Both the empirical and theoretical methods require an estimate or measurement of the temperature difference between the canopy and the air, the baseline without water stress, and the non-transpiring temperature difference between the canopy and the air. The zero water stress baseline can also only be used if the irrigator's goal is to obtain maximum yields.
LIST OF TABLES
LIST OF APPENDICES
CHAPTERl
INTRODUCTION
Shortly after the proposed empirical approach, Jackson et al. 1981) presented a theoretical method for calculating the plant water stress index. Empirical and theoretical methods were used to determine the water stress index of cereal rye and annual ryegrass crops.
INFRARED THERMOMETRY FOR MEASURING CANOPY TEMPERATURE
- INTRODUCTION
- HISTORICAL PERSPECTIVES
- METHODOLOGY
- Principles of use of infrared thermometers
- Calibration of infrared thermometers
- FACTORS AFFECTING CANOPY TEMPERATURE MEASUREMENTS
- Instrumentation factors affecting canopy temperature measurements
- Environmental factors affecting canopy temperature measurements
- Plant factors affecting canopy temperature measurements
Similarly, black body temperature was measured by averaging thermocouples placed in 1 mm holes drilled in the top, sides, and bottom of the conical housing (Bugbee et al., 1999). The location of air temperature and water vapor pressure deficit measuring equipment can also affect sky temperature measurements.
CROP WATER STRESS INDICES
INTRODUCTION
DETERMINATION OF CROP WATER STRESS INDEX
- Empirical method
- Theoretical approach
As defined by Jackson et al. 1981) the theoretical development of the crop water stress index is based on the energy balance on the crop surface, i.e. The upper limit of Te - Ta can be found by allowing the crown resistance re to increase indefinitely. example for a crop that is not transpired.
AERODYNAMIC AND CANOPY RESISTANCES
The Penman-Monteith equation can be used to determine canopy resistance (Malek et al., 1991; Lindroth, 1993). Since a large fraction of the total radiation available to a canopy is absorbed by the upper half of the canopy CAlves et al., 1996), canopy resistance of a crop can be estimated:.
APPLICATION OF THE CROP WATER STRESS INDEX
Crops in this category cannot be scheduled for irrigation based on changes in CWSI readings, as any water stress detected by a change in CWSI can reduce economic yield. Crops in this category actually have improved yield under severe water stress, and CWSI can be effectively used to monitor and control the severity and timing of water stress.
SITE AND PLANT DESCRIPTION
Common ryegrass is a bottom grass and it germinates in cooler soils than most other cover crops and pasture seeds. In general, ryegrass is adapted to irrigated agriculture and can grow on sandy soils if well fertilized, but does better on heavier clay or silt soils with adequate drainage (http://www.sarep.ucdavis.edu, Internet 2002).
INSTRUMENTATION OVERVIEW
A run interval of 60 s was used for all sensors and every 15 min the data logger converted the average of the input stored values to the final storage.
INSTRUMENT DETAILS .1 Infrared thermometers
- Vaisala CS500 Air temperature and relative humidity probe
- Propeller Anemometer
- Net radiometer
- Soil heat flux plates and soil thermocouples
- ThetaProbe
The sensor was placed at a height of 1.5 m above the ground surface in a 6-plate radiation shield (FigA.2.) on a CM6/CMlO Tripod Mast. Wind speed was calculated as a square root of the sum of the squares of the wind speeds in the U and V directions. This implies that the input impedance of the readout equipment must be at least 12.5 ohms, to give an error of less than 1.
A diagrammatic representation of the installation of the soil heat flux plates and soil thermocouples for the determination of soil heat flux density is shown in Fig. The ratio of the two voltages essentially depends on the apparent dielectric constant of the soil, which is determined by the soil's water content.
DATALOGGER AND POWER SUPPLY
A pair of 12 V batteries were connected in parallel to power the CR7X data logger, and another pair was also used to power the sensors. The batteries were replaced every seven days by charged batteries, although the parallel connection of batteries gave longer battery life. A control switch box was connected to the battery pair to power the sensors just before measurements to minimize power consumption.
To protect the data logger and sensors from lightning, the ground of the digester and the common ground of the batteries were grounded using a lightning rod. The batteries were placed inside a metal enclosure (Fig. 4.6) and the datalogger was covered using a plastic container to prevent heat and rain.
DATA COLLECTION, HANDLING AND PROCESSING
INTRODUCTION
MATERIALS AND METHODS
- Vaisala CS500 air temperature and relative humidity sensor
Water temperature was measured using a type E thermocouple immersed 10 mm below the water surface. The water temperature was changed by changing the set point of the thermostat on the hob to 45°C. The calibration of the thermocouples was done using the same materials used to calibrate the infrared thermometers, along with a mercury thermometer.
Measuring the temperature of the air stream and the set dew point of the air stream enables the calculation of water vapor pressure and relative humidity. All temperatures and relative humidities and airflow dewpoints were measured every second and averaged every 5 minutes.
RESULTS AND DISCUSSION
- Calibration of sensors
- Integrity of weather data
On the other hand, the integrity of data can also be done by comparing measurements of the used sensors with measurements of standard sensors. The deviation of the slope from 1 and the intercept from zero was due to the use of different multipliers and offsets, and the calibration factors used for the CM3 and the pyranometer were different. The comparison of data from both sensors were not statistically different from each other.
The integrity of water vapor pressure and air temperature data can be checked by transforming water vapor pressure data to relative humidity or to vapor pressure deficit (Fig. 5.] 3). The deviation of the intercept from zero was due to the initial wind speed threshold.
CONCLUSIONS
INTRODUCTION
THE DETERMINATION OF THE CROP WATER STRESS INDEX BY THE EMPIRICAL AND THEORETICAL METHODS. 1981) presented a theoretical method for calculating the crop water stress index. The theory used an estimate of net radiation and an aerodynamic drag factor, in addition to the air temperature and water vapor pressure terms required by the empirical method as briefly discussed in Chapter 3. Although the theoretical approach specified how the upper and lower bounds could be evaluated, the additional measurements of net radiation and aerodynamic drag, and perhaps some equations that look more complex than they are, meant that this method did not receive the thorough field tests have not undergone the empirical method (Jackson et al., 1988).
The objectives of this chapter are to determine: .. i) the no water stress baseline and the maximum stress baseline for cereal rye and perennial ryegrass. ii) the crop water stress index of cereal rye and perennial ryegrass using the empirical and theoretical methods ofldso et al. 1981) respectively. iii) whether the crop water stress index can be used to plan irrigation in cereal rye and perennial ryegrass to prevent water stress.
MATERIALS AND METHODS
RESULTS AND DISCUSSION
- Actual, potential, and non-transpiring surface to air temperature differential
- Crop Water Stress Index (CWSI)
- Timing of irrigation using crop water stress index
The recharge point is the soil water content below which crop growth is measurably reduced. The CWSI value of 0.24 was found to correspond to the recharge point soil water content for cereal rye by plotting CWSI against the soil water content. The cereal rye crop must be irrigated when the CWSI is above the CWSI = 0.24 equivalent to the recharge point soil water content of the soil, to relieve the water stress.
Daily variations of empirical and theoretical CWSI and soil water content for the highest rooting depth of 100 mm are shown (Fig. 6.10) for annual rye. The CWSI value of 0.29 was found to correspond to the soil water content at the recharge point by plotting CWSI against soil water content.
CONCLUSIONS
THE USE OF NON-WATER-STRESSED BASELINES FOR IRRIGATION SCHEDULING
- INTRODUCTION
- MATERIALS AND METHODS
- THE RADIOMETRIC SURFACE TEMPERATURE AS A WET BULB TEMPERATURE
- RESULTS AND DISCUSSION
- CONCLUSIONS
Wet bulb temperature was calculated from air temperature and actual vapor pressure using a Fortran program wet bulb calculator (Savage, 2002c). The program iteratively uses a wet bulb temperature and the specified dry bulb temperature to calculate the atmospheric water vapor pressure and compares this value to the actual water vapor pressure. Alternatively, with reduced accuracy, the wet bulb temperature (Tw) can be calculated from the approximation Tw ~ Ta - be /(6 + y), where be is the VPD and 6 is the slope of the saturation water vapor pressure vs temperature ratio.
Therefore, the infrared surface temperature of fully transpiring crops can be regarded as a wet bulb temperature and Eq. Therefore, the infrared surface temperature of fully irrigated cereal rye and perennial ryegrass can be regarded as a wet bulb surface temperature.
CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE RESEARCH
CONCLUSIONS
- Introduction
- Sensor calibration and the integrity of weather data
- Determination of the crop water stress index
- The use of non-water-stressed baselines for irrigation scheduling
There are no previously reported baselines in the literature for both grain rye and annual ryegrass that can be used for comparison with the non-water stressed baselines developed from this study. The CWSI method is based on two baselines: the non-water stressed baseline and the maximally stressed baseline. The non-water stressed baseline can also be used alone if the purpose of the irrigation system is to achieve maximum yield (Alves and Pereira, 2000).
However, the non-water-stressed baseline determined using the empirical method cannot be used for other locations and is only valid for clear-sky conditions. The non-water stressed baseline is a useful concept that can effectively guide the irrigator to obtain maximum yields and plan irrigation.
RECOMMENDATIONS FOR FUTURE RESEARCH
The non-water-stressed baseline determined using the theoretical method requires the calculation of aerodynamic drag and canopy drag, since knowledge of canopy drag and the values it can assume during the day is scarce. This method estimated the infrared surface temperature as a wet bulb temperature (Fig. 7.4) for cereal rye and (Fig. 7.5) for annual ryegrass. Based on this study, it is concluded that the infrared measured surface temperature of fully watered grain rye and annual ryegrass can be considered a surface wet bulb temperature.
The infrared surface temperature value can be calculated from measured or estimated values of net irradiance, ground heat flux density, aerodynamic resistance and air temperature. In addition, studies should be conducted on the determination of sensible and latent heat flux density using the surface temperature method.
Howell TA, Yazar A, Dusek DA, Copeland KS (1999) Evaluation of a crop water stress index for irrigated maize LEP A. O'Toole lC, Hatfield lL (1983) Effect of wind on crop water stress index obtained by infrared thermometry. Patel NR, Mehta AN, Shekh AM (2001) Canopy temperature and quantification of water stress in rainfed pigeonpea (Cajanus cajan (L.) Millsp.).
Wanjura DF, Hatfield JL, Up Church DR (1990) Crop water stress index relationship with crop productivity. Wanjura DF, Kelly CA, Wendt CW, Hatfield JL (1984 ) Canopy temperature and water stress of cotton crops with full and partial ground cover.
