14.3: System Types

Microirrigation includes trickle, drip, subsurface, bubbler, and spray irrigation systems (ASABE, 2019). Trickle and drip irrigation are often considered synonymous and we will refer to it as drip irrigation in this book. Here we differentiate these systems into four major types based on installation method, emitter design, or mode of operation. The four types are:

• Surface drip—water applied slowly through small emitter openings to the soil surface.
• Microspray—water sprayed over the soil surface at relatively low pressure (also called microsprinkler).
• Bubbler—a small stream of water applied to flood the soil surface in localized areas.
• Subsurface drip—water applied through emitter openings below the soil surface.

Examples of these four types of microirrigation are shown in Figure 14.1.

Figure 14.1. Examples of microirrigation systems: (a) surface drip (photo courtesy of Toro); (b) microspray (photo courtesy of Texas International Irrigation); (c) bubbler (photo courtesy of Anchor Commodities); and (d) subsurface drip (photo courtesy of Freddie Lamm, Kansas State University).

a) b)

c) d)

Surface Drip

A surface drip system consists of water applicators designed directly into or attached to lateral lines that are laid on the soil surface (Figure 14.1a). This method is also called trickle irrigation and is the most prevalent type. The maximum application rate for individual emitters is normally less than 3 gallons per hour (gal/hr). For porous tubing and other multiple outlet systems, the maximum application rate is generally less than 1 gallon per hour per foot of lateral. Advantages of surface drip irrigation over other microirrigation systems include ease of installation, inspection, repair, and cleaning of emitters. A major advantage is the ability to check soil surface wetting patterns and to measure individual emitter discharge rates. In some crops like grapes, which are trellised, the laterals are suspended above the soil surface by attachment to the trellis. This eliminates physical damage to the system from human traffic and cultural operations.

Microspray

Microspray systems apply water to the soil surface as a small spray, jet, fog, or mist (Figure 14.1b). Aerial distribution is a major component in the distribution of water by microspray systems compared to the other types where distribution of the water occurs primarily in the soil. Discharge of individual microspray applicators is typically less than 50 gal/hr. A comparison between the soil wetting pattern for surface drip and microspray systems is given in Figure 14.2. Microspray systems are used frequently to irrigate trees and other widely spaced agricultural crops. Microspray systems are vulnerable to drift and evaporation losses. Major advantages of microspray systems are minimal filtration needs and that maintenance requirements are small. Like drip, microspray systems are sometimes suspended above the soil surface.

Figure 14.2. Comparison of one half of the soil wetting pattern between surface drip and microspray irrigation systems. Image adapted from Keller and Bliesner (1990) with permission from Blackburn Press.

Bubblers

With bubbler irrigation, water is applied to the soil surface in a small stream or fountain from a single discharge point (Figure 14.1c). Application rates are much greater than for drip systems but usually are less than 60 gal/hr. The discharge, applied at one point, normally exceeds the soil's infiltration rate and, therefore, a small basin is normally created by building an earthen dike to control the distribution of water on the soil surface. Advantages of the bubbler system include minimal filtration requirements, very little maintenance or repair, easy visual inspection, and extremely low energy requirements compared to other pressurized systems. However, large-size lateral lines are normally required to minimize pressure loss associated with high discharge rates. In well-designed systems only a few feet of pressure head are required to operate a bubbler system.

Subsurface Drip

A subsurface drip irrigation (SDI) system is basically a surface system that has been buried (Figure 14.1d). Comparison between the soil wetting pattern for surface and subsurface trickle systems is illustrated in Figure 14.3. This method of irrigation is not to be confused with subirrigation where an irrigation is accomplished by raising the water table to the bottom of the crop root zone. Subsurface systems are buried at a depth of a few inches to 18 inches or more. Shallow systems are installed in planting beds that are maintained over time. Deep installations do not require the crops to be placed in the same planting beds. Advantages of SDI include a permanently installed system that can last a decade or more, little interference with cultivation and other cultural practices, and irrigation and harvesting can occur at the same time on crops like melon and tomato. Figure 14.4 shows a schematic of a subsurface drip system with laterals placed in every other row between planting beds and photographs of equipment used to install subsurface drip laterals and a manifold that delivers water from the mainline to the laterals.

Figure 14.3. Typical wetting patterns for surface and subsurface drip systems. Image adapted from Coelho and Or (1997) with permission from John Wiley and Sons.

Figure 14.4. Installation of subsurface drip irrigation laterals: (a) schematic of a typical subsurface drip installation; (b) equipment for installing three subsurface drip laterals; and (c) laterals connected to the manifold. (Photos a and b courtesy of Suat Irmak, Nebraska Extension; photo c courtesy of Steve Melvin, Nebraska Extension.)

a) b)

c)