Canopy temperature as a crop water stress indicator

Canopy temperatures, obtained by infrared thermometry, along with wet‐ and dry‐bulb air temperatures and an estimate of net radiation were used in equations derived from energy balance considerations to calculate a crop water stress index (CWSI). Theoretical limits were developed for the canopy air temperature difference as related to the air vapor pressure deficit. The CWSI was shown to be equal to 1 ‐ E / E p , the ratio of actual to potential evapotranspiration obtained from the Penman‐Monteith equation. Four experimental plots, planted to wheat, received postemergence irrigations at different times to create different degrees of water stress. Pertinent variables were measured between 1340 and 1400 each day (except some weekends). The CWSI, plotted as a function of time, closely paralleled a plot of the extractable soil water in the 0‐ to 1.1‐m zone. The usefulness and limitations of the index are discussed.