Explainer
The crop water stress index, explained
The crop water stress index (CWSI) is the most widely used thermal measure of plant water status. This page explains what it measures, how it is calculated, where it supports irrigation scheduling, and where its limits begin.
Last reviewed: September 2026
What the index measures
A well-watered plant cools itself by transpiration, so its canopy stays close to air temperature. When soil water runs short, stomata close, transpiration slows and the canopy warms. The crop water stress index turns that temperature rise into a number between 0 and 1: 0 for a fully transpiring, unstressed canopy, 1 for a canopy that has stopped transpiring altogether.
The index was introduced by Idso, Jackson and colleagues in 1981 and has been validated across many crops since.
How it is calculated
CWSI compares the measured canopy-to-air temperature difference against two baselines for the same weather: the temperature of a fully watered canopy and the temperature of a non-transpiring one. Both baselines shift with the vapour pressure deficit of the air, so a reading always needs local weather data alongside the thermal measurement.
Measure canopy temperature, usually with an infrared thermometer, thermal camera or satellite.
Measure air temperature and humidity to derive the vapour pressure deficit.
Place the reading between the wet and dry baselines for that deficit.
The position between the baselines is the index value for that moment.
Where it supports irrigation
Because the index is standardised for weather, readings can be compared across days and blocks. Growers and researchers use CWSI thresholds to time irrigation, to compare irrigation regimes in trials, and to map variability across large fields from drones or satellites.
Scheduling: irrigate when the index crosses a crop-specific threshold.
Benchmarking: compare blocks, varieties and regimes on the same scale.
Mapping: drones and satellites extend the index from single canopies to whole districts.
Where its limits lie
CWSI is a snapshot, not a trend. A meaningful reading needs clear sky, low wind and a measurement window near solar noon, and satellite passes can be days apart. Most importantly, the index only moves once the canopy has already warmed: it confirms stress that is physiologically underway. For decisions that depend on lead time, that is late.
Wind, partial cloud and mixed soil background in the thermal image are the most common sources of error.
What continuous plant measurement adds
A thermal index watches the symptom: canopy temperature. The plant's electrical behaviour changes as its water status changes, and it can be read continuously, day and night, under the canopy, in any weather. Our working hypothesis is that this signal shifts before the canopy warms. Whether it does, and by how much, is exactly what our validation programme is testing, so we present the two approaches as complements, not substitutes.
Common questions
- What is a good CWSI value?
- There is no universal threshold. Values below about 0.2 usually indicate a well-watered canopy, values above 0.4 to 0.6 indicate developing stress, but the actionable threshold depends on the crop, the growth stage and the cost of a missed irrigation.
- Is CWSI the same as NDVI?
- No. NDVI measures greenness from reflected light and responds over days to weeks. CWSI is derived from canopy temperature and responds as soon as transpiration slows, but only during a suitable measurement window.
- Can CWSI replace soil moisture sensors?
- Not on its own. Soil sensors report how much water is available in the root zone, CWSI reports how the canopy is responding. Mature irrigation programmes use both, and increasingly add a plant-based signal on top.
Measuring stress at the plant itself
If you work with crop water stress, in the field or in research, and want to compare thermal indices with a continuous plant signal, we would like to hear from you.
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