With bubbler irrigation, water is applied to the soil surface as a little stream, typically from a small-diameter tube (1–13 mm) or a commercially available emitter. Because the application rate generally exceeds the soil infiltration rate, small basins or furrows are needed to spatially control the water distribution and avoid runoff. Although bubblers are extensively utilized in landscape irrigation, they are not currently common in agricultural irrigation. Two major types of bubbler irrigation are available: Low-head or gravity (about 10 kPa) and pressurized (50–150 kPa) systems. In any case, the operating pressure is lower than that of most agricultural sprinklers. Design procedures for gravity-flow bubblers have been developed and are somewhat unique to this type of irrigation, and the design of pressurized bubbler systems is similar to the procedures for most microirrigation systems, as outlined in Chapter 6. While this chapter covers both bubbler types, the primary focus is on gravity flow.
This chapter discusses a specialized application of micro-irrigation systems— namely, use of biological effluent for crop production. The term “biological effluent” refers to wastewater that contains impurities derived from biological sources. Typical sources include human and animal metabolic waste and domestic and industrial food processing waste. Effluents from these sources contain organic and inorganic matter, in contrast to fresh water, which contains primarily inorganic matter of riparian origin. The application of the effluent back onto the grain-producing lands has conserved fresh water resources and reduced some fertilizer applications. Commonly, the livestock wastewater is applied with sprinkler or surface irrigation systems. Subsurface drip irrigation is a new technology for livestock wastewater application. This chapter considers biological effluent to be a resource. Consequently, the approach is to emphasize the efficient use of the resource rather than disposal of a waste product. Traditionally, most effluents have been applied by sprinkler or surface irrigation systems. However, continued advances in micro-irrigation system design, management, monitoring, and especially in water treatment have expanded the use of biological effluent through micro-irrigation systems. Even though effluents contain many types of materials that can readily clog emitters, the advantages strongly favor the use of such effluents for agricultural irrigation.
Microirrigation with partial soil wetting from point or line water sources often applied at high frequency gives rise to unique soil water regimes where soil transport properties are more important than soil water storage capacity. We review key soil variables and functions that are required for irrigation design and management and emphasize multidimensional flow patterns and their mathematical representation. Knowledge of wetting patterns is important for the selection of emitter discharge rates and spacing to efficiently supply crop and tree root zones without excessive water loss to runoff and deep percolation. We present steady-state-based models for common geometries used in microirrigation (surface and subsurface emitters, drippers, and lines) that can assist irrigation system designers with linking emitter characteristics with soil properties and crop requirements. The chapter contains average parameters for different soils that can be used as first estimates for use in numerical or analytical models relevant to the irrigation system of interest.
In bubbler irrigation, water is applied to the soil surface as a little stream, typically from a small diameter tube (1 mm to 13 mm) or a commercially available emitter. Because the application rates generally exceed the soil infiltration rates, small basins or furrows are needed to control the water distribution on the land. Although bubbler application is extensively utilized in landscape irrigation systems, its use in agriculture is currently limited. Two major types of bubbler irrigation systems are available; the low head or gravity (about 10 kPa) and pressurized (50 to 150 kPa) systems. Design procedures for gravity systems have been developed over the past several years and are relatively unique to this type of irrigation. Design of pressurized bubbler systems is similar to the procedures for most micro-irrigation systems. This chapter primary discusses the gravity systems. Two of the first long-term, operating bubbler irrigation systems were established in citrus groves at Tacna, Arizona and Riverside, California. Water to the systems was supplied from irrigation canals and distributed through thin-wall, corrugated polyethylene pipe to the bubbler tubes. By adjusting the elevations of the tube outlets, the gravity pressure water flow to each tree was equalized. Despite this early experimental success, the bubbler concept has not been widely adopted in agriculture. Perhaps one of the main reasons for the lack of interest is that design criteria and recommended operating procedures have not been readily available.
Chemigation is defined as the application of a chemical with irrigation water. Chemicals are injected into micro-irrigation systems to fertilize crops, to control clogging, and to control pests. A properly designed chemigation system includes a pumping plant, chemical injector, storage tanks for various chemicals, calibration devices, a backflow preventor, and adequate safety equipment. Chemigation is required for sustainable operation of micro-irrigation systems. This practice includes the injection of chlorine or other biocides, acids, or chelating agents to reduce emitter clogging due to calcium carbonate precipitation, prevent bacterial growth in emitters, laterals, filters, and pipe lines, as well as preventing root intrusion into emitters on subsurface drip irrigation (SDI) systems. Micro-irrigation systems are frequently used to apply water-soluble fertilizers. In addition, soil amendments (e.g., acids, various polymers, CaSO4), plant growth regulators, insecticides (usually systemic), herbicides, nematicides, and other compounds can be efficiently and effectively applied through micro-irrigation systems. SDI systems are particularly amenable to the application of soil fumigants, as well as other chemicals that tend to be fixed by the soil particles (e.g., some pesticides and specific fertilizer formulations). Many different chemicals can be injected into micro-irrigation systems, and each must be handled according to its intended use, physical properties, and associated legal requirements. Micro-irrigation must be managed as both a water and chemical application system. Water management is critical and personal safety protection must be assured for any successful chemigation program.
Publisher Summary Irrigation scheduling generally determines the time of the next event and the amount of water to apply. For micro-irrigation this is the decision of when to start an irrigation cycle and how long to irrigate the zone or set. Scheduling micro-irrigation is inherently different from other irrigation methods, because the application amount per irrigation is small and the applications are typically more frequent. Martin et al., Heermann et al., and Hill provide a thorough discussion of irrigation scheduling principles. This chapter discusses the principles and application techniques applicable to micro-irrigation systems. Micro-irrigation scheduling integrates elements of the system hydraulic design and maintenance together with various aspects of the soil and the crop characteristics with the atmospheric evaporative demand. It involves providing managers with the irrigation needs of the crop that must be organized together with the cultural aspects of growing and harvesting the crop. Irrigation scheduling involves long-term decisions (strategic) and short-term decisions (tactical) that must consider the producers' risks and management goals in harmony with the agronomic or horticultural requirements for the crops being grown across the irrigation block, field, farm, or even across a broader scheme (— that is, irrigation district or hydrologic basin). Micro-irrigation scheduling is generally controlled by (1) measuring or estimating crop water needs, (2) measuring a soil water status, or (3) measuring a plant water status property. The latter two conditions are frequently used to determine irrigation needs and are easily integrated into an automated control system.
A comprehensive review of published information on subsurface drip irrigation was performed to determinethe state of the art on the subject. Subsurface drip irrigation has been a part of drip irrigation development in the USAsince its beginning about 1960, but interest has escalated since the early 1980s. Yield response for over 30 crops indicatedthat crop yield for subsurface drip was greater than or equal to that for other irrigation methods, including surface drip,and required less water in most cases. Lateral depths ranged from 0.02 to 0.70 m and lateral spacings ranged from 0.25to 5.0 m. Several irrigation scheduling techniques, management strategies, crop water requirements, and water useefficiencies were discussed. Injection of nutrients, pesticides, and other chemicals to modify water and soil conditions isan important component of subsurface drip irrigation. Some mathematical models that simulate water movement insubsurface drip systems were included. Uniformity measurements and methods, a limited assessment of root intrusion intoemitters, and estimates of overall system longevity were also discussed. Sufficient information exists to provide general guidance with regard to design, installation, and management ofsubsurface drip irrigation systems. A significant body of information is available to assist in determining relativeadvantages and disadvantages of this technology in comparison with other irrigation types. Subsurface drip provides amore efficient delivery system if water and nutrient applications are managed properly. Waste water application, especiallyfor turf and landscape plants, offers great potential. Profitability and economic aspects have not been determinedconclusively and will depend greatly on local conditions and constraints, especially availability and cost of water.
Microsprinkler systems are primarily used to irrigate tree and vine crops. The system designs are similar to other microirrigation systems, but microsprinkler systems generally tend to require a higher flow rate per unit area. Typically, microsprinkler installations have one 40–75 L/h flow rate emitter per tree. The most common spray emitters have slotted caps or deflector plates that distribute water in distinct streams. Other designs (spinners) have a moving part that rotates to disperse the water stream more uniformly over the wetted diameter. During the design stage, long-term maintenance costs should be included in the economic analysis. The emitter selection process should consider uniformity as well as other factors, such as cost, wind effects, system constraints, maintenance, and soil type, so that the best emitter for a particular field condition is selected. Microsprinkler systems should be capable of applying the maximum amount of crop water needed, plus any application inefficiency. The emitters should have a coverage area sufficient to allow this volume of water to be stored in the soil without causing unintentional deep percolation. Emitters should also have a sufficient application rate in the wetted area so that the required run times are compatible with the power unit and labor availability. High application rates can lead to the leaching of nutrients and pesticides. By adjusting spray diameter, irrigation duration, and emitter flow rate, systems can be managed to meet tree water needs while minimizing overirrigation and chemical leaching.
When compared with other irrigation systems, subsurface drip irrigation (SDI) has advantages and disadvantages that should be carefully considered. There are many design and management similarities to surface drip irrigation (DI), but there are also some unique differences that affect uniformity, operation, and system longevity. Factors that affect SDI uniformity are emitter clogging, root intrusion, root pinching, mechanical and pest damage, soil overburden and compaction, soil hydraulic parameters, and, possibly, system age. A typical SDI system often requires additional components, compared to DI, such as flushlines, additional air/vacuum relief valves, and pressure gauges and a flowmeter for system monitoring. Emitter flowrate and spacing, and dripline diameter, wall thickness, spacing, and depth are all important design criteria for SDI systems. Flushing of SDI driplines is also a key design criterion, and some designers prefer to begin their design with the flushing system. SDI can potentially provide a more consistent soil water and nutrient environment for optimum crop growth, but there can also be challenges in some regions, such as crop establishment, salinity management, soil water redistribution, and application of some agrochemicals. The application of SDI for some of the lower-value grain and fiber crops has been increasing, and this trend is likely to continue.
Emitter clogging continues to be a major problem in micro-irrigation systems. For high-valued annual crops and for perennial crops, where the longevity of the system is especially important, emitter clogging can cause large economic losses. Even though information is available on the factors causing clogging, controlled measures are not always successful. Problems can be minimized by appropriate design, installation, and operational practices. Reclamation procedures to correct clogging increase maintenance costs, and unfortunately, may not be permanent. Reliable operation of micro-irrigation systems depends upon preventive maintenance. A preventive maintenance program should include water filtration, field inspection, pipeline flushing, and chemical water treatment. A suitable combination of type, size, and capacity filter unit is required. Appropriate procedures should be followed for the field inspection and flushing of micro-irrigation systems. Chemical water treatment should be properly selected for maintaining emitter performance. Because water quality is of primary importance in the design and operation of the system, adequate water analysis should be made and evaluated on the basis of past experience, such as the water classification scheme presented to evaluate the clogging potential of the micro-irrigation water source.
The farm-level economic implications of microirrigation will vary among farms and regions, and among crops and resource endowments. Farmers producing high-valued crops in arid and semi-arid regions have a greater likelihood of gaining financially by adopting microirrigation than farmers producing low-valued crops in humid and sub-humid climates. The farm-level value of water saved by reducing irrigation requirements will vary with the cost and availability of irrigation water, and with the cost of energy and the labor required to obtain and deliver the water. Farm-level gains due to increased yields will vary with the crops produced and prices received.
For full potential of microirrigation to be achieved, automation is necessary. Improved water use efficiency, improved fruit quality, and increased yield require both the accurate placement of the required volume of water and the accurate timing of the application. This is most easily achieved through automation. Automating the control of a microirrigation system may be staged as the operator gains confidence and experience with the system. Initial control may be with an open loop system with the operator setting the irrigation frequency and duration. Later, a closed loop system using a simple feedback control such as switching tensiometer to start the system for a fixed run time can be used. The final step may be the implementation of a closed loop system using soil water sensors such as TDR or FDR to both initiate and stop irrigation. Alternatively, a scheduling approach based on calculated crop water use from measured weather data or evaporation pans could be used.
There is widespread evidence for the prevalence of problems in crop production which are attributable to soil compaction caused by the passage of vehicles, implements and draft animals. Agricultural, horticultural and forestry crops are known to experience these problems in both temperate and tropical regions. Soil compaction problems were experienced in commercial production long before any coherent research was undertaken on this subject. During the early part of the 19th century, draft animals were observed to cause soil compaction during cultivation, while during the second half of the 19th and early years of the 20th centuries the use of steam engines for cultivation was accompanied by excessive compaction, unless cable traction was employed or soils were extremely dry. The introduction of the internal combustion engine for small tractors did not initially lead to widespread compaction problems but by the middle of the 20th century, and particularly during the past 30 years, mechanization has advanced to such a scale and intensity that compaction problems have become of worldwide importance. Soil compaction is now considered to be a multi-disciplinary problem in which machine/soil/crop/weather interactions play an important role and which may have dramatic economic and environmental consequences in world agriculture. However, recent progress in scientific understanding of the soil compaction process and its implications, improved insight into proper vehicle use and soil management, and the development of mechanization systems and novel running gear, provide new perspectives for reducing soil compaction problems in crop production.
Soil compaction concerns are increasing in the temperate region of North America. Increased size and weight of field equipment, plus changes in farming patterns to a predominate row-crop culture, are contributing to this problem. Soils in the region are annually subjected to a freezing and thawing cycle plus several wetting and drying cycles. However, these natural forces do not necessarily ameliorate soil structural changes resultant from compaction. Yield decreases from soil compaction have been identified for all investigated crops grown in the region, but responses have been variable and in some cases difficult to assess, although compaction problems are magnified when combined with other plant stress situations. Soil compaction effects have been identified by increased bulk density, higher soil strength and reduced pore volumes, which in turn exert a negative influence on soil aeration, root exploration, and water and nutrient uptake. Crop responses to compaction are strongly influenced by soil texture and soil water content at the time of load application. However, crop responses are not always negative, as they are also a function of growing season precipitation. Subsoil compaction, which is mainly due to high axle loads, is causing increasingly serious concerns about future productivity. Subsoil compaction is long-lasting and difficult to correct. In the temperate region of North America, efforts to reduce soil compaction have been limited and compaction will remain a production problem until changes in farm practices are made.
