Crop Water Stress Index (CWSI)
Enter measured canopy-air temperature difference dT and the well-watered lower baseline dT_ll and non-transpiring upper baseline dT_ul to compute CWSI = (dT − dT_ll) ÷ (dT_ul − dT_ll), judging water-deficit level and irrigation timing.
Input Data
Results
At a glance:The crop water stress index (CWSI) quantifies crop water-deficit level from canopy temperature, proposed by Idso, Jackson and others in the 1980s, a common remote-sensing and precision-irrigation diagnostic. Its principle comes from the energy balance: crops take up water through roots and transpire through leaf stomata; turning liquid water into vapour removes large latent heat, effectively 'cooling' the leaf. When water is ample, stomata open and transpiration is vigorous, so the canopy is cooler than (often well below) the surrounding air; once soil water runs short, crops close stomata to conserve water, transpiration drops, less heat is removed and the canopy warms toward or above air temperature. Hence the 'canopy-air temperature difference' dT = Tc − Ta becomes a sensitive deficit signal: the more positive dT (hotter canopy), the more transpiration is suppressed and the worse the deficit. But the absolute dT value is affected by weather (radiation, wind, air humidity i.e. vapour-pressure deficit VPD); the same deficit gives different dT on different days, hard to compare directly. CWSI's trick is to normalize dT to 0–1 using two baselines: the lower baseline (dT_ll, non-water-stressed baseline) is the canopy-air difference when the crop is fully watered and transpiring maximally under that weather — the coolest, dT_ll usually negative and theoretically varying linearly with air saturation deficit; the upper baseline (dT_ul, non-transpiring baseline) is the canopy-air difference when transpiration fully stops (stomata closed) — the hottest, dT_ul positive. The measured dT's relative position between these two lines is CWSI: CWSI = (dT − dT_ll) ÷ (dT_ul − dT_ll). When measured dT equals the lower limit (coolest, max transpiration) CWSI = 0, meaning no water stress; when dT equals the upper limit (hottest, no transpiration) CWSI = 1, full stress; between them proportionally, e.g. 0.5 means transpiration is about half suppressed. This way, regardless of the day's weather, CWSI consistently reflects deficit level and is comparable across days and fields. In practice, measure dT with a handheld infrared thermometer or thermal camera on the crop canopy (avoiding soil and shadow), plus air temperature; baselines are obtained either empirically (measure on a well-watered reference plant in the same area for the lower limit) or theoretically (energy balance combined with net radiation, wind and VPD for the lower limit; non-transpiring assumption for the upper limit). With CWSI you schedule irrigation: near 0 means ample water, no need yet; set a threshold (about 0.2–0.5 by crop and goal) and start irrigating when CWSI persistently exceeds it. CWSI's strengths are non-contact, fast large-area measurement and sensitive deficit response, fitting drones or satellite thermal imaging for area monitoring; caveats: aim at healthy canopy avoiding bare soil and shadow (soil background distorts readings when cover is poor); baselines must match the current crop and weather or bias occurs; signals weaken and reliability drops on cloudy, dawn or windy conditions; CWSI reflects instantaneous transpiration suppression and should be cross-checked with soil moisture and evapotranspiration, not relied on alone.
Formula
Crop water stress index: CWSI = (dT − dT_ll) ÷ (dT_ul − dT_ll).
dT = measured canopy temperature − air temperature; dT_ll lower baseline, dT_ul upper baseline.
CWSI ranges 0 (no stress) to 1 (full stress); often ~0.3–0.5 as the irrigation threshold.
$$CWSI = \dfrac{dT - dT_{ll}}{dT_{ul} - dT_{ll}}$$How to Use
- Measure canopy temperature with an infrared thermometer or thermal camera (avoid bare soil and shadow) and air temperature, compute dT = Tc − Ta.
- Enter the lower baseline dT_ll (well-watered, coolest) and upper baseline dT_ul (non-transpiring, hottest) for that weather.
- The tool returns CWSI (0–1); when it persistently exceeds your threshold (~0.3–0.5) treat it as an irrigation signal.
CWSI at different canopy-air differences (lower −1, upper 5)
| dT (°C) | CWSI | Deficit reading |
|---|---|---|
| -1.0 | 0.000 | No stress (well-watered) |
| 0.5 | 0.250 | Slight deficit |
| 2.0 | 0.500 | Moderate deficit (irrigate) |
| 3.5 | 0.750 | Clear deficit |
| 5.0 | 1.000 | Full stress (stomata closed) |
The hotter the canopy (larger dT), the higher CWSI and the worse the deficit; baselines vary with weather.
Case Studies
Canopy thermometry decides irrigation
At noon infrared reads canopy 32°C, air 30°C, dT = 2°C; baselines dT_ll = −1°C, dT_ul = 5°C.
CWSI = (2 − (−1)) ÷ (5 − (−1)) = 3 ÷ 6 = 0.5.
CWSI 0.5 exceeds the 0.4 irrigation threshold, showing transpiration clearly suppressed — schedule irrigation.
Low CWSI on a well-watered day
Next day after irrigation, canopy is cooler than air, dT = −1°C, same baselines dT_ll = −1, dT_ul = 5.
CWSI = (−1 − (−1)) ÷ 6 = 0, meaning no water stress.
Crop transpiration is vigorous and water ample; no irrigation needed yet, defer the next watering.
FAQ
What do CWSI 0 and 1 mean?
CWSI = 0 means the measured canopy-air difference sits on the 'well-watered lower baseline', crop transpiration is maximal and the canopy coolest — no water stress. CWSI = 1 means it sits on the 'non-transpiring upper baseline', stomata fully closed, transpiration stopped, canopy hottest — full stress. Between them proportionally, e.g. 0.5 means transpiration is about half suppressed. Normalizing deficit to 0–1 makes comparison easy.
How are the upper and lower baselines obtained?
Two ways. Empirical: measure the canopy-air difference on a confirmed well-watered reference plant in the same area for the lower limit, and estimate the non-transpiring upper limit. Theoretical: use energy balance with the current net radiation, wind and air saturation deficit (VPD) for the lower limit (theoretically it varies linearly with VPD), and the stomata-closed assumption for the upper limit. Baselines must match the current crop and weather.
Why does a water-stressed canopy get hotter?
When a crop transpires, water vaporizes from the leaf and removes large latent heat, cooling the leaf. With ample water, open stomata and vigorous transpiration keep the canopy cool; when short of water the crop closes stomata to conserve it, transpiration drops, less heat is removed and canopy temperature rises toward or above air temperature. So the canopy's relative warming versus air temperature is a sensitive deficit signal.
At what CWSI should I irrigate?
No absolute value; calibrate by crop and goal, usually set the threshold around 0.2–0.5: lower (e.g. 0.3) for sensitive crops or yield-protection goals, higher for drought-tolerant or deficit-irrigation management. In practice watch 'persistently above threshold' rather than a single reading, and judge together with soil moisture and weather to avoid false alarms from transient disturbance.
What to watch when measuring?
Aim at healthy, well-covered canopy, avoiding bare soil, shadow and disease spots, otherwise soil background raises readings and distorts CWSI; measure around sunny midday with light wind for the clearest signal; reliability is lower on cloudy, dawn or windy conditions. Average multiple points, use baselines matching current conditions, and cross-check with other deficit indicators for robust results.
Related Tools
References
Content reviewed by the Calculatorism editorial team. Results are for reference only; please refer to the relevant authorities for the official figures.