title: Biophysically-Based Measurement of Plant Water Status Using Canopy Temperature
Creators: name: Parry, Christopher K.
Publisher: name: Utah State University
Year: 2014

description: Precision irrigation scheduling is one approach that can conserve water by supplying crops with the minimum amount of water needed for sufficient vegetative growth and final crop yield. Improved methods for irrigation scheduling are needed for arid regions that rely mainly on irrigation for crop water needs, and humid regions that supplement water received from precipitation with added irrigation. Methods that directly determine plant physiological responses to water availability have potential to be significantly more sensitive and accurate than indirect approaches like soil moisture measurement. Stomatal conductance is a rapid physiological response to leaf water potential.
Stomatal conductance in single leaves has long been calculated using biophysical and energy balance principles. This same biophysical approach can be extended to plant communities using: 1) standard meteorological measurements, 2) accurate measurement of average canopy temperature, and 3) knowledge of canopy architecture.
Here we use a model designed to separate the energy balance of the soil and plant canopy for the calculation of stomatal conductance (gC) in row crops. This model is modified for application in row crops which differ in their spatial distribution when compared to more uniform crops such as turfgrass or alfalfa. The energy balance model requires measurement or estimation of the soil and canopy temperatures. Various methods can be used to derive these temperatures, i.e., using a composite temperature of the two and either directly measuring or estimating one of the temperatures to derive the other, or directly measuring both component temperatures. This study compares two methods to determine which one is more appropriate in determining canopy temperature for calculation of canopy stomatal conductance for the measurements taken on the fields studied.
By using the necessary environmental measurements, and model modifications, gC was continuously determined for 10 corn and 6 cotton crops throughout the Midwest and Southern United States. This gC value was then compared to a calculated reference gC for a well-watered crop. This reference gC represents the stomatal conductance of a well-watered crop experiencing no water stress. The ratio of the calculated and reference gC is an indicator of crop water status, which is called the stomatal conductance ratio (SCR). The SCR increased closer to one (indicating minimal water stress) after each irrigation or significant precipitation event, and steadily declined until the next irrigation event. Significant drought stress occurred in several of the fields.
Daily SCR values were weighted to correspond with growth stage sensitivity to drought stress. These weighted values were highly correlated with yield (r2 values up to 0.79). SCR values for cotton were also highly correlated with yield (r2 values up to 0.96).
This biophysical approach has the potential to provide a powerful tool for precision irrigation management. Growers can more efficiently apply water to their crops and more accurately determine when to apply irrigation.