Calculate Crop Water Stress Index from canopy, air, and wet-bulb temperatures, or estimate wet-bulb temperature and vapor pressure deficit.
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Crop Water Stress Index Formula
The calculator on this page uses the following simplified, wet-bulb-based form of the Crop Water Stress Index (CWSI). (The original Idso–Jackson CWSI uses crop- and site-specific lower/upper baselines; those baselines are not included here.)
Variables:
- CWSI is the Crop Water Stress Index (unitless)
- Tc is the canopy temperature (°C or °F)
- Ta is the air (dry-bulb) temperature (°C or °F)
- Tw is the wet-bulb temperature (°C or °F)
To calculate this CWSI, subtract the wet-bulb temperature from the canopy temperature. Then subtract the wet-bulb temperature from the air temperature. Divide the first result by the second result to get the Crop Water Stress Index. (If Ta = Tw, the index is undefined.)
What is a Crop Water Stress Index?
The Crop Water Stress Index (CWSI) is a unitless indicator used to describe plant water stress from canopy temperature measurements. In the standard approach, CWSI normalizes the canopy–air temperature difference between a non-water-stressed (lower) baseline and a fully stressed (upper) baseline (often dependent on vapor pressure deficit, VPD). This page’s calculator uses a simplified reference-temperature method that uses wet-bulb temperature (which can be estimated from air temperature and relative humidity) as a “wet” reference and air temperature as a simple “dry” reference. In many practical cases, values tend to fall between 0 (closer to wet-bulb) and 1 (closer to air temperature), although values outside 0–1 can occur with extreme or inconsistent inputs.
How to Calculate Crop Water Stress Index?
The following steps outline how to calculate the Crop Water Stress Index (CWSI) using the formula: CWSI = (Tc – Tw) / (Ta – Tw).
- First, determine the canopy temperature (Tc) in degrees Celsius (or Fahrenheit).
- Next, determine the air temperature (Ta) in degrees Celsius (or Fahrenheit).
- Next, determine the wet bulb temperature (Tw) in degrees Celsius (or Fahrenheit).
- Use the formula CWSI = (Tc – Tw) / (Ta – Tw) to calculate the Crop Water Stress Index.
- After inserting the values of Tc, Ta, and Tw into the formula, calculate the result.
Example Problem:
Use the following variables as an example problem to test your knowledge:
Canopy temperature (Tc) = 22°C
Air temperature (Ta) = 25°C
Wet bulb temperature (Tw) = 20°C
Solution: CWSI = (Tc − Tw) / (Ta − Tw) = (22 − 20) / (25 − 20) = 0.40
