PRINCIPLE OF AUTOMATION

Current technologies of irrigation programming consider several factors such as [2, 3]:

Duration and stage of crop growth, allowable plant water stress, soil aeration, soil wa-ter potential, soil salinity, soil moisture available to the plant, class A pan evaporation and evapotranspiration. In most cases, programming of drip irrigation has been limited to a control system that uses duration or depth of irrigation. The irrigation controller is programmed to operate solenoid valves in sequence and to verify operating pressure and flow rates, wind, temperature and other indirect variables. To obtain the minimum cost-benefit and high efficiency of water use, it is necessary to achieve high crop yield.

The water loss due to several processes (control of salinity, requirement of infiltration, evaporation, irrigation losses and runoff), must be reduced to a minimum so that the accurate application of the irrigation is limited only to the crop requirements. Four methods for automation of irrigation systems are based on: (1) Soil moisture, (2) Plant water stress, (3) Estimation of evapotranspiration and (4) Combination of one or more of these methods.

Soil Moisture Method

Irrigation based on soil water potential is perhaps the oldest method to program irriga-tion. Microprocessors along with sensors, tensiometers, heat transfer psychrometric methods, gypsum blocks and thermocouples have been used successfully for irriga-tion scheduling. The sensors can provide quick informairriga-tion to make decisions for application of irrigation depth. The microprocessor circuits combined with a computer programming can help to estimate the irrigation duration on the basis of field data, matrix potential of soil; and to calculate the number of days between two successive irrigation events.

A thermal method measures the matrix potential of soil, independent of soil tex-ture, temperature or salinity. It is based on frequent measurements of ability of a po-rous ceramic sensor to dissipate a small amount of heat. With a good calibration, the sensor can be used in any soil to automatically watch the matrix potential of the soil and for irrigation scheduling. For closed circuit automated irrigation, the soil sensor

is placed in the root zone. For an automatic control of an irrigation system based on matrix potential of a soil, we need equipments for the:

1. Automatic sampling from several sensors in sequence,

2. Comparison of the reading of each sensor at which the irrigation begins at a predetermined matrix potential of the soil, and

3. The operation of irrigation controller to control the irrigation depth. Desktop computers in combination with microprocessors have been successfully used.

There is also a commercial equipment to measure the matrix potential of the soil and for an automatic control of a drip irrigation system.

Water Content in the Plant

The water is frequently one of the limiting factors in agriculture. Transpiration loss oc-curs from the plant surface due to an evaporative demand of the atmosphere. Less than one percent of the absorbed water is retained by the plant. This small fraction of water is often used to replace the deficit between water use and transpiration. Thus any water deficiency can cause a plant water stress. The total water potential (the sum of turgor, matrix and osmotic potential) is used to indicate the condition of the plant water. The plant development and growth (cellular enlargement and photosynthesis), pollination, fruit formation, crop yield and fruits quality are affected by the water deficit. Probably, the cellular growth is most sensitive to the water deficit. There are several methods to estimate the condition of plant water. These include determination of relative water content, diffusive conductivity of the plant, water potential of the plant and surface temperature. The indirect or direct measurement of water potential is probably a good indicator of the plant water stress. There are several methods to measure the plant wa-ter stress such as: The total leaf wawa-ter potential with a leaf psychromewa-ter; temperature of leaf surface with an infrared thermometer, and the leaf water potential indirectly on the basis of the diameter of the stem.

Leaf Water Potential

The leaf water potential can be measured by psychrometer or by adhering thermocou-ples to the leaves. Although the psychrometric measurements are taken routinely for research purpose, yet the instruments are expensive and not feasible for commercial purposes.

Temperature of the Leaf

Measurements of leaf temperature can indirectly indicate status of a water stress [10].

Plant water stress index can be used to automate the irrigation system, and to indicate when to irrigate. The operating system can be easily automated to take the data, to cal-culate the index of plant water stress, to make comparisons with pre-determined values of irrigation depth and to make decisions for irrigation scheduling. Leaf temperature is measured with a non-contact infrared thermometer. The accuracy of temperature of the surface of leaf depends on the precision of calibration. The measurements are sensitive to changes in the ambient temperature, interactions with surrounding surfaces (such as soil), and leaf area index. Measurements of leaf area index of a crop vary from plant to plant. There is no standard value.

Stem Diameter

The diameter of stem and the leaf water potential are closely related to one another.

The measurements of stem diameter can be used for continuous recording of the stem growth and the condition of plant water. The periodic calibration of the changes of diameter of stem versus leaf water potential can be conducted for each phenological stage of a plant. This technique can be used for the purpose of automation.

Evapotranspiration Estimations

To program the irrigation, the evapotranspiration models have been successfully used throughout the world. The following information is needed for the evapotranspiration estimations and the criteria to decide when to irrigate.

1. Evapotranspiration of a reference crop, potential ET, etc.

2. Crop growth curve, crop coefficient and consumptive use of a crop.

3. Index to estimate the additional evaporation from the soil surface when the soil is wet or dry.

4. Index to estimate the effect of soil water loss in relation to ET.

5. Estimation of available soil moisture used by a crop: Consumptive water use.

6. Relation between expected crop yield and crop water use.

To estimate the ET, many of the variables are not well defined and must be es-timated. Although the ET models can be useful to accurately estimate the irrigation needs, yet these are not viable for irrigation scheduling as available weather data are limited for a particular location.

Direct Measurement of Essential Evapotranspiration

The weighing lysimeter in a given crop can serve as a guide to provide an adequate irrigation depth for the crop need. A water tank is connected to a lysimeter so that the weight of the irrigation depth is included in the daily weight of lysimeter. Whenever one millimeter of ETc is registered, lysimeter is automatically watered by drip irriga-tion system to maintain the soil water potential. The tank is automatically filled daily to a constant depth. Therefore, the daily changes in the weight of lysimeter represent the crop growth. The water potential of the soil is almost maintained constant by the drip irrigation system (see Figure 7.1) [8, 11].

INSTRUMENTATION AND EQUIPMENTS

The automation of a drip irrigation system at an operating pressure can potentially pro-vide an optimum crop yield and optimum water use. A system of controls in an automated irrigation system must use sensors to measure variables, such as: Depth and frequency of irrigation, flow rate, operating pressure; and environmental conditions such as wind speed, ambient temperature, solar radiation, rain fall, soil moisture, leaf temperature, leaf area index, etc. Maximum irrigation efficiency is possible with the continuous monitor-ing and control of the operation of the system with measurements of flow (solenoid valves) and operating pressure (pressure regulators) at strategically important locations in the field. The data or control of functions can be transmitted by electrical cables, la-ser or hydraulic lines, rays, radio frequency signals, remote control or by satellites. A

wide variety of instrumentation and equipments with characteristics are available com-mercially. These can be subdivided in six categories: (1) Controls, (2) Valves, (3) Flow meters, (4) Filter, (5) Chemical injectors, and (6) Environmental Sensors.

Controls

The controls receive feedback about the volume of water for the field, pressure in the line, flow rates, climatic data, soil water, plant water stress and from the field sensors.

This information is then compared with the predetermined values and the irrigation is reprogrammed to adjust for the new values, if necessary. The controls, volumetric valves, hydraulic valves, fertilizer or chemical injectors, flushing of filters, etc., can be operated automatically or manually.

Valves

Automatic valves (Figures 7.2 to 7.9) can be activated electrically, hydraulically or pneumatically and these are used to release or to stop the water in the lines; to flush the mains and laterals; to continue the water from one field to another field and to regulate flow or pressure in main, sub-main or lateral lines. The type of valve will depend on the desired purpose. Valves receive feedback to verify the precision of operation.

Automatic Volumetric Valve: Flow meters

The flow metering valve (Figure 7.10) allow programming the predetermined values.

Usually these meters are calibrated to measure applied volume of water or to measure the flow rate.

Figure 7.1. Logic diagram to measure weight of lysimeter sensors and to control the irrigation sequence with three depths of irrigation.

Figure 7.2. Automatic irrigation controller (Rain Bird).

Figure 7.3. Logic hydraulic valve.

Figure 7.4. Automatic metering valve along with a hydraulic valve.

Figure 7.5. Fertilization and irrigation programmer for six different valves (for green house or field).

Figure 7.6. Automatic controller (Nirim electronics), using a programmer with a perforated tape or card.

Figure 7.7. Fertigation and chemigation equipments.

Figure 7.8. Logic diagram for an automatic controller in a drip irrigated field.

http://www.bermad.com/

Figures 7.9 and 7.10. Bermad automatic volumetric valve.

Ambient Sensors

Various types of instruments are available to determine the soil moisture (ceramic den-sitometry, ceramic cup, heat dissipater sensor, soil psychrometer); to measure climatic parameters (weather station, automated evaporation tank, etc.), plant water stress or leaf temperature of the crop (leaf psychrometer, porometer for stomate diffusion, in-frared and sensorial thermometer to measure stem diameter). These can be used as feedback for the management of irrigation. If the soil at a particular field station is wet, the sensor opens the circuit of the hydraulic or solenoid valve and this station is bypassed. If the soil at this field station is dry, the closer the circuit and the field at this station are irrigated for a specified duration.

Filters

The obstruction in the drippers caused by clogging agents (physical, chemical or bio-logical) is a common problem and is considered a serious problem in the maintenance of the drip irrigation systems. The suspended solids may finally clog or reduce the filtration efficiency. The automatic flushing valve is available for different types of filters. The flushing is done by means of back flow of water [4, 8, 11], thus allowing the water to move through the filter in an opposite direction (Figure 7.11).

Figure 7.11. Automatic flushing of filters by inverse or back flow.

Chemical Injectors

The chemigation methods to inject the fertilizers, pesticides and other inorganic com-pounds are: (1) Pressure differential, (2) Venturi meters and (3) Injection Pumps. In all these cases, digital flow meters can be used for the chemigation by allowing a known amount of chemicals in a known amount of water to maintain a constant concentration of chemicals-in-the-irrigation-water [8, 11].