Chapter 12

Greenhouse Irrigation Systems

Maintaining Irrigation Efficiency in Greenhouses

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. Two important factors must be known in order to determine irrigation efficiency. First, the amount of water required by the plant, which is determined using irrigation scheduling techniques, such as weighing pots or using evapotranspiration values. Second, the irrigation application amount must be known, which requires measuring the applied irrigation water. In formula form, irrigation efficiency is:

Irrigation Scheduling Techniques

Irrigation scheduling is generally explained as “applying the right amount of water at the right time.” Most plants use more water on hot, sunny days than cool, overcast days. The moisture needs of crops vary with their stage of growth with larger plants needing more water than smaller ones. The season of the year also affects the water requirements of the crop. Other factors affecting irrigation requirements include greenhouse humidity, the type of irrigation system, the type of growing media used, the type of containers used, and the crop itself. Some plants require much less water than others to produce the same amount of growth. Not to mention scheduling irrigation in greenhouses is especially challenging because different crops may be grown together in a single irrigation zone. In such cases, the best approach may be to estimate the water needs of the crop that requires the most water and over-irrigating the other crops.

Weighing Pots

The most common method of determining a watering regime is by simply weighing the containers with a scale or by hand on a daily basis. Plant water use in greenhouses can be measured directly by weighing plant containers.

Evapotranspiration

Estimates of evapotranspiration for greenhouse-grown plants can be made using evaporation pans, automated weather stations, or atmometers, all of which can provide estimates of the amount of water loss in a given surface area. Evapotranspiration values are expressed in units of inches per day or centimeters per day. Evaporation pans measure water evaporated from a standard sized pan of water and then a correction factor must be applied to estimate evapotranspiration. Atmometers measure the amount of water that evaporates from a porous surface designed to represent a leaf surface.

Soil Sensors

Using soil sensors to collect quantitative information about crop water requirements can provide objective information to improve irrigation management. The basic idea behind using soil sensors to control irrigation is simple: when plants use water, they take it up from the substrate, so the water content of the substrate decreases. Soil sensors detect these changes and can be used to open an irrigation valve when the substrate water content drops below a user-determined set-point. This results in frequent applications of small amounts of water, and the frequency of irrigation is adjusted automatically based on the rate of substrate water depletion.

Tensiometers. Tensiometers measure moisture by measuring the water tension in the soil (See Figure 12.11). A tensiometer has a vacuum-pressure gauge mounted to the water-filled tube to observe the soil tension measurement at any time. The unit requires the tube to be filled with water and serviced to remove any entrapped air before installation into the soil. The tensiometer is installed in the soil with the porous ceramic tip placed at the desired depth. As the soil dries, water is pulled out the tube and the vacuum gauge indicates the soil water tension at that moment.

Granular Matrix Sensors. Granular matrix sensor (GMS) technology reduces the problems inherent in gypsum blocks (i.e., loss of contact with the soil by dissolving, and inconsistent pore size distribution) by use of a granular matrix confined in a metal case (See Figure 12.12). Granular matrix sensors operate on the same electrical resistance principle as gypsum blocks and contain a wafer of gypsum embedded in the granular matrix.

Dielectric Sensors. Dialectic methods determine the water content from the dialectic constant of the growing substrate; dry substrate has a lower dielectric constant than that of a moist substrate. This technique is relatively simple and highly reproducible. There are two main types of dielectric sensors that measure the volumetric water content (VWC) in the growing substrate: Time Domain Reflectometry (TDR) and Frequency Domain (FD) (Reflectometry and Capacitance). Both types of sensors measure the dielectric constant of the substrate. The dielectric constant is 1 for air, 3-5 for mineral soils, and 80 for water; thus, any change in those three parameters would change the dielectric constant of the substrate. Dielectric sensors generally require substrate-specific calibrations, because the dielectric properties of different soils and substrates differ, affecting sensor performance.

Measuring the Amount of Water Used for Greenhouse Irrigation

It is important to measure the amounts of water beneficially used and delivered to a greenhouse in order to document improvements in irrigation efficiency due to management changes and/or upgrades in the irrigation system. Careful measurements of crop water use make it possible to determine the volume of water beneficially used.

Improving Greenhouse Irrigation Efficiency

Many irrigation management practices can be implemented in greenhouses to increase irrigation efficiency. The following is a list of some of these 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.

Know Your 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.

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 sub-irrigation 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.

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 in excess of that amount simply runs through or off the pots.

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.

Eliminate Off-Target Application

Off-target application is essentially the water you apply that misses the pot or, in the case of automated systems and new employees, the excess water that runs through the pot during periods it should not. Leaching soil is a valid activity in the greenhouse. Leaching every day, however, and/or over-watering a crop each day through poorly timed automation or lack of employee training not only wastes water, it can lead to pollution problems by runoff and soil percolation, disease humidity and water condensation, especially in cooler seasons.

Automation

In greenhouse management, use of proper method of irrigation and its control is important. Automation of irrigation system has the potential to provide maximum water use efficiency by monitoring soil moistures and other crop parameters at optimum level. In conventional irrigation system, the grower has to keep watch on irrigation time-table, which is different for different crops and dependent on type of soil and other parameters.

Click on the following topics for more information on greenhouse irrigation systems.