Automated measurement of canopy stomatal conductance based on infrared temperature

• Published in: Agricultural and forest meteorology 2009-11, Vol.149 (11), p.1931-1945
• Main Authors: Blonquist, J.M., Norman, J.M., Bugbee, B.
• Format: Article
• Language: English
• Subjects: Agricultural and forest climatology and meteorology. Irrigation. Drainage; Agricultural and forest meteorology; Agronomy. Soil science and plant productions; air temperature; alfalfa; Biological and medical sciences; Boundary layer conductance; canopy; cloudiness; Crop water stress index; diurnal variation; Energy budget; Evapotranspiration; forage crops; Fundamental and applied biological sciences. Psychology; General agronomy. Plant production; Irrigation scheduling; lawns and turf; leaves; measurement; Medicago sativa; Plant canopy water stress; plant height; relative humidity; remote sensing; seasonal variation; simulation models; soil water content; soil-plant interactions; solar radiation; stomata; stomatal conductance; surface temperature; turf grasses; Water balance and requirements. Evapotranspiration; wind speed
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2009.06.021

Decreased water uptake closes stomates, which reduces transpiration and increases leaf temperature. The leaf or canopy temperature has long been used to make an empirical estimate of plant water stress. However, with a few supplemental measurements and application of biophysical principles, infrared measurement of canopy temperature can be used to calculate canopy stomatal conductance ( g C ), a physiological variable derived from the energy balance for a plant canopy. Calculation of g C requires an accurate measurement of canopy temperature and an estimate of plant height, but all of the other measurements are available on automated weather stations. Canopy stomatal conductance provides a field-scale measurement of daily and seasonal stomatal response to prevailing soil water and atmospheric conditions, and facilitates a comparison of models that scale conductance from single leaves (measured with porometers) to canopies. A sensitivity analysis of the input measurements/estimates showed g C is highly sensitive to small changes in canopy and air temperature, and less sensitive to the other required measurements (relative humidity, net radiation, wind speed, and plant canopy height). The measurement of g C becomes increasingly sensitive to all of the component factors as the conditions become cloudier, cooler, and more humid. We determined g C for alfalfa and turfgrass by making the necessary environmental measurements and coupling them with a two-source (plant canopy layer and soil layer) energy balance model. We then compared these g C values to maximum single leaf values scaled-up to the canopy level ( g CP , defined as potential canopy stomatal conductance herein) for the two crops. For both crops, g C matched g CP within approximately 10% after irrigation. The turfgrass g C measurements were also compared to mean single leaf values measured with a porometer. At mid-day, g C values were typically about double the single leaf values. Because this approach for determining g C allows continuous, non-contact measurement, it has considerable potential for coupling with measurements of soil moisture to better understand plant–soil water relations. It also has potential for use in precision drought stress and irrigation scheduling.