Canopy Surface Temp Δ (leaf−air)

Definition

Canopy Surface Temp Δ is the canopy-minus-air temperature difference (leaf−air), in °C, measured by the thermal-infrared channel of the Spectral Canopy Array against co-located air sensors. Transpiring foliage evaporatively cools itself below air temperature, so a healthy canopy runs negative; as stomata close under water stress, cooling stops and the canopy warms toward and above air temperature. It is an increasing-is-worse stress index.

Why it matters

This Δ is the facility's spatial water-stress thermometer: it covers the whole canopy area at high cadence and reveals stress hot-spots that point hydraulic probes cannot localize. A warming Δ is an independent corroboration of falling stomatal conductance and forms the basis of CHLORA's crop-water-stress index used to steer irrigation zoning. Because it integrates the same physiology as g_sw over a wide area, the two are read together to confirm genuine stress versus a single-leaf artefact.

Formula

ΔT = T_canopy − T_air        (°C, leaf−air)

where:
  T_canopy = thermal-IR canopy surface temperature
             (Spectral Canopy Array, 8–14 µm, ε = 0.98)
  T_air    = co-located shielded air temperature

emissivity & atmosphere correction:
  T_canopy from radiance with leaf emissivity 0.98 and
  short-path atmospheric correction (closed environment).

masking:
  computed over live-canopy pixels only (NDVI > 0.40);
  cell value = median ΔT of masked pixels.

reference:
  also feeds CWSI = (ΔT − ΔT_wet) / (ΔT_dry − ΔT_wet)

Reported to 0.1 °C.

Inputs

Units & Scale

°C, reported to 0.1, signed (negative = cooler than air = transpiring well). Computed as the masked-canopy median per cell, then leaf-area-weighted to a facility value. Strongly diurnal and light-dependent; thresholds apply during the photoperiod. Confidence degrades when masked-pixel fraction drops below 60%, when air-sensor pairing is stale, or under condensation events that bias the thermal read.

Sampling & Source

• Computed every 5 minutes from the Spectral Canopy Array thermal-IR channel.
• Paired with co-located PhytoSense Mesh shielded air sensors.
• Thresholds evaluated during the photoperiod; feeds the crop-water-stress index for zoning.
• Stale handling: flagged STALE after 15 min or on air-pair loss.

Thresholds

Recent Trend

Facility leaf-area-weighted ΔT (photoperiod), last 14 samples:

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — thermal-IR emissivity & air-pair calibration.
• Crop-water-stress zoning & WARN/CRIT triage — Operations SOP set in the SOP Library.
• HVAC-fault discrimination & alarm escalation — Monitoring Systems.