Crop Water Stress Index.

Every other input on this site measures the plant's environment: the light on it, the air around it, the water and salt at its roots. This one measures the plant's response. A well-watered canopy transpires and runs a few degrees cooler than the air; a thirsty one closes its pores, stops cooling, and heats up. The Crop Water Stress Index turns that temperature gap into a single number, and the plant tells you it is stressed before it ever wilts.

Leaves cool themselves the way skin does: by evaporating water. When a plant is well supplied, its stomata, the pores on the leaf, stay open, water vapor escapes, and that evaporation carries heat away, holding the canopy below air temperature. When water gets hard to reach, the plant defends itself by closing those stomata to stop losing water. The cooling stops, and the leaf warms toward, then above, the surrounding air. Canopy-minus-air temperature is therefore a direct readout of whether the plant is actually managing its water, integrating everything happening at the roots and in the air into one honest signal from the plant itself.

Ask the plant, not the soil.

This is what makes CWSI special. You can have perfect moisture, perfect tension, perfect EC, and still have a plant under water stress because the air is pulling water out of the leaf faster than the roots can supply it, a problem no root-zone sensor can see. CWSI sees it, because it reads the outcome at the leaf. It is also the earliest objective stress signal: the canopy warms hours before a leaf visibly droops, so you can act while it's still cheap to act.

Why the leaf runs cool.

The same transpiration that cools the leaf is the engine that lifts water and dissolved nutrients up from the roots, so a canopy that has stopped cooling has also stopped pulling its feed. That links CWSI directly to the rest of the system: it is the plant-side mirror of VPD (which sets how hard the air pulls), of tension (how hard the roots must pull back), and of root oxygen (whether the roots have the energy to pull at all). When any of those go wrong, the canopy warms.

The index needs context: it's computed, not raw.

Raw canopy temperature alone can mislead, because how cool a healthy leaf should run depends on the air. On a hot, dry day the air pulls hard and even a well-watered plant can't cool as much; on a cool, humid day the baseline is different again. CWSI corrects for this by scaling the measured gap between two references, the gap a fully-watered crop would show and the gap a fully-stressed crop would show under the same conditions, using VPD to set the well-watered baseline. The result is a normalized number from 0 (no stress) to 1 (maximum stress) that means the same thing across weather. This is why CWSI lives downstream of air temperature and humidity: it is derived from canopy temp plus the atmospheric demand, not read straight off a sensor.

How to read it.

The hardware is simple: a non-contact infrared thermometer aimed at the canopy reads leaf temperature without touching it (the inexpensive MLX90614 is the DIY part), and you already have air temperature and humidity from the environment side. A small node that reads the IR sensor and reports canopy temperature, air temperature, and the gap between them is enough to compute the index wherever VPD is known; that node emits canopy and air temperature as plain oat-ods readings, and the index is derived from them downstream. The gatherer lands the temperatures in your own data, tied to the zone.

The trap: reading canopy temperature without the air.

The trap is treating a bare canopy temperature as the answer, panicking on a hot afternoon when the leaf is warm for a perfectly legitimate reason, or aiming the infrared sensor at the pot, the floor, or a sunlit wall instead of clean leaf. CWSI exists precisely to remove that ambiguity: feed it the air conditions, point it at canopy, and a rising index is a real signal you can irrigate by, not a thermometer reacting to the weather.