). The overhead irrigation system included impact rotor type sprinklers with aluminum pipes and 12-inch risers spaced 40 feet apart in the field. This system is referred to as a solid set sprinkler system and is the most common irrigation system used by vegetable growers in the region. Irrigation rate was 0.25-inch of water per acre per hour, or 6,789 gallons per acre per hour. The drip irrigation system included T-Tape TSX Tree & Vine 515 model with emitters spaced 36 inches apart. Irrigation rate was 0.570 gallons of water per plant per hour, or 1,3794 gallons per acre per hour. In the drip system, irrigation was applied twice a week for 4 hours each application, and in the overhead system, irrigation was applied once a week for 4 hours. Drip irrigation was applied more frequently than sprinkler irrigation due to the slower water delivery rate of the drip system.
We evaluated and compared weed growth in the overhead and drip irrigation systems. We collected ten samples (50 in2 per sample) from both irrigation systems and sorted, identified, counted and weighed weeds in each sample. We also measured weed fresh and dry weights. After weed samples were collected, the winter squash plots were thoroughly weeded.
Figure 1. Drip irrigation was installed in the field (left), plants were transplanted next to emitters (center), and plants and drip tape were buried in a raised bed.
Figure 2. Ten weed samples (50 in2 per sample) were collected from both drip (left) and overhead (right) irrigation systems in late June 2002.
In 2001, drip and overhead sprinkler systems worked well and without mishap. In both 2002 and 2003, the drip irrigation system developed problems that resulted in a significant reduction in water delivery to many plants early in the study (Figure 2). As a result, crop yields were significantly reduced both these years in the drip system. Yield reduction was due to human error and not due to the irrigation system. However, our mistakes point out what are likely the biggest challenges of using drip irrigation. First, it is necessary to appropriately design and constantly monitor the system throughout the field to assure adequate water delivery. And second, adequate irrigation every week for the first 3–4 weeks following transplanting is critical for crop survival and production. In 2002, drip tape emitters were blocked following weeding in the rows. In 2003, the header pipe was not large enough to provide adequate pressure to deliver water down the rows. Both years, plants were not sufficiently irrigated for the first two weeks following transplanting, and this error resulted in dead plants or plants that were significantly reduced in growth and delayed in maturity. Although this error significantly impacted crop yield, we feel the affect on water usage and weed growth both years was minimal as water delivery was only restricted the first two weeks of the trial.
Figure 3. Winter squash grown under drip (left) and sprinkler overhead (right) irrigation in 2002. Under drip, water delivery the first two weeks after transplanting was somewhat adequate for the first half of each row and was heavily constricted for the remainder of each row, resulting in dead plants where water was not adequate.
Water Use. On average over the three years in this study, we used 50% less water with drip irrigation as compared to overhead irrigation (Table 1). The amount of water used in 2001 was less than half of what we used in 2002 and 2003. This was due to greater summer rainfall in 2001. We scheduled our irrigation each week based on precipitation that week, and if it rained more than 1-inch we did not irrigate. If it rained less than 1-inch, we applied enough irrigation water so that combined with precipitation only 1-inch of water was applied to the field.
Weed Control. On average over the three years in this study, there were 50% fewer weeds, and weed biomass was 75% less in the drip system as compared to the overhead system (Table 2, Figure 3). In the drip system, water is only delivered to the root zone of each winter squash plant whereas in the overhead sprinkler system, the entire field receives a uniform amount of water resulting in more weed growth throughout the field.
Figure 4. Weed growth in mid-summer under drip (left) and overhead sprinkler winter squash plots in 2003. Weed samples were collected in early July and plots were then thoroughly weeded.
Winter Squash Yield. Plot size and winter squash varieties varied in this study each year, therefore it is not possible to compare yield among years. Drip and overhead irrigation plot sizes were the same each year, therefore it is possible to compare yield between treatments each year. In 2001, yield and fruit number under drip irrigation were greater than under overhead irrigation (Figure 5). In 2002 and 2003, technical errors in the water delivery system interrupted drip irrigation for the first two weeks of this study, and this greatly impacted squash yields. These two years, yield and fruit number were greater under overhead irrigation than under drip irrigation. Average fruit weight in all three years was greater under drip than under overhead irrigation. That is, winter squash were larger when drip irrigation was used.
Figure 5. Winter squash yield under drip (left) and overhead sprinkler (right) irrigation in 2001. Squash are lined up by variety and both total fruit weight and total fruit number were greater under drip irrigation.
The drip system used 50% less water, and produced 50% fewer weeds with 75% less biomass than the overhead system. In this study, winter squash yield was only greater in the first year under drip irrigation than under overhead irrigation. This was due to problems that arose in the field in the second and third years of the study that resulted in restricted water flow in the drip system for the first two weeks after transplanting. The disadvantages of drip irrigation systems include: more time required for initial installation; greater up-front costs; and regular monitoring and maintenance to assure adequate water delivery. The drip tape emitter holes can become clogged with soil particles, algae or mineral precipitates. Insects, rodents and mammals can damage emitters or the drip tape. The disadvantages of the overhead sprinkler system include greater water usage, greater weed growth, soil erosion potential (Figure 6), and surface water or ground water potential contamination. We feel that with adequate design and monitoring in the field, the advantages of the drip system far outweigh its disadvantages.