The attempt was to design and develop an automatic closed canopy chamber (ACCC) having dimension of 1.2 m×1.2 m× 1.2 m for crop evapotranspiration measurement by using transparent acrylic sheet of 4 mm thickness. Between two small fans a temperature and relative humidity sensor was used to measure vapor density. The intermediate circuit was developed for making automation system in ACCC. The arduino based coding was developed as per desired logic operation. The top lead of chamber was automatically closed for 2 minutes when inside and outside temperature and relative humidity of ambient air were similar. During measurement mode of ACCC, the two fans were started automatically. After measurement mode, fans were automatically stopped and top lead was opened. The ACCC was calibrated by evaporating mass of water from water filled tray which was placed inside the automatic closed canopy chamber. The validation of the developed ACCC were made using micro-lysimeters (MLS) having size of 0.2 m × 0.2 m × 0.2 m by growing shallow rooted crop like fenugreek. The depth of irrigation was computed based on soil moisture content before irrigation and field capacity. The field testing of ACCC was made by placing chamber in plots of fenugreek crop. The irrigation was applied by drip irrigation as per crop water consumption. The sensor sensed and recorded the instantaneous temperature and relative humidity at 1 second interval for 24 hours. Two sample t-tests were done to compare the data pair of crop evapotranspiration obtained by the MLS inside the ACCC with that of outside the ACCC to ascertain whether there is any effect of the change in micro-climate for a short period of 2 minute on the crop growth physiological processes. Also, the data pair of crop evapotranspiration measured by the MLs, ACCC using the sensor data of temperature-relative humidity were compared and statistically analyzed through t-test. Similarly, the data pair of ETC measured by the FWB (Field Water Balance Method) and ACCC using the sensor data of temperature-relative humidity were also compared and statistically analyzed through t-test. The calibration factor of the ACCC was found as 1.666. The results revealed that there was no significant difference in the crop evapotranspiration measured by the MLs inside and outside ACCC. Also in case of validation and field testing of ACCC, there were no significant difference between the ETC measured by the ACCC, MLs and FWB at 95 percent confidence level. This implies that there are no effects of the change in micro-climate for a short period of 2 minutes in the chamber, on the plant physiological processes. The ETc rate of fenugreek increases as sun rises and reaches the peak after one to two hour from mid-day and then continuously decreases with time. During validation and field testing of ACCC, the fenugreek crop coefficients varied from 0.72 to 1.04 and 0.69 to 1.02 respectively. The developed ACCC is portable as well as more comfortable and cost effective compared to the lysimeter for the measurement of the actual crop evapotranspiration and the crop coefficient.
The calibration factor of the ACCC was found as 1.666.
There was no significant difference in the crop evapotranspiration measured by the MLs inside and outside ACCC.
Also in case of validation and field testing of ACCC, there were no significant difference between the ETC measured by the ACCC, MLs and FWB at 95 percent confidence level.
There were no effects of the change in micro-climate for a short period of 2 minutes in the chamber, on the plant physiological processes.
The developed ACCC is portable as well as more comfortable and cost effective compared to the lysimeter for the measurement of the actual crop evapotranspiration and the crop coefficient.
Land and water are two essential requirements for improvement in agriculture and economic expansion of any country. Demand of water increases continuously, while its supply decreases. India has about 16% of the world’s population as compared to only 4 % of its water resources. Therefore, these resources are need to be managed efficiently, optimally and sustainably in a country like India, where demand of these natural resources for ever increasing population outdoes the availability of these resources. India occupies only 329 Mha (Million hectare) area. India has 2 % of the world’s land area. India has about 140 Mha of cultivable land. 42% of the country’s cultivable land lies in drought-prone areas/districts. Irrigated area of India is 64.7 Mha. Moreover, 54% of India’s net sown area depends on rain, rain fed agriculture plays an important role in the country’s economy. India’s annual rainfall is around 1183 mm, out of which 75% is received in a short span of four months during monsoon (June to September). This result is run offs during monsoon and calls for irrigation investments for rest of the year. India receives an average of 4000 BCM (Billion Cubic Meters) of precipitation every year. However, only 48% of it is used in India’s surface and groundwater bodies. It is estimated that owing to topographic, hydrological and other constraints, the utilizable water with conventional approach is 1137 BCM which comprises of 690 BCM of surface water and 447 BCM of replenishable ground water resources. In India per capita surface water availability in the years 1991 and 2017 were 2309 and 1544 m3 and these are projected to reduce to 1401 and 1191 m3 by the years 2025 and 2050 respectively (
Evapotranspiration (ET) is the combined loss of water due to evaporation and transpiration. The main climatic factors affecting evapotranspiration are Solar radiation, air temperature, air humidity and wind speed etc. Soil water content, which relates to soil water movement, vapour transfer and soil root relationships, is another vital factor for ET (
The weighing lysimeter allows the mass or volumetric soil-water content change to be measured by weighing the lysimeter and finding its mass change over time. Lysimeters are used for accurate measurement of crop evapotranspiration ETC. However, there are some limitations for using lysimeters on field scale due to its higher initial cost, need for regular maintenance, complex and difficult construction and usage limitations to a single crop at a time. Lysimeters cannot be transported from one place to another place due to its immovable establishment.
Indirect and direct methods have been used to quantify ET fluxes. Indirect methods include the use of micrometeorological techniques such as eddy covariance (EC) (
The direct measurement of crop evapotranspiration and crop coefficient can be obtained easily and accurately by canopy chamber due to its simple construction and portability. With the help of canopy chamber, one can estimate the crop evapotranspiration and crop coefficient for many crops through diurnal planning of ETc measurement. An automatic transient state closed system canopy chamber is used for gas exchange determinations of field crops to allow unattended day and night, high frequency H2O exchange measurements, with short measurement time and high scanning rate. An essential characteristic of the transient-type chambers is their portability from place to place in the field, achieved by mounting them on carriers such as farm tractors, fork lifts, gantries, or simply However, in all cases the need for human intervention during night-time imposes strong limitations on the monitoring program. In order to overcome these problems, an automated closed system chamber has been developed to allow day and night high frequency measurements of H2O gas-exchanges. After 2 or 3 days of gas-exchange flux monitoring, the chamber is removed manually and placed at a different location. The advantages obtained in terms of automated and unattended monitoring of the gas-exchange are, of course, at the expense of a reduced number of replicates, or sampled sites. However, such studies have not been conducted yet in India. Therefore, keeping in view the above issues and importance of ETC measurement, present study was carried out .
The experiment was conducted at the Instructional farm of the Soil and Water Conservation Engineering Department, College of Agricultural Engineering and Technology, Junagadh Agricultural University, Junagadh. It is located at 21.5° N latitude and 70.1° E longitude with an altitude of 82 m above mean sea level on the western side foothills of Mountain Girnar. The location map of study area is show in
The climate of the study area is subtropical and semi-arid type with an average annual rainfall of 900 mm and average annual pan evaporation of 5.6 mm/day. The area is characterized by climatic condition of fairly cold and dry winter, hot and dry summer and warm and moderately humid during monsoon. Winter sets in the month of November and continues till the end of February. Summer commences in the second fortnight of February and ends in the middle of June. April and May are the hottest month of Summer. According to last 35 years weather data recorded at the JAU observatory located near to experimental site, the monthly mean of daily maximum temperature, minimum temperature, relative humidity, wind speed, bright sunshine hours and pan evaporation during the crop period ranged from 30.2 °C to 38.9 °C, 12.2 °C to 22.2°C, 62.2 % to 74.4 %, 3.5 km/hr to 6.6 km/ hr, 8.1 to 9.5 hours and 4.6 to 9.5 mm, respectively.
Location map of study area
Physiochemical properties including fertility status of soil in the experimental soil are presented in
Physiochemical properties of the soil of experimental field
Ground water was used to irrigate the Fenugreek crop. The analysed quality of irrigation water was depicted in
Quality analysis of irrigation water
The resources and materials used during the experiment are described in subsequent heads.
Transparent acrylic sheet having 4 mm thickness and 95% transmittance was used for fabrication of chamber.
The supporting frame was made using 0.75” GI square pipe to support the acrylic sheet and the mounting of components like sensors and fan.
HTC easy log Temperature and Humidity datalogger was used for sensing and recording the temperature and relative humidity inside the automatic closed canopy chamber. Temperature sensor was used for measuring temperature of air within the chamber. This compact datalogger has a built-in LCD screen to monitor the current temperature, relative humidity, logging status, battery use and the memory consumption in between the readouts.
Two small fans were used as per requirement for proper air and water vapour mixing inside the chamber.
The battery of 12 Volt and more than 476 Ampere hour was used for power supply to the 12 volt dc geared motor for opening and closing of top lead of chamber and two small fans.
For making automatic closed canopy chamber, intermediate circuit was developed by using following components:
The Arduino Mega 2560 is a microcontroller board based on the ATmega 2560. The arduino Mega 2560 has hardware parts and software parts. The hardware parts include 54 digital input/output pins. Out of 54 digital input/output pins 14 can be used as PWM (Pulse Width Modulation) outputs, 16 analog inputs, 4 UARTs (Universal Asynchronous receiver Transmitter) hardware serial ports, a 16 MHz crystal oscillator, a USB (Universal Serial Bus) connection, a power jack, an ICSP (In Circuit Serial Programming) header and a reset button. The software parts include IDE (Integrated Development Environment) and Sketch.
A breadboard is a solderless device for temporary prototype with electronics and test circuit designs. Breadboard is a board on which electronic circuits can be built. The breadboard has strips of metal underneath the board and connect the holes on the top of the board. The metal strips are laid as shown below. The top and bottom rows of the holes are connected horizontally and split in the middle while the remaining holes are connected vertically. The set of connected holes can be called a node. Most electronic components in electronic circuits can be interconnected by inserting their leads or terminals into the holes and then making connections through wires where appropriate.
Jumper wires are simply wires that have connector pins at each end, allowing them to be used to connect two points to each other without soldering. Jumper wires are typically used with breadboard and other prototyping tools to make it easy to change a circuit as needed.
USB cable allows to connect Arduino Mega 2560 to personal computer for programming. It also provides power to the Arduino Mega2560.
The AC to DC adaptor having barrel connector was used for supplying an input of 7-12 V. This is regulated to 5 V by the onboard voltage regulator and the board is powered on.
High voltage electronic devices can be controlled by using relays. A relay is actually a switch which is electrically operated by an electromagnet. The electromagnet is activated with a low voltage, for example 5 volts from a microcontroller and it pulls a contact to make or break a high voltage circuit. 4 channel relay has 4 relays with rating of 10 A @ 250 and 250 V AC and 15 A @ 125 V AC. The high voltage output connector has 3 pins, the middle one is the common pin and one of two other pins is for normally open connection and other one for normally closed connection.
On the other side of the module has 2 sets of pins. The first one has 4 pins, a ground and a VCC pin for powering the module and 4 input pins IN1, IN2, IN3 and IN4. The second set of pins has 3 pins with a jumper between the JDVcc and the Vcc pin. With this configuration, the electromagnet of the relay is directly powered from the arduino board and if sometimes goes wrong with relay the microcontroller could get damaged.
DC gear motor is also called as DC geared Motor, Geared DC motor or Gearbox motor. DC Geared motor consists of a electric DC motor and a gearbox or gear head. DC motor was used for converting electrical energy into mechanical energy when electricity is applied to its leads. Coils of wire inside the motor become magnetized when current flows through them. These magnetic fields attract and repel magnets, causing the shaft to spin. If the direction of the electricity is reversed, the motor was spinned in the opposite direction. The gear heads are used to reduce the DC motor speed, while increase the DC motor torque. Therefore, lower speed and higher torque can be generated from gear motor. 12 V DC Geared Motor was used for opening and closing of top cover of Automatic closed canopy chamber.
The electronic balance was used for weighting the micro-lysimeters having capacity of 20 kg and least count of 0.1 g.
Ten weighing type micro-lysimeters of size approximately 0.20 × 0.20 × 0.20 m fabricated from aluminium sheet of 1 mm thickness were used. It was used for growing crops by filling soil inside.
A square aluminium tray was used for filling the water and measuring the evaporation by the automatic closed canopy chamber as well as the pan evaporation method.
The chamber was constructed according to a design approach used in a previous study (
The Automatic closed chamber had top cover of 10 kg weight. The force of gravity pulling it down from centre of mass is
The lifting force (T) was applied to top cover with angle of 45° with horizontal. Then vertical component of lifting force (T) was T sin 45°. Therefore, moment of vertical component of lifting force (T) at (X = 1.2 m) distance from hinge point balanced moment of force due to weight (W) at centre of mass located at 0.6 m distance from hinge point of top cover of automatic closed chamber.
Since force was applied tangentially on pulley having radius of 0.02 m. Therefore, torque (τ) needed at pulley due to tension in string was,
After considering the friction, the actual torque was taken as 0.7 Nm.
Physically, power is defined as the rate of doing work. For rotational motion, the power is the product of torque multiplied by the rotational distance per unit time.
Where,
ω = Angular velocity (rad/sec)
From equation (
The overall efficiency of DC geared motor depends on the number of reduction stages: on average is 90% per stage. Therefore, a two stage reduction gives 90 × 90 = 81%; 3 stage was 72.9% and a 4 stage reduction was 66%. Therefore, efficiency of motor was considered as 66%. So the motor should be having capacity of as below:
Therefore, the direct current geared motor having torque of 0.7 Newton-meter and power rating of 0.33 Kilowatt was selected.
Since, DC motor was runned 10.8 minutes out of 1 hours. Therefore, working hour was 4.32 hour per day.
Energy consumption = 330 W × 4.32 hr = 1425.6 Watt hour
Considering 30% energy lost in the system.
So, total Energy consumption = 1425.6 × 1.3 = 1853.28 Watt hour/day
The solar panel converts the sunlight into electricity as direct current (DC). These are typically categorized as Monocrystalline or Polycrystalline. Monocrystalline is costlier and efficient than the polycrystalline panel. Solar panels are generally rated under standard test conditions (STC): irradiance of 1000 W/m2, the solar spectrum of AM 1.5 and module temperature at 25°C.
The solar panel was selected in such a way that it charges the battery fully during the day time.
During the 12 hr day time, the sunlight is not uniform. It also differs according to location around the globe. Therefore, 4 hours was assumed as effective sunlight hours which will generate the rated power.
Total Wp of PV panel capacity needed = 1853.28 Wh /4h = 463.25 W.
By taking some margin, the solar panel of 470 W, 12 V was chosen.
Battery capacity was rated in terms of Ampere Hour.
Power = Voltage × Current
Watt Hour = Voltage (Volts) × Current (Amperes) × Time (Hours)
Battery Voltage = 12 V (as our system is 12 V)
Battery capacity = Load/Voltage = 1853.28 /12 = 154.44 Ah
Practically battery is not ideal, so 15% will be considered as battery loss.
So, battery capacity required was 154.44/.85 = 286 Ah
For better battery life, flooded lead-acid battery 60% depth of discharge (DOD) was considered as good practice.
So capacity required = 286/.6 = 476.6 Ah
Therefore, Battery of more than 476.6 Ah capacity was selected.
A solar charge controller is a device that was placed between a solar panel and a battery. It regulated the voltage and current coming from solar panels. It was used to maintain the proper charging voltage on the batteries. As the input voltage from the solar panel raised, the charge controller regulated the charge to the batteries preventing any overcharging.
Usually, the solar power systems used 12-volt battery, however, solar panels could deliver far more voltage than is required to charge the battery. By, in essence, converting the excess voltage into amperes, the charge voltage could be kept at an optimal level while the time required to fully charge the battery was reduced. This allowed the solar power system to operate optimally at all times.
Since solar system was rated 12 V, the charge controller was of 12 V.
Current rating = Power output of panels/ Voltage = 470W/12V = 39.16 A
By taking a 20 % margin, 39.16 × 1.2 = 47 A charge controller.
Therefore, Charge controller of 12 V and a current rating of 47 A was chosen.
The chamber consisted essentially of a parallelepiped with five transparent Acrylic walls (4 mm thick), was held together by a narrow metallic angular frame (1 cm wide). The same materials was for constructing adapter frames for different heights. Four different heights of canopy chambers were designed to match different crop growth stages. A short chamber was more portable to be transported from one sample site to another sample site, while a large chamber has large capacity of water vapor for large ET flux and reduce the possibility for internal condensation occurring on the acrylic film. Therefore, it was suitable to use chambers of different heights to measure ET fluxes for different canopy heights. The ground surface area of chamber was 1.44 m2 (1.2 m
The height of chamber was changed by placing another chamber over previous chamber assembly and it was fitted by using nuts and bolts. The gasket air tight rubber was placed between top of lower section and bottom of upper section of chamber for avoiding air leakage. All corners was enforced with angle iron using a drill and iron rivets. A 1.44 m2 acrylic glass sheet was glued on top of a frame which was serves as the chamber cover.
A rubber sealing was placed on the closing edge of the top-cover, with a magnetic strip inside, to allow tight closure during measurements upon release by the rotating pulley. Two water proof fans were installed at 45
The bottom side of the chamber consisted of a metallic frame with lateral sharp blades sunk in the soil. The top-cover was pivoted by using exterior door hinges on one side so that a simple pulley driven string can pull up the other side. The alternate motion of pulling and releasing the string (to open and close the top cover) was run by a simple rotating 12 V Direct current motor with relay by using microcontroller (Arduino MEGA2560) through an intermediary circuit. Between the fans, a combination temperature and relative humidity sensor was mounted inside the chamber to monitor internal atmosphere. A data-logger was used for the data collection and storage for all of the data of sensors connected to the canopy chamber. The measured values were used to verify that the climate condition inside the chamber is similar to the ambient climate condition during measurement. The air temperature inside of the chamber increased during measurements, because of the greenhouse effect of the chamber. Changes in water vapor concentration inside the chamber was measured to evaluate the ET flux. A portable rechargeable battery was utilized as power supply.
The arduino based intermediate circuit was developed for making automatic closed canopy chamber. The programming code was based on required logic of operation of canopy chamber. Developed intermediate circuit of automatic closed canopy chamber is shown in
Developed intermediate circuit of automatic closed canopy chamber
Two operational mode was recognized: stand-by mode, when the top cover was open, fans were off and another operational mode was measurement mode, when fans were on, the top cover was closed and fans are set off and top cover opens again. Temperature and relative humidity sensor sensed temperature and relative humidity continuously. When inside and outside temperature and relative humidity were same, then measurement mode started. Similarly, this cycle was repeated. The conceptual design of automatic closed canopy chamber was shown in
Conceptual design of automated closed canopy chamber
Generally, the transparency of 4 mm acrylic sheet is 95%. The transparency of the acrylic sheet with and without overlapping was checked using lux meter at different time intervals.
Micro-lysimeters were fabricated from light aluminium sheets of 1 mm thickness. The size of each micro-lysimeters was 20 cm
All the 10 micro lysimeters were filled with soil of Instructional Farm of Soil and Water Conservation Engineering Department in such a way that the bulk density of the soil was maintained at the field value. The average weight of the soil in the micro lysimeters were 11.4 kg and the volume in each micro lysimeter was 7600 cm3. The bulk density of soil in the micro-lysimeter was 1.5 g/cm3 which was same as the field value. The faces of adjacent micro lysimeters were kept just touching each other.
The Fenugreek crop was sown manually in the micro lysimeters on 7/12/2019. The seed to seed distance was maintained at 2-3 cm and the depth was maintained at 1 cm. The average germination percentage of seed was 90%.
Irrigation was applied manually with a measuring flask as per the crop water requirement calculation. The crop water requirement was obtained from the lysimeter and irrigation water depth was calculated. Application of irrigation water in micro-lysimeter was in such a way that there was no drainage due to deep percolation.
Seedbed was prepared by tractor operated cultivator. The dimension of seed bed was 1.8 × 10 m. Three seed beds were prepared for fenugreek crop.
Crops were sown manually in seedbed by keeping 30 cm and 10 cm for row to row and plant to plant spacing respectively.
The calibration of water meter was done by using volumetric bucket. Water was passed through the water meter. The actual amount of water filled in bucket was compared with water meter reading. The ratio of the actual amount of water collected in volumetric bucket to water meter reading was considered as calibration factor of water meter.
Water meter was connected between submain and laterals. The inline lateral spacing was 0.6 meter and dripper spacing of 0.4 m having discharge of 2 lph.
Fertilizer was applied as per recommended dose in kg/ha of fenugreek crop. The recommended dose of Nitrogen, Phosphorus and Potassium for fenugreek crop was considered as 20, 25 and 20 kg/ ha respectively. The amount of application of DAP, Urea and Murate of Potash were 308.1, 126.51 and 189 gm respectively. Before irrigation DAP was applied as basal dose according to recommended dose of fertilizer.
Irrigation was applied through drip irrigation once in three days according to soil moisture depletion from field capacity. Soil sample before irrigation and 24 hour after irrigation was collected by auger hole method. The augers were penetrated upto effective root zone depth. Soil samples were taken from slots of augors and placed in moisture box. The initial weight of soil sample was taken and placed in oven dryer at 105°C for 24 hours. The final weight of oven dried sample was recorded. Dry basis moisture content was calculated. This moisture content was converted into volumetric moisture content by multiplying bulk density of soil sample. The effective root zone depth of fenugreek was considered as 20 cm. The water was applied according to difference between volumetric moisture content of soil sample after irrigation and before irrigation. The amount of delivered water to the field was recorded by water meter reading by considering calibration factor of water meter.
When inside and outside microclimate is same, the chamber was automatically closed for 2 minutes and chamber was opened. When chamber was closed, the fans were automatically started and circulated air water mixture for 2 minutes. This cycle was repeated for 24 hours. The sensor sensed and recorded the instantaneous temperature and relative humidity at 1 second intervals for 24 hours continuously.
The crop evapotranspiration data were collected over a period of 2 minutes during closed condition of chamber throughout the day. The automatic closed canopy chamber was positioned over the fenugreek crop grown in the micro-lysimeters. The chamber was automatically closed for 2 minutes under similar micro-climate condition of inside and outside of automatic closed canopy chamber.
Measurement of fenugreek crop evapotranspiration in the automatic closed canopy chamber
After 2 minutes, chamber was automatically opened to obtain micro-climate similar to open atmosphere.
While a data logger measured the temperature and relative humidity at every 1 second for 24 hours continuously. Efforts were made not to block solar radiation during measurement. Datasets were collected within 2 minutes and datasets were repeated whenever chambers were closed. The measurement of fenugreek crop evapotranspiration in automatic closed canopy chamber is shown in
Automatic closed canopy chamber was placed in different plots having fenugreek crops once in two weeks. The chamber was placed in such a way that crops inside the chamber should not disturb. The evapotranspiration of field crops was measured under closed condition of chamber for 2 minutes by automatic closed canopy chamber. The measurement of fenugreek crop evapotranspiration in the automatic closed canopy chamber is shown in
Measurement of fenugreek crop evapotranspiration of field crops in the automatic closed canopy chamber
Temperature and relative humidity measurements were converted into saturation vapour pressure (Es), actual vapour pressure (Ea), and vapour density (ρv) as per (
Where,
Actual vapour pressure (Ea) was calculated using relative humidity as:
Where,
The density of water vapour inside the chamber was calculated as,
Where,
Where,
The soil surface area in micro-lysimeters was 0.2 m2 and the base area of automatic closed canopy chamber was 1.44 m2. There was no evapotranspiration from the remaining area and that much less water vapour was generated. So the area factor for calculation of ETc under automatic closed canopy chamber was taken as 1.44/0.2 = 7.2. It was multiplied by the chamber ETc for correction of automatic closed chamber ETc.
The area factor of automatic closed canopy chamber was calculated by following equation,
The chamber calibration factor, which account for the slight hydrophilicity of the chamber material, was determined using method described by (
A square aluminum tray having sides of 84 cm was used for calibration of automatic closed canopy chamber by measuring the water evaporation throughout the day. Known weight of water was filed in the tray and its evaporation was measured by the automatic closed canopy chamber and compared with water volume lost from the tray.
This procedure was repeated several times at each of the several known evaporation rates. The chamber and known evaporation rates were plotted against one another and the slope of the best-fit line was taken as the calibration factor. The calibration of automatic closed canopy chamber is shown in
Calibration of automatic closed canopy chamber
The fabricated and calibrated automatic closed canopy chamber was put on tray having 5 micro-lysimeters. The ET values were estimated using data of water vapour flux measured and recorded by the temperature – RH sensor using the equation (
The weight of each of the 10 micro-lysimeters were taken as 3 day interval. Irrigation water as per requirement was applied to micro-lysimeters during the experimentation. Amount of water drained was considered zero because no drainage water was found from the micro-lysimeters during experiment period because required amount of water was applied. The fenugreek crop evapotranspiration measured by micro-lysimeters is shown in
The crop evapotranspiration was calculated as below;
Where,
The ETc measured by automatic closed canopy chamber was compared with that obtained by the water balance in micro-lysimeters.
The field water balance method uses soil moisture, precipitation and drainage data to estimate crop ET. The soil water content in the field plot was measured by oven dry method. Soil samples were taken at different depths according to root zone of crops grown in field. Soil water storage in the root zone was computed.
ETc measurements by micro-lysimeters
The water was given in precise amount to prevent drainage. The field water balance method is based on the following equation,
Where,
The aim of field water balance method is to quantify all water balance components on the left hand side of the equation number (12) to compute ET. Accurately monitoring short term changes in soil moisture is one of the challenges of this method. Timlin
Fundamentally, the crop coefficient is defined as the ratio of crop ET (ETc) to some reference ET (ETo) as defined by weather data. In FAO-56, values listed for Kc represent ET under growing conditions having a high level of management and with little or no water or other ET reading stress and thus represent what are referred to as potential level of crop ET.
Kc curve comprises of four straight line segments representing the initial period, the development period, the mid season period, and the late season period. These segments are defined by three primary Kc values; Kc during initial period Kc ini, Kc during mid season (full cover) period Kc mid, and Kc at harvest (or at the end of late season) Kc end. The Kc ini defines the horizontal portion of the Kc curve during the initial period until approximately 10% of ground is covered by vegetation. The Kc mid defines the value for Kc during the peak period for the crop, which is normally when the crop is at “effective full cover”. This period is described by a horizontal line extending through Kc mid. The development period is defined by a sloping line that connects the initial and mid season period. The late season has a sloping line that connects the end of the mid season period with the harvest (end) date.
The crop coefficient can be calculated by the following equation.
Where,
Where, ETO = Reference evapotranspiration (mm/day)
A two sample t test was used to compare the 2 sets of data to check whether they are significantly different or not. It is rigorous test of the hypothesis that the two samples are drawn from a population having the same variance. t – Test is significantly used only with small samples. If m is the mean of the sample of size n and s is the standard deviation as estimated from the sample and if we are testing the deviation of m from a hypothetical value µ then the t-Test used to test whether the means are different or not that can be calculated as follows (
Where,
In these formulae,
Daily maximum and minimum temperature observed during December to March 2019-20 is shown in
Daily maximum and minimum temperature during December 2019 to March 2020
Daily maximum and minimum relative humidity during December 2019 to March 2020
Daily wind speed during December 2019 to March 2020
Daily bright sunshine hours during December 2019 to March 2020
The calibration of automatic closed canopy chamber was done by evaporating a known mass of water and then placing the chamber over the water to determine evaporation rate based on temperature and relative humidity measurement. The evaporation of water estimated by measuring the volume of evaporated water from tray and automatic closed canopy chamber are shown in
Calibration of automatic closed canopy chamber
The chamber and known evaporation rate was plotted against one another as shown in
Calibration of automatic closed canopy chamber
The weight of micro-lysimeters were measured at the required intervals. Two sets of 5 micro-lysimeters were used for fenugreek crop. One set of 5 micro-lysimeters was used for estimating the crop evapotranspiration of fenugreek inside the automatic closed canopy chamber and another set was kept outside the automatic closed canopy chamber having condition of continuous open atmosphere. The input of irrigation was kept similar in both the sets. Using the water balance of each micro-lysimeter, the fenugreek ETC was measured for both set.
Comparison of ETc of fenugreek crop by ML inside the ACCC and open atmosphere
Comparison of ETc of fenugreek crop by ML inside the ACCC and open atmosphere are graphically represented in
The ETc by micro-lysimeter inside the automatic closed canopy chamber and open atmosphere were recorded for statistical comparison. Statistical analysis of ETc by micro-lysimeter inside the automatic closed canopy chamber and open atmosphere was carried out to check whether these two values are statistical at par or not. The data of ETc by micro-lysimeter inside the automatic closed canopy chamber and open atmosphere value were considered as two separate groups and two sample t-test was conducted assuming equal variance at 95 percent confidence level of each group.
The Statistical comparison by t-test of ETc by MLs inside the ACCC and open atmosphere for fenugreek crop is presented in
The t-test result of statistical comparison of fenugreek ETc by MLs inside the ACCC and open atmosphere
The present investigation was carried out in Rabi Season (7th December 2019 to 30 March 2020). The fenugreek crop was sown in MLs as well as in field plots. Water balance method was adopted to measure the crop evapotranspiration. The volume of water balance components consists of irrigation, precipitation, variation of soil water contents, drainage and finally actual ETC values during experimental study. The precipitation component of water balance method was considered zero during Rabi season because no effective rainfall were observed during study period. The drainage component was also considered as zero because precise amount of water was applied inside the MLs as well as field plots.
Fenugreek crop evapotranspiration measured by MLs and FWB during Rabi season
The crop evapotranspiration (ETc) of fenugreek measured by automatic closed canopy chamber at 11 days interval and its comparison with micro-lysimeters measured ETc is shown in
Comparison of fenugreek crop evapotranspiration measured by ACCC and MLs
Comparison of fenugreek crop Evapotranspiration measured by automatic closed canopy chamber (ACCC) and Micro-lysimeters (MLs) during Rabi season is represented in
Relation between Fenugreek crop evapotranspiration measured by ACCC and MLs
Relation between crop evapotranspiration measured by ACCC and MLs for fenugreek crop is represented by plotting
The crop evapotranspiration of fenugreek measured by ACCC and MLs was statistically analysed. Statistical analysis of crop evapotranspiration of fenugreek measured by ACCC and MLs was carried out to check whether these two values are statistically at par or not. The data of crop evapotranspiration of fenugreek measured by ACCC and MLs value were considered as two separate groups and two sample t-test was conducted assuming equal variances at 95 percent confidence level of each group.
The t-test result of statistical comparison of Fenugreek crop evapotranspiration measured by ACCC and MLs
The Statistical comparison by t-test of ETc measured by ACCC and MLs for fenugreek by ACCC and MLs are presented in
Comparison of fenugreek crop evapotranspiration measured by ACCC and FWB during
Comparison of fenugreek crop evapotranspiration measured by ACCC and FWB
It can be seen from
Relation between fenugreek crop evapotranspiration measured by ACCC and FWB for fenugreek is represented by plotting
Relationship between fenugreek crop evapotranspiration measured by ACCC and FWB
From
The fenugreek crop evapotranspiration measured by ACCC and FWB were statistically analysed. Statistical analysis of fenugreek crop evapotranspiration measured by ACCC and FWB was carried out to check whether these two values are statistically at par or not. The data of fenugreek crop evapotranspiration measured by ACCC and FWB value were considered as two separate groups and two sample t-test was conducted assuming equal variances at 95 percent confidence level of each group.
The t-test result of statistical comparison of fenugreek crop evapotranspiration measured by ACCC and FWB
The Statistical comparison by t-test of fenugreek crop evapotranspiration measured by ACCC and FWB are presented in
The diurnal variation of fenugreek crop evapotranspiration rate under validation and field testing of ACCC is shown in
From
Diurnal variation of fenugreek ETc rate on 11th February 2020 under validation of ACCC
Diurnal variation of fenugreek ETC rate on 5th February 2020 under field testing of ACCC
It was found that during night time, evapotranspiration was very less as compared to the day time evapotranspiration. This is consistent with the previous study with the enclosed portable chamber (
The crop coefficient (KC) was taken as the ratio of the actual evapotranspiration to reference evapotranspiration. The actual evapotranspiration (ETC) was measured by MLs and FWB and reference evapotranspiration (ET0) was estimated using Penman Monteith equation.
According to FAO 56, the growing period of fenugreek crop is divided in 4 sections as initial (0 to 7 DAS), development (7 to 38 DAS), mid (38 to 65 DAS) and late (65 and onwards) period (
The seasonal variation of fenugreek crop coefficient obtained by MLs under validation of ACCC
From
The growth stage-specific KC value of fenugreek crop were determined based on KC curves that represent the distribution of KC over time throughout the season (
The seasonal variation of fenugreek crop coefficient obtained by FWB under field testing of ACCC
The developed automatic closed canopy chamber is portable as well as more convenient, cost effective and reasonably accurate as compared to the lysimeter to measure the actual crop evapotranspiration and crop coefficient.
The calibration factor of ACCC was found as 1.666.
The crop evapotranspiration measured by lysimeter inside and outside the automatic closed canopy chamber were found reasonably comparable to each other indicating that there are no effects of changes in micro climate for short span of time inside the automatic closed canopy chamber on the plant growth processes.
The crop evapotranspiration measured by the micro-lysimeter kept inside the automatic closed canopy chamber and that computed by sensed temperature and relative humidity inside the automatic closed canopy chamber were found reasonably comparable to each other.
The crop evapotranspiration measured by field water balance method and that computed by sensed temperature and relative humidity inside the automatic closed canopy chamber were found reasonably comparable to each other.
The following mathematical models are proposed to measure actual crop evapotranspiration based on ML results using the ETC measured by ACCC.
Where;
The following mathematical models are proposed based on FWB results to estimate actual crop evapotranspiration using the
Where;
Author thankful to Department of soil and water conservation Engineering, College of Agricultural Engineering and Technology, Junagadh Agricultural University, Junagadh-362001, Gujarat, India.