Sap Flow Rate
VC-MET-007 — the mass rate of xylem sap ascent through the stem, the whole-plant integral of transpiration and the supply side of the facility's water balance.
Definition
Sap flow rate is the mass of water transported upward through the stem xylem per hour, expressed in grams per hour (g·h⁻¹), integrating the whole plant's transpirational demand. At GreenWorld Sector-7 it is measured by PhytoSense Mesh heat-ratio / heat-pulse sap-flow sensors inserted in the stem, which infer sap velocity from the convective transport of an applied heat pulse and scale it by conducting sapwood area.
Why it matters
Where stomatal conductance reports a single leaf, sap flow integrates the entire plant — it is the truest whole-plant transpiration signal and the supply-side counterpart to turgor. A sap-flow that fails to rise with the morning light ramp is an early flag for root-zone or hydraulic-pathway failure that point sensors miss. CHLORA reconciles sap flow against irrigation delivery to detect leaks, blockages, and root loss; it supports CVI hydraulics without a direct weight.
Formula
F = V_s · A_sapwood · ρ_sap · 3600 where: V_s = sap velocity (cm·s⁻¹), heat-ratio method A_sapwood = conducting sapwood cross-section (cm²) ρ_sap = sap density ≈ 1.00 g·cm⁻³ 3600 = s→h conversion heat-ratio velocity: V_s = (k / x) · ln(v1 / v2) k = thermal diffusivity of green wood x = probe spacing v1,v2 = temperature ratios up/downstream of heater calibration: zero-flow at predawn equilibrium; sapwood area by allometry per cultivar, verified by core sampling at potting. Reported to 1 g·h⁻¹.
Inputs
| Symbol | Quantity | Reference Band | Source |
|---|---|---|---|
| V_s | Sap velocity (heat-ratio) | 0 → 30 cm·h⁻¹ | PhytoSense Mesh sap probe |
| A_sapwood | Conducting sapwood area | per cultivar | Allometry / core sample |
| ΔT | Up/downstream temp ratio | device-relative | PhytoSense Mesh |
| F_irrig | Irrigation delivery (reconcile) | per schedule | Irrigation Control |
Units & Scale
g·h⁻¹, reported to 1 g·h⁻¹. Strongly diurnal, near-zero at night by design, so thresholds apply during the photoperiod plateau. Facility value is summed per canopy unit and biomass-normalized for comparison. Confidence degrades when zero-flow predawn drift exceeds 3 g·h⁻¹ (suspected wound response or probe misalignment) or when sapwood-area allometry is overdue for verification.
Sampling & Source
- Sampled every 5 minutes by PhytoSense Mesh heat-ratio sap-flow probes.
- Thresholds evaluated on the photoperiod plateau; night flow logged, not alarmed.
- Reconciled against irrigation delivery to detect leaks and root loss.
- Stale handling: flagged STALE after 15 min; predawn-drift fault routed to Mesh maintenance.
Thresholds
OK
Healthy whole-plant transpiration. No action.
WARN
Suppressed flow; CHLORA advisory, check root zone & supply.
CRIT
Near-stalled transport; auto-dispatch, hydraulic failure.
Recent Trend
Facility biomass-normalized daytime sap flow, last 14 samples:
Interpretation Guidance
| Flow Band | Reading | Likely Driver | Action |
|---|---|---|---|
| 100–140 | High demand | Bright, high-VPD day | Confirm irrigation keeps pace. |
| 40–99 | Healthy | Normal transpiration | None; normal range. |
| 25–39 | Soft suppression | Cloud, low light, or mild stress | Cross-check conductance (VC-MET-004). |
| 12–24 | WARN suppressed | Root deficit or partial blockage | Check supply & Ψleaf (VC-MET-005). |
| < 12 | CRIT stalled | Embolism or root failure | Auto-dispatch; cultivation triage. |
Related Metrics
Stomatal Conductance
Leaf-level driver of flow.
VC-MET-006Stem Turgor Pressure
Demand/supply balance partner.
VC-MET-005Leaf Water Potential
Tension generated by flow.
VC-MET-014Root Electrical Capacitance
Root-mass status behind flow.
VC-MET-001Canopy Vitality Index
Supports CVI hydraulic sub-signals.
VC-MET-102VPD
Atmospheric driver of transpiration.
Related SOPs
- SOP Library — sap-flow probe insertion & predawn zero calibration.
- Sap-flow WARN/CRIT triage & irrigation reconciliation — Operations SOP set in the SOP Library.
- Leak/blockage detection & alarm escalation — Monitoring Systems.