Infrared Thermometry I. History, B. R. Gardner, Development of portable infrared thermometers and the defi- nition of the Crop Water Stress Index (CWSI) have led to widespread interest in infrared thermometry to monitor water stress and schedule irrigations. But the CWSI concept is still new and poorly understood by many. The purpose of this paper is to review the definition of CWSI, and the determination and interpretation of the non-water-stressed baselines used to com- pute CWSI. The non-water-stressed baseline equation normal- izes the canopy minus air temperature differential for variations in vapor pressure deficit. Non-water-stressed baselines can be determined empirically from measurements of canopy and air temperatures and vapor pressure deficit, made diurnally on a single day, or at a single time of day over many days, on well- watered plants. The value of the maximum canopy minus air temperature differential under maximum water stress should also be determined empirically. Causes for CWSI values fall- ing outside of the defined 0 to 10 unit range are reviewed. Non- water-stressed baselines may shift with plant growth stage. Ef- fective use of CWSI is dependent on understanding the defini- tion of CWSI, and the proper determination and use of non-water-stressed baselines. and the Crop Water Stress Index. Theory, and Baselines D. C. Nielsen,* and C. C. Shock ~~ ~ ~ NFRARED THERMOMETRY was first used to measure tem- I peratures of vegetative surfaces in the early 1960s (Fuchs and Tanner, 1966), and became more widely used during the 1970s with development of small, hand-held, portable infrared thermometers. During this period, the stress-degree-day parameter (the accumulation of posi- tive values of the difference between canopy and air tem- peratures [dT]) was used to effectively quantify water stress. This worked well in wheat (Triticum aestivum L.) where it was observed that dT was negative when plants were well-watered, and positive when wheat was water- stressed (Jackson et al., 1977). But it is obvious from ex- amination of the energy balance that variations in vapor pressure deficit have a significant effect on the magni- tude of dT. This effect was observed in the early 1970s for cotton (Gossypium hirsutum L.) by Ehrler (1973). In the 1980s more routine use was made of infrared ther- mometry to quantify water stress in plants when Idso et al. (1981) and Jackson et al. (1981) defined and demon- strated the use of the CWSI. In the past 5 yr makers of infrared thermometers have incorporated software into instruments that automatically calculate CWSI for the user. Many growers, researchers, and extension agents use CWSI, but for many the concept is still new and poor- B.R. Gardner, BP Research, 4440 Warrensville Center Road, Cleveland, OH 44128. D.C. Nielsen, USDA-ARS Central Great Plains Res. Stn., P.O. Box 400, Akron, CO 80720. C.C. Shock, Malheur Exp. Stn., Ore- gon State University, 595 Onion Ave., Ontario, OR 97914. Oregon Agric. Exp. Stn. Tech. Paper no. 9855. Received 19 Mar. 1992. *Corresponding author. Published in J. Prod. Agric. 5:462-466 (1992). ly understood. The purpose of this palper is to review the definition of CWSI and the determination and interpre- tation of the non-water-stressed baselines used to com- pute CWSI. CWSI DEFINITION CWSI was defined by Idso et al. (1981) as dT - MIN CWSI = - MAX - NUN where dT = Tc = Ta = MIN = VPD = A= B = MAX = Tc - Ta ("C) crop temperature ("C) air temperature ("C) non-water-stressed baseline = A + B*VPD ("C) vapor pressure deficit (kPa) intercept of non-water-,stressedbaseline ( "C) slope of non-water-stressed baseline ( "C kPa - ') upper limit of dT ("C) Tc is obtained from measurements made with an infrared thermometer. Ta and VPD have been obtained in sever- al ways, including use of a psychrometer to get dry and wet bulb temperatures, or use of other temperature and humidity measuring devices and accompanying software built into an infrared thermometer and data logging system. The value of CWSI can range from 0 (no stress) to 1 (maximum stress). Jackson et al. (1981) described this as "estheticaIly pleasing," since scienlists studying plant- water relations often consider the ratio ET/ET,, which similarly ranges from 1 (ample water) to 0 (no available water). Gardner and Shock (1989), on the other hand, described commercial development of infrared thermo- metry to monitor water stress and schedule irrigations, and reported that users of this technology did not readi- ly accept the 0 to 1 scale of CWSI. Apparently users had difficulty interpreting the magnitude and significance of a change in CWSI that was reported in tenths or hun- dredths of a unit on a 0 to 1 scale. They reported that multiplying the scale by 10 to make a range from 0 to 10 was a practical improvement that allowed CWSI to be more easily understood and accepted. Equation 1 is sometimes referred to as the empirical form of CWSI. Jackson et al. (1981) pointed out that CWSI is a crop-ET-based index defined by CWSI = 1 - (ET/ET,), where ET is actual crop evapotranspira- Abbreviation: CWSI, Crop Water Stress Index. 462 J. Prod. Agric., Vol. 5, no. 4, 1992 Published April 19, 2013