Applying Biophysical Principles to the Measurement of Canopy Stomatal Conductance
Abstract
Decreased water uptake closes stomates, which reduces transpiration and increases leaf temperature. The leaf or canopy temperature can thus be used to quantify plant water stress. Multiple methods to measure water stress using canopy temperature have been proposed. Nearly all of them are modifications of a 27- year-old index called the crop water stress index, which is based on empirically-established baselines. Here we propose to abandon this relative index and replace it with canopy stomatal conductance, a plant physiological parameter derived from the energy balance for a plant canopy that does not rely on empirical coefficients for specific plants and climates. Direct calculation of canopy stomatal conductance requires the meteorological measurements found on most weather stations and a measurement of canopy temperature and height, and provides a field-scale measure of diurnal and seasonal stomatal response to prevailing soil water and atmospheric conditions. Canopy stomatal conductance also facilitates scaling conductance from single leaves (measured with porometers) to canopies. A sensitivity analysis of the equation to calculate canopy stomatal conductance shows it is most sensitive to canopy and air temperature measurements compared to the other required measurements (relative humidity, net radiation, soil heat flux, wind speed, and plant canopy height). Canopy stomatal conductance becomes more sensitive to all measured parameters as the conditions change from sunny, warm, and dry to cloudy, cool, and humid. An alternative method for calculating canopy stomatal conductance is also presented. The second method uses a dual temperature difference scheme to reduce the absolute accuracy required for canopy and air temperature measurements. Measurements in full cover alfalfa are used to calculate and compare canopy stomatal conductance from both methods.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.B43C0455B
- Keywords:
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- 0426 Biosphere/atmosphere interactions (0315);
- 0452 Instruments and techniques;
- 0495 Water/energy interactions (1878);
- 1843 Land/atmosphere interactions (1218;
- 1631;
- 3322)