Canopy Heat & Reads the Thirst, Triggers the Water

CWSI is a 0–1 index that scales how water-stressed a crop is from its canopy temperature. It is defined as CWSI = (dT − dT_lower)/(dT_upper − dT_lower), where dT is the measured canopy-minus-air temperature, dT_lower is the cool baseline of a fully-watered canopy, and dT_upper is the hot limit of a non-transpiring, fully-stressed canopy. A value near 0 means well watered; near 1 means maximally stressed.

A crop with plenty of water transpires freely, and evaporating water cools the leaves below air temperature — a 'canopy temperature depression.' As the crop runs short of water it closes its stomata, transpiration drops, and the canopy warms toward and then above air temperature. CWSI quantifies where the canopy sits between those two extremes.

The NWSB is the canopy-minus-air temperature of a fully-watered crop, and it falls linearly as the air gets drier: dT_lower = a + b·VPD, where a is the intercept, b is a negative slope, and VPD is the vapour-pressure deficit. Each crop has its own a and b. This baseline is the 0-stress reference; the further the measured canopy sits above it, the higher the CWSI.

Vapour-pressure deficit is the dryness of the air. The tool computes saturation vapour pressure from air temperature with the Tetens equation es(T) = 0.6108·exp(17.27·T/(T+237.3)) kPa, then VPD = es(Tair)·(1 − RH/100). For example, 30 °C and 40% relative humidity give a VPD of about 2.5 kPa. VPD sets where the baseline sits, so it must be entered correctly.

A common rule is to irrigate when CWSI rises above a crop threshold, often around 0.3–0.4 for full production (lower for sensitive vegetables, higher where deficit irrigation is intended). The tool compares your CWSI against the crop's trigger and gives an irrigate-now or hold verdict. Crossing the threshold means the canopy has warmed enough to indicate real water shortage.

CWSI is only valid for a clear, sunny midday — roughly 12:00 to 14:00 solar time — over a canopy that fully covers the ground. At that time radiation is high and the canopy-air contrast is largest and most repeatable. Readings under cloud, in early morning or late evening, or over partial cover with hot soil in view are unreliable and will mislead the index.

Cotton's trigger is about 0.3, so a CWSI of 0.45 is above it — the crop is moderately stressed and irrigation is warranted. At that level the canopy is running well above its well-watered baseline, indicating the root zone is roughly 40–50% depleted. Cool the canopy back toward the baseline by irrigating, then re-read the next midday.

The tool maps the CWSI linearly onto an approximate fraction of available soil water used, anchored at CWSI 0 = full profile and CWSI 1 = fully depleted. It is an indicative figure, not a soil-moisture measurement, because the canopy-to-soil-water relationship varies with crop, rooting depth and soil. Use it as a guide alongside the irrigate/hold verdict.

Yes — CWSI has crop-specific baselines for grapes, citrus, almonds and pistachios, with triggers around 0.4. Tree and vine canopies are patchier than field crops, so aim the infrared sensor at sunlit, fully-leafed canopy and avoid the gaps. Many growers deliberately run vines and nut trees at a mild deficit, in which case a higher CWSI target is appropriate.

A handheld infrared thermometer (IRT) aimed at the canopy gives the canopy temperature; a simple thermometer or weather station gives air temperature and humidity. Drone- or tractor-mounted thermal cameras give the same canopy temperature over many plants at once. The math is identical regardless of sensor — what matters is a representative canopy reading at solar noon.

The raw formula can compute values outside 0–1 when the canopy is cooler than the baseline (negative) or hotter than the stressed limit (above 1), usually from measurement noise or an off-spec reading. The tool clamps the reported CWSI to the 0–1 range. A clamped 0 means well watered; a clamped 1 means at or beyond the fully-stressed limit.

Tensiometers and soil probes infer stress from the soil; CWSI reads the plant's own response directly from its leaf temperature, integrating the whole root zone and the current atmospheric demand. The two are complementary — CWSI catches stress the plant is actually feeling, while soil sensors tell you how much water to refill. Using both gives a robust schedule.