Substrate Moisture (VWC)
VC-MET-106 — the volumetric water content of the root-zone substrate, the supply side of the plant water balance and the primary controlled variable for irrigation.
Definition
Substrate Moisture (VWC) is the volumetric water content of the growing medium: the fraction of total substrate volume occupied by water, expressed as a percentage. It is sensed directly by capacitance / time-domain-reflectometry probes on the PhytoSense Mesh, inserted into the root zone of representative containers. Reported VWC is substrate-calibrated — the raw permittivity reading is mapped through a medium-specific curve so that the percentage reflects actual plant-available water, not just dielectric response.
Why it matters
VWC is where irrigation control lives. Below the refill point the substrate cannot meet the canopy's transpiration demand — set by VPD (VC-MET-102) and light — and water stress propagates into turgor, conductance and the Canopy Vitality Index (VC-MET-001). Above field capacity the root zone becomes anaerobic, starving roots of oxygen and inviting rot. CHLORA schedules irrigation shots to ride VWC within its band, and reads the dry-down slope between shots as a live proxy for whole-plant water use.
Formula
A directly measured signal. Zone VWC is the representative-probe mean after substrate calibration and temperature correction:
VWC_i = f_substrate( ε_i ) substrate cal curve
VWC_zone = trimmed_mean( { VWC_i + tc_i(T_root) } )
ε_i measured permittivity, probe i
f_substrate medium-specific permittivity→VWC mapping
tc_i temperature correction from VC-MET-109
Dry-down rate (water-use proxy):
dVWC/dt over the inter-shot interval; steeper slope ⇒
higher transpiration demand.
Refill / field-capacity references are set per substrate
recipe and stored with the irrigation plan.
Inputs
| Channel | Sensor | Placement | Reference Band | Source |
|---|---|---|---|---|
| M1–M4 | Capacitance/TDR moisture probe (±2 % vol) | Root zone, 4 representative pots | 28 → 42 % vol | PhytoSense Mesh |
| T_root | Root-zone temperature | Probe co-location (correction) | VC-MET-109 | PhytoSense Mesh |
| W | Container load cells | Gravimetric cross-check | per substrate mass | PhytoSense Mesh |
| EC | Nutrient conductivity | Solution-strength context | VC-MET-107 | PhytoSense Mesh |
Units & Scale
Reported as percent of substrate volume, whole-percent precision. VWC is intensive per container and is not summed; zone roll-up is the mean of representative probes, cross-checked against gravimetric load cells. Confidence degrades to DEGRADED when probe and load-cell estimates diverge by > 4 % vol (a sign of channelling, root binding, or a probe pulled loose), or when the substrate calibration curve has not been refreshed for the current medium recipe.
Sampling & Source
- Sampled every 5 minutes from the PhytoSense Mesh moisture probes.
- Substrate-calibrated and temperature-corrected against Root-Zone Temperature (VC-MET-109).
- Stale handling: a probe silent > 15 min is dropped; loss of the probe set fails over to gravimetric load cells with a DEGRADED flag.
- Calibration: per-recipe substrate cal at media change; annual probe verification against oven-dry gravimetric samples.
Thresholds
OK
Between refill point and field capacity; aerated and supplied. No action.
WARN
Drought stress (low) or waterlogging (high); irrigation-control review.
CRIT
Wilting-point approach (low) or root anoxia (high); auto-escalation.
Recent Trend
Zone-A representative-probe VWC, last 14 sampling roll-ups (% vol):
Interpretation Guidance
| Band | Reading | Likely Driver | Action |
|---|---|---|---|
| 28–42 % vol | Optimal | Irrigation riding the band | None; log dry-down slope as reference. |
| 22–27.9 % vol | Drying drift | High VPD demand, missed shot, fast dry-down | Advance next shot; check VPD (VC-MET-102). |
| 42.1–48 % vol | Wet drift | Over-irrigation, poor drainage | Skip a shot; verify drainage & root-zone temp. |
| < 22 / > 48 % vol | WARN | Supply fault or drainage fault | Irrigation-control review; CHLORA advisory. |
| < 15 / > 55 % vol | CRIT | Wilting risk or root anoxia | Auto-dispatch; emergency shot or drain/aerate. |
Related Metrics
Nutrient Conductivity
Solution strength delivered with water.
VC-MET-108Root-Zone pH
Co-measured root-zone chemistry.
VC-MET-109Root-Zone Temperature
Drives anoxia/rot risk when wet.
VC-MET-102VPD
Demand side of the water balance.
VC-MET-001Canopy Vitality Index
Reflects water-stress propagation.
VC-MET-110Dissolved Oxygen
Root oxygen status in hydro loops.
Related SOPs
- SOP Library — irrigation scheduling & substrate-management procedures.
- Drought- and waterlogging-response on CRIT — see Environment SOP set in the SOP Library.
- Irrigation-valve interlock & alarm escalation — Monitoring Systems.