Irrigating Greenhouse Crops
Irrigation Efficiency
Declining water supplies, drought, increased competition from other users, and either existing or anticipated restrictions on the amount of water that can be applied over a specified time, are encouraging many greenhouse operations to improve the irrigation efficiency. Irrigation efficiency is a measure of how much of the applied irrigation water is used to satisfy crop water requirements. Other beneficial uses can include salt leaching, pesticide or fertilizer applications, or crop cooling. Water lost to runoff that is not reused or unused by plants is considered non-beneficial uses. Many irrigation management practices can be implemented in greenhouses to increase irrigation efficiency. The following sections list of some of the more common practices. Most of these suggested practices can be done in concert, and it may well be that some of the suggestions are not appropriate for some operations.
Crop Water Needs
First, it is important to understand the water needs of the crops being grown. The amount of water each crop uses from the soil profile each day depends on the daily temperatures and amount of wind during the growing season, the growth stage of the crop, the rooting depth of the crop, and the amount of residue on the surface of the soil. Generally, the substrate should be watered thoroughly to the point that at least a small amount of water can be seen coming out of the bottom of the containers.
Irrigation Design
Water-use efficiency begins with greenhouse irrigation design. The first choice is that of application method, whether open irrigations systems (i.e., boom, drip, etc.) or subirrigation systems (i.e., ebb-and-flow and trough systems). Open irrigation systems apply water directly to the container substrate and are the most efficient in terms of water application. However, these systems typically require high quality water and higher time inputs for management and maintenance.
Maintenance of Irrigation Systems
The next step in efficient irrigation is regular maintenance and audits of your irrigation system. Maintenance entails detecting and repairing leaks, flushing to unclog lines, replacing worn nozzles (avoiding unintentional mixing of different-size sprinkler nozzles and drip emitters), and checking to ensure that appropriate pressure throughout the system is maintained.
Pulse Irrigations
Applying water in short, “pulsed” irrigation durations can significantly reduce runoff and improve irrigation efficiency. Pots hold only a certain volume of water, and any irrigation more than that amount simply runs through or off the pots.
Irrigation Zones
Establishing zones in which irrigation is constant and can be controlled improves irrigation water management. It allows a nursery manager to operate only those zones needing irrigation for only the time needed.
Grouping of Plants for Efficient Irrigation
Placing plants with similar irrigation needs together in an irrigation zone allows efficient irrigation water management if based on container size, substrate, plant type, plant water requirements, stage of growth, plant leaf type, and plant canopy architecture.
Avoid Irrigating Where No Plants Are Present
Avoid applying water where there are no plants seems like common sense, but it can be difficult to accomplish. It is made easier when numerous irrigation zones, each of which can be controlled separately, provide increased irrigation control. As plants are sold, group remaining plants that have similar needs together and shut off irrigation in unused areas.
Automation
Irrigation automation can improve water management, but it should be used in conjunction with good human management. In automated irrigation, a controller operates solenoid-equipped valves at specified times and durations. While the controller may be scheduled to turn the irrigation system on and off, it takes human management to determine whether there is too much runoff or too few or no plants in an irrigation zone being irrigated.
Monitoring Leaching Fraction
Leaching fraction (LF) is the amount of water that drains out of a container immediately after an irrigation event compared to the amount applied. This measurement is a quick and effective way to judge whether the length of the irrigation event is sufficient to replace the amount of water lost from the substrate. Large volumes of leachate and thus high leaching fractions indicate over-irrigation while little to no leachate and low leaching fractions indicate under-irrigation.
Measuring the Amount of Water Used for Greenhouse Irrigation
Whether the method of irrigating is by micro-irrigation or sprinkler irrigation, a flowmeter is an essential tool for every irrigation system. They provide valuable information about water use and irrigation system efficiency and help irrigators manage and schedule irrigation more precisely, in turn maximizing crop quality and yields while conserving the world’s most precious natural resource. 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. Water flowmeters can measure the flow rate of water or the total water that has passed by the measuring point. Flow rate is the volume of water per unit of time moving past the measuring point.
Propeller Flowmeters
The most commonly used flowmeters are propeller meters. They should be installed in a section of pipeline that is straight and unobstructed for 8 to 10 times the pipe diameter upstream of the meter and 4 to 6 times the pipe diameter downstream. This minimizes turbulence caused by elbows, valves, and other fittings, which leads to inaccurate measurements.
Magnetic Flowmeters
Magnetic flowmeters have the advantage of not creating an obstruction in the pipe. This feature eliminates the problem of possible entanglement from debris in the water as well as any pressure loss across the metering device. They should be installed in a section of pipeline that is straight and unobstructed for 3 to 5 times the pipe diameter upstream and downstream of the meter in order to minimize turbulence caused by elbows, valves, and other fittings, which leads to inaccurate measurements.
Ultrasonic Flowmeters
Ultrasonic flowmeters provide the capability of measuring water flow rate accurately, by utilizing the dynamics of ultrasonic energy transmission. A portable ultrasonic flowmeter enables one to determine water flow rate through a pipe without having to interrupt the flow (non-intrusive testing). The popularity of portable ultrasonic flowmeters has been largely attributed to their convenient size, weight and ease of use. Ultrasonic flowmeters consist of clamp-on transducers that are attached on the outside wall of the pipe with flowing water. These transducers transmit and receive ultrasonic signals that result in the computation of corresponding flow rate, which is displayed by the meter.
Turbine Flowmeters
Turbine flowmeters operate on the principle of a rotor assembly, turning at a rate proportional to the flow rate in the pipelines. The rotor is suspended near a magnetic pickup that records a pulse on its readout unit as each rotor blade passes. The accuracy of turbine flowmeters is comparable to that of propeller flowmeters (within 2%) under the correct flow conditions, and they require a section of pipe that is straight for 10 times the pipe diameter upstream of the meter and 6 times the pipe diameter downstream.
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