Plant growth and soil water deficit can vary spatially and temporally in crop fields due to variation in soil properties and/or irrigation and crop management factors. We conducted field experiments with cotton (Gossypium hirsutum L.) over two seasons during 2007-2009 to test if infrared thermography can distinguish systematic variation in deficit irrigation applied to various parts of the field over time. Soil water content was measured with a neutron probe and thermal images of crop plants were taken with a thermal infrared camera. Leaf water potential and stomatal conductance were also measured on selected occasions. All measurements were made at fixed locations within three replicate plots of an irrigation experiment consisting of four soilwater deficit treatments. Canopy temperature related as well with soil water within the root zone of cotton as the stomatal conductance index derived from canopy temperature, but it neglected the effect of local and seasonal variation in environmental conditions. Similarities in the pattern of spatial variation in canopy temperature and soil water over the experimental field indicates that thermography can be used with stomatal conductance index to assess soil water deficit in cotton fields for scheduling of irrigation and to apply water in areas within the field where it is most needed to reduce water deficit stress to the crop. Further confidence with application of infrared thermography can be gained by testing our measurement approach and analysis with irrigation scheduling of other crops.
We used twelve load cells (20 kg capacity) in a mini-lysimeter system to measure evapotranspiration simultaneously from twelve plants growing in separate pots in a glasshouse. A data logger combined with a multiplexer was used to connect all load cells with the fullbridge excitation mode to acquire load-cell signal. Each load cell was calibrated using fixed load within the range of 0-0.8 times the full load capacity of load cells. Performance of all load cells was assessed on the basis of signal settling time, excitation compensation, hysteresis and temperature. Final calibration of load cells included statistical consideration of these effects to allow prediction of lysimeter weights and evapotranspiration over short-time intervals for improved accuracy and sustained performance. Analysis of the costs for the mini-lysimeter system indicates that evapotranspiration can be measured economically at a reasonable accuracy and sufficient resolution with robust method of load-cell calibration.
Geonics EM38 is a portable, non-invasive equipment that induces an electrical current in the soil for rapid measurement of apparent electrical conductivity (EC a ) in the field. We used an EM38 in a wheat field to evaluate the effects of systematic variation in soil water content (within three replicate plots of four irrigation treatments) and seasonal variation in soil temperature on EC a . The effective depth of sensing of EM38 could be varied by using it in both vertical and horizontal dipole modes and by placing it at various heights above the ground. Accumulated water within various soil depths was measured with a neutron probe throughout the season. Values of EC a over the season for a soil depth related linearly or nonlinearly with soil water within that depth with a coefficient of determination (R 2 ) of 0.70-0.81. EC a values were also influenced by variation in soil temperature within 5-25 cm depth (range 10.1-29.3 °C) and air temperature (range 14.2-34.0 °C), but to a smaller extent than soil water. The overall relationship between EC a , soil water and soil temperature improved considerably when multiple regression was used (R 2 = 0.82-0.98). Good correspondence between maps of EC a and soil water content over the experimental field suggests that EC a maps could be useful in determining spatial distribution of soil water within crop fields so that variable rather than uniform quantity of irrigation water can be applied to improve water use efficiency.
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