Micro-Irrigation for Greenhouse Crops
Micro-Irrigation System Components
Micro-irrigation systems can be arranged in several ways. The arrangement of components in Figure 17.1 represents a typical layout. For a system to irrigate satisfactorily, the application of water must be uniform. There should be no more than a 10 percent variation in discharge between the emitters with the lowest and highest output. To achieve this, pipes and tubing must be sized correctly. A basic drip irrigation system consists of a water supply, back flow prevention valve, fertilizer injector (optional), water filtration system, pressure regulator, a main line to transport water to the greenhouse, submains to distribute water within the greenhouse, laterals or poly tubes to distribute water down a row, and emitters to meter water to the plants. There may also be valves for zone control; various pipeline appurtenances such as vacuum relief valves, air relief valves, and pressure relief valves; flushing valves; pressure gauges; and system controllers.
Pumping Station
The pumping station consists of the power unit (internal combustion engine or electric motor) and a centrifugal, deep-well, or submersible pump and appurtenances. In the design and selection of pumping equipment for a micro-irrigation system, high efficiency is the principal requirement.
Main, Submain, Manifolds, and Laterals
The main objective of a micro-irrigation system is to provide an irrigation system such that when properly managed, each plant will receive the same amount of water and nutrients, in sufficient quantity, at the proper time, and as economically as possible. For this goal to be realized, the system must deliver the needed pressurized amount of water to each emitter. If the headworks and other previously described components are performing properly, mains, submains, and manifolds must then deliver the water to the laterals and emitters.
Main Lines and Submains
Main lines are typically high-density polyethylene or polyvinyl chloride (PVC). The main line to the greenhouse is buried underground at a safe depth below the frost line. The valve assembly that makes the transition from the main lines to submains at each block location may be above ground or in a box below ground level. Pipe size should be selected based primarily on the economic trade-off between power costs and pipe installation costs. Factors to be considered in the design and installation of pipelines include pipeline velocity, energy losses due to fittings, pressure ratings, surge pressures, temperature effects, and flushing mode. The pipe should be sized to minimize friction loss at the maximum expected flow rate and have a maximum flow velocity of less than 5 feet per second.
Manifolds
The manifold, or header, connects the mainline to the laterals. Manifold pipelines are frequently installed underground with laterals on the surface, although laterals may also be underground. Underground pipelines last longer and do not interfere with greenhouse operations. Main and manifold pipelines are often set 16 to 24 inches below the surface, but site conditions may require other depths. The manifold is usually connected to the main line at the midpoint of the manifold to provide more uniform pressure to all laterals in the system.
Laterals
Laterals or emitter lines supply water to the emission devices from the manifolds. Black polyethylene (PE) is generally used as the laterals and ranges in size from 0.5 to 1 inch (1.25–2.54cm), but the clear majority of greenhouse irrigation systems use 0.5-inch tubing. The PE material is used because of its high strength and impact resistance properties.
Flow Control Devices
The term "valve" applies to a variety of devices for controlling the flow of water. Various valves allow for on-off control, modulation of the flow rate through the system, and prevention of backflow. They can also be used for pressure relief or as a safety device. In general, valves can vary from simple manual on-off devices to sophisticated control equipment that acts as metering instruments and delivers predetermined amounts of water to the system. For normal on-off control, the best choices are gate, ball, and plug valves. The on-off service valves function by sliding or by turning a flat, cylindrical, or spherical flow control element over an orifice in the valve body. Leakage past the flow control element is prevented by sealing or seating surfaces at the orifice. In the fully open position, a passage through a gate, ball, or plug valve is unrestricted, resulting in a low-pressure loss through the valve.
Control Valves
On-Off Control Valves. On-off control valves can be operated manually or electrically by using a solenoid to control the flow of water in the mainline.
Pressure Gauges
The performance of micro-irrigation systems depends on consistent control and knowledge of water pressure. Regardless of how well the micro-irrigation system is designed or how well the emitters are manufactured, operating pressures must remain at design specifications to maintain the desired performance and distribution uniformity. Manually monitoring pressures often or continuously with automation is important because changes in pressure can indicate a variety of problems. Depending on the location of the instrument, a pressure drop may indicate a leak, a component or line break, a blocked filter, or a malfunctioning pump.
Water Flow Meters
An important device for measuring water movement between the water source and the greenhouse is the water flow meter. Close monitoring and accurate recordkeeping with this device will allow the irrigator to make fundamental adjustments and detect problems before they can have serious effects on the plants. Flowmeters can either be monitored manually or automatically by computerized monitoring and control systems. A key requirement of operating a micro-irrigation system is knowing how much water is being supplied to the crops in the greenhouse.
Irrigation Controller
Control, monitoring, and data management equipment are increasingly important in simple to complex irrigation systems. Pump control and monitoring can be accomplished remotely, so the grower may not need to drive out to a field to turn on or off the irrigation system. The system can also be monitored for operating pressure, flow, and chemical dosing.
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