ELSEVIER Agricultural Water Management 33 ( 1997) 169- 18 1

Agricultural water management

Design and development of an auto irrigation system

S.K. Luthra * , M.J. Kaledhonkar, O.P. Singh, N.K. Tyagi Central Soil Salinity Reseurch Institute. Karnal, India

Accepted 20 September 1996

Abstract

In scientific irrigation scheduling water should be applied to a crop at an appropriate soil water tension to fulfil its evapotranspiration requirement. Automatic control of water application at predecided soil water tensions is an effective irrigation scheduling technique. This can be achieved through an indigenously developed auto irrigation system.

In this system soil water tension is sensed through a modified manometer type tensiometer. The design provides control of irrigation at the predecided soil water tensions and preprogrammed timer. The circuitry can be operated with a 12 V d.c. storage battery for a long period. 0 1997 Elsevier Science B.V.

Keywords: Modified tensiometer; Gate valve; Micro irrigation water meter; Auto pumping unit; Auto irrigation system

1. Introduction

In developing countries, most of the available water is used for agriculture. With the increase in agricultural activity and competitive demand from different sectors, it has become essential to economise on the use of water. This calls for optimal utilisation of available resources by adopting innovative methods of irrigation with scientific methods of scheduling.

Soil moisture measurements, evapotranspiration estimates, leaf water potential and canopy temperature are various parameters employed for irrigation scheduling. For effecting automation in irrigation based on the discrete values of these parameters, a datalogger and control panel are required. In the case of the infrared thermometer

* Corresponding author.

0378-3774/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO378-3774(96)01292-9

170 S.K. Luthra et al./Agriculturai Water Mamgement 33 (1997) 169-181

method of canopy temperature measurement, misleading information may be obtained as a result of stomata1 closure during periods of peak solar radiation. Automated field irrigation studies have been primarily limited to the gravity system. Typical are those by Haise and Kruse (1969) and Fischbach et al. (1970). Phene et al. (1973) used heat dissipating soil matric potential sensors with an automated plot irrigation system. Austin and Rawlins (1977) described an optoelectronic level detector for mercury manometers, and reported that detectors and controller using Transistor Transistor Logic (TTL) circuits were used to successfully control irrigation in 29 plots. Irrigation was initiated when any two or more tensiometers in the plot indicated a matric potential at or below the set point (Hollis and Dylla, 1980).

In the present study automatic irrigation scheduling is effected through real time values of soil moisture stress sensed by a modified tensiometer. The tensiometer controlled scheduling is particularly suited for a micro irrigation setup. The auto irrigation system is indigenously developed through commercially available components.

1.1. Principle

The design of the tensiometer has been modified to generate proportionate electrical signals for different values of soil water tension. At a preset value of soil water tension, the control circuitry generates a signal for operation of a bidirectional d.c. motor which controls the valve for initiation or termination of irrigation. The speed of the d.c. motor is reduced to 2 rev min.’ through a reduction gear mechanism. The magnetic controls for regulation of irrigation are placed on the body of the valve. A clutch mechanism has been included in the design to prevent damage due to overshoot of motor drive. The motor and control circuitry work on 12 V d.c. The inlet of the valve is connected to an overhead tank. The level of water in the overhead tank is sensed continuously by electronic sensors. The auto pumping unit maintains a constant level in the overhead tank. The design also provides for automatic measurement of the amount and rate of application of irrigation water. The automatic unit has been tested with a drip irrigation setup laid out in a 0.25 ha area at CSSRI, Karnal.

2. Materials and methods

The main components of the auto irrigation system (Fig. 1) are a soil moisture sensor (modified tensiometer), control circuitry , gate valve, auto pumping unit, timer and power supply. The components are described briefly below.

2.1. Modified tensiometer

For introducing automation in irrigation, it is necessary that a soil moisture sensor should generate an appropriate electrical signal for sensed values of soil water tension (SWT). The performance of the manometer type tensiometer has been reported to be better than that of the pressure gauge type tensiometer (Trotter, 1984). In the present setup, the design of the manometer type tensiometer has been modified (Fig. 2) for

SK. Luthra et al./Agricultural Water Management 33 (1997) 169-181 171

Resetting Pulse

Motor -Clutch

Fig. 1. Schematic diagram of the auto irrigation system.

automatic operation of the valve developed for regulation of irrigation. An acrylic tube replaces the mercury cup. A number of equidistant copper electrodes have been installed on the body of the acrylic tube. In saturated condition (when soil water tension is zero) all electrodes are in contact with the mercury in the acrylic tube, so - 12 V (ground) applied to the lowermost electrode extends to all the electrodes through the mercury. With the rise in value of soil water tension, the level of the mercury in the acrylic tube starts to fall and ground is progressively disconnected from subsequent electrodes. This disconnection signal is processed in the control circuitry for operation of the valve. Electrodes are extended to sockets on a terminal strip. For initiation of irrigation, a particular value of soil water tension is selected by plugging the corresponding socket.

2.2. Control circuitry

The signal generated from the modified tensiometer for initiation and termination of irrigation is processed in the control circuitry for opening and closing of the valve. The control circuitry consists of two I.C. chips (Fig. 3). The output of the soil moisture sensor is fed to pin-4 of IC-4. The IC-4 is 555 I.C. having high input impedance. The removal of the earth at pin-4 extends the supply to pin-4 through a resistance of 10 kR. Alternatively, if initiation of irrigation is to be controlled through a timer, then positive voltage is applied to pin-4 of IC4 through pin-25 of IC-5387 of the timer. The pin-2 of IC-4 is connected to the ground through magnetic contact M2. On the application of positive supply at pin-4, 12 V is applied on pin-8. The delay circuit, consisting of 1 MR resistance and 5 p_F condenser is connected at pin-6, pin-7 and pin-8 of IC-4. The positive output at pin-3 drives the ‘0’ relay. The 12 V is extended to the port of the motor through operated contact 02 of relay ‘0’. The motor starts to move in an

172 S.K. Luthra et d/Agricultural Water Management 33 (1997) 169-181

anticlockwise direction. The valve which is coupled to the motor is thus opened. The magnet is displaced from the front of magnetic contact M2 to the front of magnet contact Ml. Thus, M2 becomes open and Ml is closed. The ground on pin-2 of IC-4

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gets disconnected. ‘0’ relay releases, contacts 01 and 02 become open. Thus, supply is disconnected to the motor. When sufficient amount of irrigation has been applied, a negative signal generated from the modified tensiometer is applied to the pin-2 of IC-3, through closed magnetic contact Ml. On application of positive voltage at pin-4 (through tensiometer switch or timer), 12 V becomes available at pin-3 of IC-3. This voltage operates ‘L’ relay through contact 01 and ground. Now 12 V is connected to the opposite terminal of the motor through operated contact Ll of ‘L’ relay. The ground is connected to the other motor terminal through operated contact L2 of L relay. As the voltage on the motor terminal is reversed, the motor moves in a clockwise direction, closing the valve coupled to it and terminating the irrigation. The magnet in front of contact Ml is displaced and contact Ml opens, - 12 V is disconnected from pin-2 of IC-3 after a delay of a few seconds provided by resistance of 10 kfl and a condenser of 0.1 ~.LF. The 12 V at pin-3 of IC-2 is removed, ‘L’ relay releases, Ll and L2 become open, 12 V supply to the motor terminal is disconnected. Thus, the motor stops after closing the valve.

2.3. Gate valve

The valve system consists of a 2” diameter PVC gate coupled to the shaft of the d.c. motor through the clutch mechanism (Fig. 4). The inlet port is connected to an overhead tank through a PVC pipe and the outlet port is connected to the main or submain drip irrigation system. This type of gate valve is commercially available in different standard dimensions to suit the on-field requirements of the irrigation system. The drive shaft of the valve is connected to a d.c. motor. A reduction gear mechanism is used to reduce the speed of the motor to 2 rev min.‘. A clutch mechanism (Fig. 5) has been designed and incorporated in the device to avoid damage to the gate valve from the overshoot of moving parts in either direction. For smooth functioning of the valve mechanism, the shaft of the gate valve, the shaft of the motor and the centre of the clutch gear should be properly aligned.

A small ring magnet has been embedded on the shaft of the gate valve and two reed contacts, Ml and M2, have been installed on the outer body of the PVC valve. Ml is positioned in front of the magnet in the completely open condition of the valve, while contact M2 faces the magnet in the completely closed condition (Fig. 3).

2.4. Auto pumping unit

To maintain a predecided level of water in the overhead tank, an auto pumping unit has been designed, developed and installed (Fig. 6). The main components of the auto pumping unit are level sensors, control circuitry and a three phase on-line starter. When the water level in the overhead tank falls below the lower sensor ‘L’, an electrical signal is generated by the control circuitry to energise the starter to connect three phase supply to the motor pumping unit, which starts to pump water to the overhead tank. As the level of water touches the upper sensor ‘T’, a signal is generated by the control circuitry to switch off the motor pumping unit. Thus, the water level in the overhead tank is maintained at a constant level automatically.

SK. Luthra et al./Agricultural Water Management 33 (1997) 169-181

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Fig. 5. Clutch assembly (auto irrigation valve).

176 S.K. Luthra et ul./Agriculturul Water Munagement 33 (1997) 169-181

Fig. 6. Auto pumping unit.

2.5. Micro irrigation water meter

Irrigation water supplied to the drip irrigation system can be measured by counting the number of times the overhead tank is emptied (Fig. 7). When the level of water in the overhead tank falls below mark ‘L’, the IC-7 senses this condition and generates a pulse in conjunction with IC-5 which is counted by the set of ICs (l-4). The number of pulses generated by IC-7 is in direct proportion to the number of times water in overhead tank reaches the mark ‘L’.

Five light emitting diodes (LEDs) with sensors have been fixed on the overhead tank, between marks T and L equidistantly. When the water in the overhead tank reaches mark T all LEDs glow. As the water level starts to fall, the LEDs go off sequentially. The period between the ‘glowing off’ of two consecutive LEDs gives the rate of application of water.

2.6. Timer

The timer unit consists of a crystal, frequency divider (IC-1) and frequency counter (Fig. 3). Alternatively, the timer can be used for control of irrigation. The timer is preprogrammed for initiation and termination of irrigation. When the preset time is attained, a signal is extended from the timer to control circuitry for opening or closing of the gate valve by applying a positive supply on pin-25 of IC-2.

S.K. Luthra et al./Agricultural Water Management 33 (1997) 169-181 177

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Fig. 7. Micro irrigation water meter.

2.7. Power supply

The power supply for operation of the d.c. motor and electronic components is obtained from a 12 V storage battery in float with 12 V solar panel. Since the d.c. power requirement of the auto irrigation system is limited to 5 min duration for an irrigation cycle, the life of the battery is quite long and requires minimum maintenance. Protective devices like fuses and diodes have been incorporated into the circuit to avoid damage to the solar panel and electronic components due to a short circuit or any other defect.

Working of auto irrigation system

The drip irrigation system can be operated in three modes: tensiometer controlled; timer controlled; tensio-timer controlled.

178 S.K. Luthru et al./Agriculturd Water Munugement 33 (1997) 169-181

3.1. Tensiometer controlled

In the tensiometer controlled mode, the soil water tension at which irrigation has to be initiated or terminated is selected by plugging the corresponding socket on the terminal strip of the modified tensiometer. The control circuitry processes the signal generated by the modified tensiometer for opening and closing of the valve for initiation and termination of irrigation as explained earlier.

3.2. Timer controlled

In timer controlled irrigation, timer Tl extends positive voltage to pin-4 of IC-4 when the preset time for initiation of irrigation has been reached. Pin-2 of IC-4 is already on earth voltage due to closed Ml contact. Pin-3 becomes high. ‘0’ relay is operated. Control circuitry opens the gate valve as in the case of the tensiometer. As and when the preset time in timer T2 is attained, the timer extends positive supply to pin-4 of IC-3. Pin-2 of IC-4 is already negative because of closed M2 contact, which is in front of the ring magnet during the open condition of the valve. Pin-3 becomes high and the valve is closed by the control circuitry as described in tensiometer control setup.

3.3. Tensio-timer controlled

In this mode of control, irrigation is provided to the plants at a certain rate and the tensiometer does not respond instantaneously to the changing moisture conditions (Klute

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and Gorden, 1952). Irrigation is initiated with respect to a preset value of soil water tension. At the instant of initiation of irrigation, the timer is reset by the resetting circuit (Fig. 3) 12 V is extended to point ‘P’ through operated 02 contact. The point ‘P’ is connected to pin-32, 33, 34 of IC-1 through diodes which reset the display to 0O:OO and the counter restarts counting. The termination of irrigation is effected through timer control as described earlier.

4. Calibration of modified tensiometer

The modified tensiometer was calibrated with respect to standard manometer type tensiometer. After initial charging both tensiometers were installed at similar locations

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Fig. 9. Layout of the drip irrigation system.

180 SK. Luthra et al./Agricultural Water Management 33 (1997) 169-181

and the same depth. The calibration setup is shown in Fig. 8. Electrodes are connected to 12 V through individual LEDs and potentiometer control. In the saturated condition, mercury touches all the electrodes. Therefore, all the LEDS light up. The current through an LED is controlled by an attached potentiometer. With a rise in soil water tension, the level of mercury in the acrylic tube falls. The fall in the level of mercury from the first electrode is indicated by the ‘glowing off’ of the first LED. At this instant, a reading is taken on a standard manometer type tensiometer. Subsequent readings were taken at the ‘glowing off’ of the other LEDs for the calibration of the modified tensiometer.

4.1. Testing of auto irrigation system

The auto irrigation setup was tested in a drip system laid out in a 0.25 ha area at the experimental farm of CSSRI, Karnal (Fig. 9). The drip system consisted of one main, two submains and 15 laterals. Three drippers of standard pressure compensating type were provided for each plant.

To test the control circuitry and valve setup, different soil water tension values were selected for initiation of irrigation. For termination of irrigation, the timer in the control circuitry was programmed for different durations of irrigation.

4.2. Test results

The valve and control circuitry were tested under field conditions. Various soil water tension values were selected for automatic initiation of irrigation. Observed values at the instant of initiation of irrigation were obtained through a standard manometer type tensiometer installed at the same depth and location as that of the modified tensiometer. Values of duration of irrigation programmed on the timer were recorded.

Table 1 shows that the selected values of soil water tension closely matched observed

Table 1 Results of testing the auto irrigation system

Sr. no. Date S WT,, SWT,, SWT,, SWT,, -SWT,, T, To r, - T,

2. 3. 4. 5. 6.

8. 9. 10. Il. 12.

24/4/1995 412 450 452 25/4/1995 408 450 450 26/4/ 1995 368 450 448 27/4/ 1995 342 450 446 l/5/1995 470 500 504 3/5/ 1995 450 500 502 14/5/1995 304 350 350 15/5/1995 208 350 348 16/5/1995 256 300 302 17/5/1995 216 250 252 19/6/1995 382 400 404 2/7/ 1995 414 450 450 0

2 2 0 4 4 0 4 4 0 5 5 0 6 6 0 8 8 0

12 12 0 10 10 0 6 6 0 8 8 0 6 6 0 4 4 0

SWT,, observed value of soil water tension at setting (mbar); SWT,,, set value of soil water tension for initiation of irrigation (mbar); SWT,,, observed value of soil water tension at the instant of initiation of irrigation (mbar); T,, set value of duration of irrigation (h); I”,, observed value of duration of irrigation (h).

SK. Luthra et al./Agriculturd Water Management 33 (1997) 169-181 181

values. This implies the satisfactory performance of the control circuitry and timer. The volume of discharge through the valve with automatic control matched that with manual control. Therefore, the performance of the valve in completely open and closed conditions was satisfactory.

5. Summary and conclusions

The auto irrigation system developed monitors soil water stress at the root zone continuously and controls irrigation as per preset values of soil water tension and duration of irrigation. The gate valve is motor controlled and works against any head. The magnetic contacts and PVC material used in the construction of the gate valve prevent corrosion and sparking. The low power requirement of the system can be easily met through a 12 V storage battery in float with a solar panel. The auto pumping unit ensures the storage of water in an overhead tank for the supply of irrigation water to the drip system. This indigenously developed low cost auto irrigation system ensures better returns to Indian farmers through considerable savings in labour and other farm inputs.

Acknowledgements

The authors gratefully acknowledge the support and guidance of Director C.S.S.R.I., Karnal. The help of Sh. Munish Kumar (technician) in design and fabrication of the valve is also gratefully acknowledged.

References

Austin, R.S. and Rawlins, S.L., 1977. Optoelectronic level detector for mercury manometer. J. Agric. Eng. (Instr. News Sect.), 58: 29-30.

Fischbach, P.E., Thompson, L.T. and Stetson, L.E., 1970. Electric controls for automatic surface irrigation system with reuse system. Trans. ASAE, 13: 286-288.

Haise, H.R. and Kruse, E.G., 1969. Automation of surface irrigation systems. ASCE J. Irrig. Drain. Div., 95: 503-516.

Hollis, S. and Dylla, AS., 1980. Irrigation automation with moisture sensing system. Trans. ASAE, 23: 649-656.

Klute, A. and Gorden, W.R., 1952. Tensiometer response time. Soil Sci., 93: 204-207. Phene, C.J., Hoffman, O.J. and Austin, R.S., 1973. Controlling automated irrigation with soil matric potential

sensor. Trans. ASAE, 16: 773-776. Trotter, C.M., 1984. Errors in reading tensiometer vacua with pressure transducer. Soil Sci., 138 (4): 3 14-3 16.