Chapter 14

Drip Irrigation for Greenhouse Crops

Drip Irrigation Control Systems

Drip irrigation systems require a higher level of management expertise. Drip irrigation systems are more complex, require greater filtration and water treatment, and typically have high maintenance costs compared to other types of irrigation. One way of managing the higher demands of drip irrigation is the use of automation and central control systems. These technologies allow efficient control of water flows to various zones; injection of water conditioners, fertilizers, and agricultural chemicals; allow remote checks of system performance; control filter backflushing; and provide extensive records of water use. In many drip irrigation systems, a controller is an important and integral part of the irrigation system. Controllers can help to achieve labor savings in addition to applying water in the necessary quantity and at the right time to achieve high efficiency in water, energy and chemical uses. Irrigation controllers have been available for many years in the form of mechanical and electromechanical irrigation timers. These devices have evolved into complex computer-based systems that allow accurate control of water, energy and chemicals while responding to environmental changes and crop demands.

Types of Control Systems

A controller is an integral part of an irrigation system. It is an essential tool to apply water in necessary quantity and at the right time for greenhouse crop production and to achieve high levels of efficiency in water and energy uses.

Open Loop Systems

In an open loop system, the operator makes the decision on the amount of water that will be applied and when the irrigation event will occur. This information is programmed into the controller and the water is applied according to the desired schedule. Open loop control systems use either the irrigation duration or a specified applied volume for control purposes.

Closed Loop Systems

In closed loop systems, the operator develops a general control strategy based on a set of environmental parameters (such as soil moisture, temperature, radiation, wind-speed, humidity, etc.) as well as system parameters (pressure, flow, etc.). Once the general strategy is defined, the control system takes over and makes detailed decisions of when to apply water and how much water to apply.

Controllers

These are automatic timing devices which supply the actuating power to operate the remote control (electric) valves, i.e. to open and close on a pre-set program. They contain a transformer that provides a low-voltage output (typically 12 or 24 volts DC or 24 volts AC ) to the valves and control devices for specific zones.

Electromechanical Controllers

Electromechanical controllers (See Figure 13.12) use an electrically driven clock and mechanical switching (gear arrays) to activate the irrigation stations. These types of controllers are generally very reliable and not very sensitive to the quality of the power available.

Electronic Controllers

Electronic controllers (See Figure 14.13) rely on solid state and integrated circuits to provide the clock/timer, memory and control functions. These types of systems are more sensitive to power line quality than electromechanical controllers, and may be affected by spikes, surges and brownouts.

Sensors

A sensor is a device placed in the system that produces an electrical signal directly related to the parameter that is to be measured. In general, there are two types of sensors: continuous and discrete. Continuous sensors produce a continuous electrical signal, such as a voltage, current, conductivity, capacitance, or any other measurable electrical property. Continuous sensors are used when just knowing the on/off state of a sensor is not sufficient. For example, to measure pressure drop across a filter or determine tension in the soil with a tensiometer fitted with a pressure transducer (Fig. 8) requires continuous-type sensors

Computer-based Irrigation Control Systems

A computer-based control system consists of a combination of hardware and software that acts as a supervisor with the purpose of managing irrigation and other related practices such as fertigation and maintenance. Generally, the computer-based control systems used to manage drip irrigation systems can be divided into two categories:

Interactive Systems

Interactive systems are usually built around a microcomputer, either a standard personal computer (PC) or a specially designed unit. The information is transferred into a central unit either directly from sensors in the pipeline or from intermediate units which collect the data from a number of sensors and then process and store them temporarily for further transfer to the central computer.

Automatic Systems

In fully automated systems, the human factor is eliminated and replaced by a computer specifically programmed to react appropriately to any changes in the parameters monitored by sensors. The automatic functions are activated by feedback from field units and corrections in the flow parameters by control of devices in the irrigation system until the desired performance level is attained.

Centralized Irrigation Control

Water shortages and rising power costs demand increased attention to sound water management by greenhouses operations. Centralized irrigation control is not only an appropriate tool for improving water management but other objectives can be accomplished at the same time.

Central Control Components

Central control systems consist of a central computer, communications equipment, greenhouse controllers, and sensors. The central computer is usually located in the manager's office. The communications equipment is located both at the computer and at the greenhouse devices. Communications equipment can consist of telephone modems, radio modems, or fiber-optic modems.

Communication between Central Control Components

Communication between various units of the control system can be by wire or wireless. Data encoded by pulse requires two- or three-wire telephone cable in which one strand usually serves for energy supply from the source. For radio transmission the units are provided with radio transceivers, operating on the usual personal phone frequencies.

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