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/Ep, 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.

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Canopy temperatures were measured on durum wheat grown in six differentially irrigated plots. Soil water content was measured by using a neutron‐scattering technique at two locations within each plot. Water contents, in 20‐cm increments to 160 cm, were determined two to five times per week. Using a sliding cubic smoothing technique, we calculated daily water contents and thus water depletion rates for the entire growing season. Canopy temperatures were measured daily between 1330 and 1400 hours. Air temperatures measured at 150 cm above the soil surface were subtracted from the canopy temperatures to form the difference Tc – Ta. The summation of Tc – Ta over time yielded a factor termed the ‘stress degree day’ (SDD). The SDD concept shows promise as an indicator for determining the times and amounts of irrigations. An expression relating evapotranspiration (ET) to net radiation and Tc – Ta was simplified and tested by using ET measurements with a lysimeter. The expression was used to predict water use by wheat in the six plots. Predicted ET and measured water used agreed reasonably well. The expression may be useful in determining amounts of irrigation water to apply.

A comprehensive experiment was conducted at Phoenix, Arizona, involving the monitoring of full spectrum thermal radiation and those fractions of that flux that are contained within the 8-to 14-/ma and 10.5-to 12.5-/am subregions. Also monitored were surface air temperature (To) and vapor pressure (Co). equations ((15)-(17)) based on this physical model were thus developed for the effective emittance of the atmosphere in both the 10.5-to 12.5-/ma and 8-to 14-/ma wavebands, as well as for the full thermal spectrum. Equation (17) was then shown to be a significant improvement over previous equations that have attempted to model this phenomenon. 20 ;)5 30 35 40 45 • VAPOR PRESSURE (rnb) Analyses of the data established the source of water vapor associated thermal emittance ({) variations of the cloudless sky as being due to the variable atmospheric concentration of water dimers•pairs of water molecules linked together by weak hydrogen bonds. New

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