CROP WATER STRESS INDEX

Theory of the CWSI

• The CWSI is a measure of the relative transpiration rate occurring from a plant at the time of measurement using a measure of plant temperature and the vapor pressure deficit which is a measurement of the dryness of the air. Jackson et at 1981 presents the theory behind the energy balance that separates net radiation from the sun into sensible heat that heats the air and latent heat that is used for transpiration. As the crop undergoes water stress the stomata close and transpiration decreases and leaf temperature increases. When a plant is transpiring fully the leaf temperature is 1 to 4 degrees below air temperature figure 1. and the CWSI is 0. As the transpiration decreases ,the leaf temperature rises and can reach to 4 to 6 degrees above air temperature. When the plant is no longer transpiring the CWSI is 1.

Use of the CWSI

The crop water stress index is calculated by determining the relative distance between the lower base line representing non stress conditions and the upper base line representing no transpiration on a plot of air temperature-canopy temperature vs. vapor pressure deficit. Fig 1. Plot the upper and lower baseline data from the following list of crops. click here These baseline are appropriate for calculating the CWSI in the southwest during summer months. Measure the plant temperature between 1200 noon and 2:00 p.m. only on non cloudy days following the procedure for use of the infrared gun. To learn how to take a canopy temperature using the infrared thermometer click here Measure the air temperature and vapor pressure deficit using the psychrometer To learn how to measure relative humidity air temperature and vapor pressure deficit using the psychrometer click here> Plot the point of measurement on the graph and measure or calculate the relative distance of the point between the lower and upper base line Fig. 1. The CWSI is equal to a/b. (Itso et al 1981) The relative transpiration or the measured transpiration (T) divided by the potential transpiration (Tp) is: T/tp =1-CWSI.

Because of the scatter in the measured canopy minus air temperature vs vapor pressure deficit, the crop does not need to be watered until the CWSI reaches 0.1 to 0.15. At this time the crop is transpiring at less than the optimal rate and crop yield will start to decrease. The lower baseline for a crop in a specific area can be determined by making canopy minus air temperature and vapor pressure deficit measurements two days after a crop has received a deep watering. The measurements need to be made from 10:00 AM to 2:00 PM resulting in a range of vapor pressure deficit measurements. If this process is conducted for several time periods when the weather results in dry and humid days, a complete range of temperature and vapor pressure deficit measurements can be made and a liner regression of the data will produce the equation for the lower base line. The upper base line can be determined by cutting off the plant and then wiring the plant back in place and waiting 1 day, till the plant is no longer transpiring and then measuring the upper base temperature. In Las Cruces New Mexico during the summer months, the upper base line is usually close to 6 degrees C. above air temperature

```References
Idso, S. B., Jackson, R.D., Pinter, P.J.Jr., Reginato, R. J. and Hatfield, J. L.
1981 Normalizing the stress-degree-day parameter for environmental variability. Agric.
Metorol., 24:45-55

Jackson, R. D., S. B. Idso, R. J. Reginato, P. J. Printer,. 1981. Canopy temperature
as a crop water stress indicatior. Water Res 17:1133-1138

Sammis, T.W. and D. Jernigan.1992. Crop water stress index of ornamental plants.
American Soc. of Agric. Eng., Vol. 8, No. 2

Sammis, T.W. W.R. Riley, and D.G. Lugg.  1988.  Crop water stress index of pecans.
Applied Eng. in Agric. 4(1):39-45.

Azzam, A. Tabaileh, T.W. Sammis, D.G. Lugg.  1986.  Utilization of thermal infrared
thermometry for detection of water stress in spring barley.  Agricultural
Water Management 12:75-86.
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tsammis@nmsu.edu