Subsurface drip irrigation (SDI)

Subsurface drip irrigation (SDI) has been a part of agricultural irrigation in the world for about 40 years but interest has increased rapidly during the last 20 years. Early drip emitters and tubing were somewhat primitive in comparison to modern materials, which caused major problems, such as emitter plugging and poor distribution uniformity. As plastic materials, manufacturing processes, and emitter designs improved, SDI became more popular but emitter plugging caused by root intrusion remained a problem. Initially, SDI was used primarily for high-value crops such as fruits, vegetables, nuts, and sugarcane. As system reliability and longevity improved, SDI was used for lower-valued agronomic crops, primarily because the system could be used for multiple years, reducing the annual system cost. Design guidelines have also evolved to include unique design elements for SDI, including air entry ports for vacuum relief and flushing manifolds. Specific installation equipment and guidelines have also been developed, resulting in more consistent system installation, improved performance, and longer life. Crop yields with SDI are equal to or better than yields with other irrigation methods, including surface drip systems. Water requirements are equal to or lower than surface drip and fertilizer requirements are sometimes lower than for other irrigation methods. Interest in the use of wastewater with SDI has increased during the last decade. The future of SDI is very promising, including its use in wastewater systems, and especially in areas where water conservation is important or water quality is poor. SDI is a very precise irrigation method, both in the delivery of water and nutrients to desired locations and the timing and frequency of applications for optimal plant growth.

CURRENT STATUS AND USE

Design of SDI systems is similar to that of surface drip systems, especially with regard to hydraulic characteristics. However, special attention to water filtration, proper number and location of air-vacuum relief and check valves, pressure regulation, flow measurement, and flushing is required for successful SDI systems. Air-vacuum relief valves are needed to prevent aspiration of soil particles into emitter openings when the system is depressurized. Water filtration is often more critical for SDI systems than for surface drip systems because the consequences of emitter plugging are more severe and more costly. The specific crop and soil essentially determine the system capacity, emitter spacing, and lateral depth and spacing. If water supplies are not limiting, system capacity, along with effective precipitation and stored water, must satisfy peak crop water requirements.

For multiple-year, long-term SDI systems, the lateral depth should be deep enough to prevent damage by tillage equipment but shallow enough to supply water to the crop root zone without wetting the soil surface. Generally, laterals in SDI systems are installed at depths of 0.1-0.5m, with shallower depths on coarse-textured soils and slightly deeper on finer-textured soils. In some cases, surface wetting is required for seed germination and seedling establishment. In SDI systems, surface wetting occurs primarily from longer irrigation durations and when the emitter flow rate exceeds the hydraulic conductivity of the soil surrounding the emitter. It is affected by many variables, including soil texture, lateral depth, emitter flow rate, and soil compaction. Emitter spacing and flow rate are determined by crop rooting patterns, lateral depth, and soil characteristics, but emitters should provide overlapping wetted areas along the lateral for most row crops. Lateral spacing is determined primarily by the soil, crop, and cultural practices and should be narrow enough to provide a uniform supply of water to all plants and to manage salinity, if necessary. For row crops, laterals should be parallel to crop rows and crops should be planted at the same location relative to the row each year, usually at consistent row spacings. Generally, laterals are spaced 1- 2 m apart in row crops but may be spaced closer for irrigation of pasture, forage crops, and turf.

Many types of tubing have been used successfully for SDI. Thin-walled (0.15 mm to 0.30 mm), flexible tubes are typically used more for short-term installations and typically are installed at shallow depths. Thicker-walled (0.38 mm to 0.50 mm), flexible tubes have been used successfully for several years provided that they are installed deep enough to avoid tillage, cultivation, and harvest equipment. In non-bridging soils where the soil profile does not provide sufficient support, thicker-wall tubing (>1.50 mm) may be required to prevent deformation or collapse of the tubing by equipment or soil weight. Rigid tubing with even thicker walls is often used on perennial crops or on annual crops for longer time periods (>10 years).

Most SDI laterals are installed by tractor-mounted shanks equipped with feed tubes mounted on the backside of the shank to install the tubing at the proper depth. During installation, care should be taken to avoid stretching or kinking of the tubing. Care should also be taken to install the drip tubing at a uniform depth throughout the field, especially around the field perimeter, so that there is no question about the maximum allowable tillage depth. During installation, drip laterals should be oriented so that the emitters are on top to minimize plugging from particulate matter that accumulates along the bottom of the lateral. Manifold or header pipes are installed deeper than laterals to reduce interference with tillage operations, to prevent them from draining, to accelerate pressurizing the system, to avoid damage from field equipment, and to prevent particulate matter from entering the lateral. Manifolds, mains, and sub-mains are plastic, usually PVC, and are connected to laterals using a variety of connectors, depending upon the type of tubing. When flushing manifolds are a part of the system, these manifolds should be carefully sized to provide sufficient velocity to transport particles out of the system.

SDI is currently being used on a wide variety of crops, including tree, fruit, vine, agronomic, pasture, landscapes, and turf. In a review of published research results for SDI, Camp (1998) listed over 30 different applications. Most applications were for food and fiber crops, but some applications included trees, turf, and landscape plants, especially with recycled or waste water sources. While this review related primarily to published research results, the mix of applications should be somewhat representative of commercial practices. For SDI use on fruit and vegetable crops, tomato (fresh market and processing) was the most popular, followed by lettuce, potato, and sweet corn. Other crops included apple, asparagus, banana, bell pepper, broccoli, cabbage, melons, carrot, cauliflower, pea, green bean, okra, onion, papaya, rape, squash, and floriculture. For agronomic crops, cotton and corn were the most popular. Others included alfalfa, grain sorghum, peanut, pearl millet, and wheat. There are multiple reasons for adoption of SDI on specific crops. For example, plant diseases may be reduced on crops such as strawberry by using plastic mulch and SDI, which keeps the soil surface and foliage relatively dry. Multiple-year use of SDI may reduce the annual system cost so that it is profitable for use with lower-value crops such as cotton and corn. The precise placement and management of water and fertilizers, an inherent capability of SDI, is an important factor with tree and vine crops.

The water supply capacity directly affects the design and operation of a SDI system. The size of the irrigated field or zone is often controlled by the available capacity of the water supply. For example, in some humid areas, when high-capacity wells are not available multiple low-capacity wells can be distributed throughout a farm. Fortunately, with SDI, the size and shape of fields can be economically adjusted to correspond to water supply capacity and other factors (O=Brien et al., 1998). Quality of the water supply is extremely important and significantly influences the type of water filtration required. Generally, the better the water quality the less complex the filtration system, but surface, recycled, or waste water supplies require the most elaborate filtration. However, good filtration is the key to good system performance and long life, and should be a major concern in system design. In some cases, chemical injection may be required to adjust acidity, to control biological activity, or to correct other water supply deficiencies. Several reports with special emphasis on water supplies (saline, deficit, and wastewater) for SDI were listed by Camp (1998).

Emitter plugging caused by root intrusion can be a major problem with SDI systems, but can be minimized by chemicals, emitter design, and irrigation management. Chemical-based control techniques include the use of herbicides, either slow-release growth-retarding compounds embedded into emitters and filters or periodic injection of low-concentration solutions into the irrigation stream, or injection of other chemicals, such as fumigants, into the irrigation stream. Periodic injection of phosphoric acid and chlorine can modify the environment immediately adjacent to emitters and reduce root intrusion. Emitters that are plugged by roots may be cleaned using periodic injections, such as acids and chlorine, but some of these chemicals can cause long-term, adverse soil effects and/or damage other system components.

Emitter design may also affect root intrusion. Smaller orifices tend to have less root intrusion but are more susceptible to plugging by particulate matter. Some emitters are constructed with physical barriers to root intrusion. Root intrusion appears to be more severe where emitters are located in areas of preferential root growth, such as along seams of thin-walled tubes. However, root intrusion problems appear to be greater for emitters, tubes, and porous tubes that are not chemically treated.

Irrigation management can influence root intrusion by controlling the environment immediately adjacent to the emitter. High frequency pulsing that frequently saturates the soil immediately surrounding the emitter can discourage root growth in that area for some plants but not others. Conversely, deficit irrigation, sometimes practiced to increase quality or maturity or to control vegetative growth, can increase root intrusion because of high root concentrations in the emitter area. When injecting chemicals into SDI systems, the entire system should be thoroughly flushed after each injection event.

SDI systems offer the potential for precise placement and management of water, nutrients, and pesticides if the system is properly designed and managed. Water can be applied in a variety of modes, varying from multiple continuous or pulsed applications each day to one application in several days. Likewise, fertilizers can be injected into the irrigation water and delivered to the plant root zone at similar frequencies. The application frequency used depends upon several factors, including soil characteristics, crop requirements, water supply, system design, and management strategies. When proper flow measuring devices are provided, SDI systems can be managed to optimize use of limited water supplies. Camp (1998) reviewed several reports comparing application frequency using SDI, reporting that some crops responded to high-frequency irrigation while others did not.

Some systemic pesticides and soil fumigants can be safely injected via SDI systems. This technique has the potential to minimize chemical exposure of workers and environment contamination, to reduce the cost of pesticide rinse water disposal, and to improve precision of application to the desired target (e.g. root pests). However, a high level of management with system automation and feedback control is required to minimize chemical movement to the ground water when chemicals are used.

SDI must be managed properly to obtain an acceptable (economical) system life (>10 years), especially when used in lower-valued crop production systems. A consistent, proactive management strategy of preventing problems, such as emitter plugging, is required instead of one where components are repaired or replaced after they fail. Locating and repairing or replacing failed components is more difficult and more expensive with subsurface systems than with surface systems. In fact, a major complaint with SDI from system managers is that most system components are buried and cannot be directly observed. Consequently, operational parameters such as frequent measurements of flow rate and pressure must be used as indicators of system performance. Global positioning systems (GPS) can now be used to better locate specific points. A major factor in maintaining consistently good system performance is constant attention to maintaining good water quality and proper filtration. Periodic system flushing to remove particulate matter is also used to prevent emitting plugging. Some commercial systems in the southwestern USA have been in use for almost 20 years. Research-based SDI systems have performed well for 8-11 years with little indication of degradation (Phene et al., 1992; Camp et al., 1997; Lamm and Trooien, 1999).

When properly managed, subsurface drip is one of the most efficient irrigation methods with typical application efficiencies exceeding 90% . In a review, Camp (1998) found that yields for subsurface drip irrigated crops were equal to or greater than yields from other methods of irrigation. He also found that the water requirement for SDI systems was generally similar to or slightly less than for any efficient, well-managed irrigation system. Some investigators reported irrigation water requirements as much as 40% less than for other irrigation methods.

Typically, irrigation applications are very uniform for SDI systems that have been properly designed, installed, and maintained. Assessment of system application uniformity is more difficult for SDI systems than for surface drip irrigation because most components are buried and not available for direct assessment. Indirect methods, including computer models, are available to assist in uniformity evaluations (Phene et al., 1992; ASAE, 1999b; Burt et al., 1999). However, once significant emitter plugging has occurred, many of the statistical sampling methods are unreliable because basic assumptions used in their development are no longer satisfied (Camp et al., 1997).

Compared to conventional surface drip systems, concentration of salts on or near the surface causing germination and other problems tends to be reduced under properly designed and managed SDI systems. However, salinity may still be a problem with SDI in arid and semi-arid areas since any leaching above the tubing occurs only as the result of rain. Thus, salts tend to accumulate in this area during the season as the plants extract water and leave the salts behind. High salt concentrations exceeding 10 dS/m have been found in the top 6-10 cm of the soil profile (Ayars et al., 1995). Salinity distribution measurements have shown that salts were moved to below the plant row when the laterals were placed under the furrows rather than under the beds (Ayars et al., 1995). The chloride concentration increased as the distance from the lateral increased.