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.
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.
