AUTOMATIC SYSTEMS

With the exception of a volumetric metering valve that operates according to the time or the discharge rate, the automatic irrigation systems can be divided in three groups on the basis of operation: (1) Sequential hydraulically operated system, (2) Sequential

electrically or hydraulically-electrically operated systems and (3) Non-sequential elec-trically operated system with or without programming: With the possibility of using information of the field (feedback) by remote control.

Sequential Hydraulically Operated System

This system controls the valves in sequence (Figure 7.12). The valves open and close based on the water pressure in the line. The pressure arrives at the valve by means of a flexible hydraulic tube (micro tube: polyethylene tube of small diameter) to provide a required pressure. The diameter of the micro tube is generally between 6 and 12 mm and is connected to the hydraulic valve at one end and the other end is connected to the automatic control or the line of water.

Figure 7.12. Sequential hydraulically operated system for green houses, gardens, nurseries, and fruit orchards.

Some hydraulic systems can be connected to the main valve of the line or to the system that replaces the water. In this case, the main valve is connected automatically to open when the system in series is in operation and to close at the end of the irriga-tion cycle. Electrically operated automatic system activates the pump and deactivates the pump, when the irrigation cycle is over.

Sequential hydraulically operated system is controlled by a predetermined amount of water. The amount of water can be different for each valve and can be adjusted by a regulator mounted in same valve. The hydraulically sequential system can be used to water fruit orchards, gardens, green houses and nurseries, establishing low flow rates through tubes of small diameter and for flow rates in any diameter of tube. The system includes automatic metering valve, hydraulic valve and hydraulic tube.

Sequential System: Operated Electrically or Operated Hydraulically-Electrically

These systems supply an electrical current through cables for the remote control of the valves (Figure 7.12). The current from the “control panel” to the valves, usually

passes through a step down transformer to supply a voltage of 24 V. For safety reasons, a current of 220 V should not be used when the subsurface cables extend to the field valves. The regular solenoid valves are mainly used for low flow rates. For pipes of larger diameters, the solenoid valves are used only as controls to activate the hydraulic valves and all the automation process is hydraulically-electrical. The control of the second valve is always hydraulic. In the hydraulic sequential system, the opening is controlled electrically by a timer mounted next to the main valve. In such cases, the current source is direct and not alternating.

Programming irrigation with solenoid valves

The solenoid valves can be used to program the irrigation (Figure 7.13). In order to calculate the crop water use, a computer program can be used with the information such as: The soil moisture, evapotranspiration, the date of the next irrigation and the amount of water to be applied. The irrigation programs are based on evapotranspira-tion estimaevapotranspira-tions, complex water budgets in several dimensions or crop growth models.

The ET models use crop and climatic data such as crop coefficient, root zone depth, allowable depletion, drainage rates, air temperature, sun radiation, precipitation and constants in the evapotranspiration equations, and so forth.

Figure 7.13. Electrically operated tensiometer and solenoid valve.

Then the model incorporates the climatic information to calculate the evapotrans-piration rates and to adjust the water balance in the soil as the water is being used.

The evapotranspiration model requires an irrigation criteria based on the allowable depletion or the irrigation interval. Actual field data after the irrigation can be helpful to compute the infiltration and immediate drainage for correction of estimated soil moisture. The rate of computed ET can be used to indicate the required amount of ir-rigation or to specify the time for irir-rigation interval. This method is more practical for drip irrigation than for other irrigation methods. The records of field data are kept in the office files for the irrigation programming, so that the data can be used to-update-the-inputs-in-the-program.

Automatic valves

The automatic valves are commonly used for the pump house and filters; for regu-lating the pressure in the main line; to control the flushing cycles in the filters, or to control the volume of water through the secondary or lateral lines. The solenoid valves can be used in the secondary or lateral lines to control the volume of water to the individual blocks. The primary function of a solenoid valve is to switch on or switch off the system. However these valves can be equipped with pressure regula-tors and check valves. The solenoid valves are operated electrically from the “Cen-tral Control Panel.” Automatic control valves can also be equipped with manual valves for better efficiency. Automatic valves require periodic maintenance to assure a satisfactory operation. The maintenance program depends on the use of the valve and the flushing operations.

At least, it is recommended that all the diaphragm valves are disarmed and cleaned at least once a year. It is important to clean the deposits on the stem of the valve.

Almost all the manufacturers provide a service or fast replacement of most of the components. This can usually be done without removing the valve from the irrigation line. A number of auxiliary controls can be adapted to the diaphragm valves to provide flexibility and convenience.

Pressure reducing valve

This valve responds to changes in the pressure at the exit of the main valve and adjusts to the pressure in the cap or valve cover to compensate for any change. A trouble in the operation can be caused by contamination, obstructions, incorrect assembly, and damages or worn out parts.

Pressure-regulator-valve

This valve is used to separate the system from the pressure in the main line. It must be open during the normal operation. Whenever the pressure exceeds a preset value, the valve releases the excess pressure.

Controls-to-adjust-the-velocity-of-the-main-valve

These are small adjustable controls in the pilot control system. These regulate the speed of opening and closing of the main valve by blocking or strangling the flow that

enters or leaves the casing. These can be subjected to obstructions by fine sediments if tightly fit.

Controls

Several electromechanical and electronic controls in the drip irrigation system are automated. The controls with mechanical time clocks open and close only a single valve at one time. These are programmed based on series of climatic and soil sensors:

to decide when to begin and to end the irrigation cycle; start and to put off pump; to open and close the valves to supply an irrigation depth and to remember how much water and fertilizer was applied to each block within the field. The controls are also available to diagnose operation and identify the troubles and to take remedial steps.

Others put off the system during rainfall and restart the system when necessary. A timer uses a clock to program the beginning and sequence of irrigation. The control is a source of electric or hydraulic signal to activate by remotely located valves to allow or to stop the flow.

The communication between the irrigation controller and the valves is by means of electrical wires, hydraulic lines or radio signals. The microprocessors and microcomputers also can be programmed using data of tensiometers, pan evaporation, thermocouples, soil moisture tension gages, anemometer, flow meter, pressure transducer, etc. These controls are based on the climatic and soil sensors or according to the program specified by the irrigator. Using these data, the con-troller uses a program to compute irrigation requirements for each crop and block within a field.

The data from the flow meters and pressure gages is used to determine the flush-ing time and to detect any troubles in the system. In most of the cases, the controller has a calendar programmer, so that the cycle of irrigation begins automatically on a particular day of the week and at a particular time of the day. Most of the controllers can be programmed for fourteen days, while others are only limited to seven days.

Practically all automatic controllers have a station selector on the outer surface of the panel (Figures 7.4–7.7). This station selector shows a green light to show the station in operation. In addition, it can also be set manually so that the irrigation operator can start and put if off whenever desired.

Sequential System: Electrically Operated

In these systems, the amount of water distributed to the different blocks is determined by a flow meter. A timer determines the duration of operation: 14 days and 24 hours per day. Sensors based on tensiometers or pan evaporation can activate these. Al-though this type of system was developed mainly to water green houses, yet it can be used for the drip irrigation system.

Non-sequential System

These systems are completely automatic and are controlled electrically. These non-sequential systems are controlled by hydraulic or electrical valves that can operate the valve in the desired block at random, and can supply known amount

of water for a known duration to a desired block. Each unit can supply a known flow at different hours during the day, in response to soil moisture status in each block. The “Control Panel” consists of electrical circuits that operate the pump, main valve, adds fertilizer according to a pre-established schedule and measures the soil moisture to estimate the crop irrigation requirements. This system usually operates by a remote control system and is designed to provide feedback of field data, so that the automatic adjustment can be made and adjustments for changes in pressure and flow rates can be made to the discharge flow in the distribution lines.

Central panel

The central panel controls all the operations of the field, sending instructions to the valves and receiving continuous data on the operation of the irrigation system. It con-sists of a programmed unit of irrigation, a unit for transmission of information, a unit for the control of flow in the laterals and a unit for warning signals.

Field panel

The field panel is placed centrally in the field and operated by remote control unit. The signals of the main panel are sent by an individual communication channel and these are transmitted to individual field panel. The field panel can collect the data on water meters, operating pressures and warning signals. Then the data can be transmitted to the main panel (control panel).

Use of Sensors to Program Irrigation

In addition to the above mentioned instruments, sensors are available to determine the soil moisture tension or the soil moisture. Tensiometers and gypsum blocks are simple and economical to use. Another method is a neutron scattering method, but it is quite expensive and is used for research purpose only.

Use of Gypsum Blocks and Tensiometers

The gypsum blocks can measure the soil moisture tension in the range of 1–15 atmospheres. There are two electrodes inserted in each block and the changes in the soil moisture are calibrated with variations in the resistance. The precision of this method is based on the temperature, salt concentration in the soil solution, physical characteristics of the gypsum block and the electrical resistivity of the soil. For tensions of 80 cbars, a tensiometer is recommended instead of a gypsum block. Tensiometer (Figure 7.14) measures the tension and the reading is given in cbars. The main disadvantage of a tensiometer is a relatively low critical tension of 85 cbars after which the air enters the plastic stem of a tensiometer. The soil moisture by any method will show variations in the soil moisture within the same field. A sample of the soil in a given location represents only the soil condition of that location. Therefore several observations of soil moisture at various locations in the field are desirable.

Figure 7.14. Automatic unit for control of irrigation based on the gypsum blocks (sensors).

Neutron Scattering Method

The neutron scattering method consists of a neutron radiation source of high energy and a neutron detector. Neutrons travel through the soil medium, loose energy, and the speed is reduced when these hit the elements that are present in the soil. The hydrogen, a component of the water, is dominant in the reduction of the speed of fast neutrons.

Due to other factors that can affect the reading, the calibration of this method is done in a location where the equipment will be installed and used. The use of the neutron scattering method requires the installation of access tubes at the beginning of planting and removal of these tubes after the last harvest.

It is recommended to install one sensor at each 30 cm depth. Periodically the oper-ator will obtain the readings of the tube at the desired depth. A minimum of three read-ings are taken: at shallow root depth, at middle depth, and at a deeper depth. The water content of these readings is added and the water content at field capacity is deducted from the sum. The difference between these two estimates will be the amount of water that should be applied. The readings can be recorded automatically and are stored in the memory of the neutron scattering equipment. Then these can be downloaded on the computer of the Control Panel. With this information, the computer will give the necessary commands to the drip irrigation system so that the crop water requirements are met in the desired block.

Class A Pan Evaporation to Automate the System

The relationship between pan evaporation and the water loss have been well estab-lished. Both are exposed to similar climatic conditions in the same field. This correla-tion can be used to schedule the irrigacorrela-tion. If electrodes in the tank can be installed at a depth (based on previous experience), the irrigation can be controlled automatically.

The irrigation will begin when the surface of the water in the class A pan lowers to a predetermined level and will stop when the level raises to certain level in the tank [7].

PREVENTIVE MAINTENANCE