Beyond the Crop Water Stress Index: Applying Biophysical Principles to Plant Stress Measurement.
James Mark Blonquist Jr., Apogee Instruments, 721 W 1800 N, Logan, UT 84321 and Bruce Bugbee, Utah State University, Plants Soils & Climate Department, Logan, UT 84322-4820.
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 26-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 measurement of canopy stomatal conductance, a biophysical parameter that does not rely on empirical measurements derived for specific plants and specific climates. We perform a sensitivity analysis to determine the necessary measurement accuracy of each parameter required to calculate canopy conductance, and compare canopy conductance measurements with the crop water stress index over turf grass at two field sites. Modeled results indicate the necessary measurement accuracy increases in cloudy, cool, and humid conditions. Measured results showed that the canopy conductance is predictive, facilitating its use in irrigation scheduling. We also review and compare methods for calculating boundary layer conductance (a component of canopy conductance), discuss methods to account for soil heat flux, and evaluate procedures to correct for the effect of emissivity on canopy temperature.