Crop Water Stress Index Calculator

Calculate CWSI using calibrated baselines, retain the simplified wet-bulb ratio and its inverse calculations, or estimate wet-bulb temperature and vapor pressure deficit.

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Crop Water Stress Index Formula

The calibrated-baselines mode uses CWSI = [(Tc − Ta) − LL] / (UL − LL), where LL and UL are crop- and condition-specific canopy-minus-air temperature differences for non-stressed and non-transpiring references, and UL must exceed LL. The simplified modes retain the wet-bulb reference ratio shown below; that shortcut is not a validated replacement for calibrated CWSI.

CWSI = (Tc - Tw) / (Ta - Tw)

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 the simplified ratio, 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). The simplified modes use a 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. For this ratio, 0 corresponds to the wet-bulb reference and 1 to air temperature. Values outside 0–1 are retained and require checking the references; the ratio does not establish an irrigation threshold or percentage water deficit.

How to Calculate Crop Water Stress Index?

The following steps outline how to calculate the simplified wet-bulb ratio 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 simplified wet-bulb ratio.
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 (simplified ratio): ratio = (Tc − Tw) / (Ta − Tw) = (22 − 20) / (25 − 20) = 0.40

Crop Water Stress Index Calculator screenshot