Evaluating canopy temperature-based indices for irrigation scheduling

Summary

Since the development of commercial versions of infrared sensors, they have been increasingly used to determine canopy temperature and schedule irrigations. However, some shortcomings of the technique have been identified, among them the sensitivity of canopy temperature measurements to weather fluctuations. Based on field and computer simulated data, an analysis of the suitability of crop water stress indices (CWSI's) developed from canopy temperature under variable weather conditions was done. Important day to day fluctuations of CWSI values determined using an empirical baseline (empirical CWSI) appeared common for nonstressed crops, particularly under low vapor pressure deficit conditions. These fluctuations generate uncertainty in the use of this empirical index to determine needs for irrigation. The use of an improved index (theoretical CWSI) requiring measurements of net radiation, soil heat flux and wind speed, and estimates of aerodynamic and canopy resistances reduced but did not eliminate these fluctuations. Results using a simulation model showed that the empirical CWSI provided late indication of irrigation needs, after some water stress has developed, which may limit its application for crops sensitive to water stress. These simulations also indicated that the theoretical CWSI was able to track the development of water stress and provide reasonable indication of irrigation needs. However, this result may not be fully realized in field applications where the determination of CWSI may be affected by various sources of variability which are not accounted for by the model.