Key takeaway
Water content describes how much water is present; tension concerns how tightly soil holds it. The numbers cannot be pooled as one moisture scale.
Decide which quantity the sensor history contains
An irrigation dashboard exports moisture values of 25 and 35 from two sensor types. The first could mean 25% volumetric water content; the second could be 35 kPa tension. They are not two observations on a common percentage scale. Recover the measurement type before calculating an average, a deficit or a trigger crossing.
University of Minnesota Extension distinguishes sensors that estimate volumetric water content from those that measure soil-water tension. It defines volumetric content as water volume per soil volume and describes tension as the energy associated with extracting water from the soil. A relation between them depends on the soil; a unit conversion alone cannot supply it.
Account for layer thickness in a fictional profile
Suppose a hypothetical profile has two layers, each represented exactly by its assigned sensor for this calculation. The upper layer is 200 mm thick and changes from 0.30 to 0.25 m³/m³. The lower layer is 400 mm thick and changes from 0.20 to 0.18. Both observations concern the same two times, and no layer is missing.
Multiplying content by layer thickness gives 60 to 50 mm of stored water in the upper layer and 80 to 72 mm below. The profile total falls from 140 to 122 mm: an 18 mm net storage decrease. An unweighted mean of the two content changes would ignore their unequal thicknesses. These are stipulated representative layers, not evidence that two real probes characterize an entire farm.
| Invented layer | Thickness | Content before → after | Stored water before → after |
|---|---|---|---|
| Upper | 200 mm | 0.30 → 0.25 m³/m³ | 60 → 50 mm |
| Lower | 400 mm | 0.20 → 0.18 m³/m³ | 80 → 72 mm |
| Profile | 600 mm | Layer-weighted interpretation | 140 → 122 mm; decrease 18 mm |
| Separate tension sensor | Depth/location retained | 35 kPa | No universal VWC equivalent assigned |
A storage decrease is not the irrigation instruction
The example does not retain rainfall, irrigation, drainage, runoff or evapotranspiration. It therefore cannot attribute all 18 mm to plant water use or prescribe an 18 mm irrigation application. The Extension checkbook method explicitly retains water inputs, evapotranspiration and losses in its water balance.
A deficit relative to field capacity needs a separately supported field-capacity reference for the relevant soil/layers. A crop decision can also depend on root depth, growth stage and operating constraints. Keep those assumptions separate from the historical sensor readings. Do not manufacture a universal trigger by relabelling the largest value in the file as field capacity.
Preserve where the sensor looked
Record measurement type, original unit, sensor/model, depth, installation location and calibration/reference information. A local sensor has a spatial support; its reading is not automatically a field-average observation. Preserve replacements, moved probes and gaps rather than joining all records under one location name.
For tension sensors, retain the soil-specific interpretation curve or leave a requested content conversion unresolved. For content sensors, retain the layer-representation assumption used in any storage calculation. If a probe moves from 150 to 300 mm depth, a trend needs that change exposed. An apparently continuous timestamp series cannot repair the change in what was observed.
Return a usable interpretation boundary
The completed fictional result is 18 mm of stipulated profile storage change, with a separate 35 kPa tension observation and unknown water-balance components. It is not measured crop consumption, a field-wide estimate or an irrigation recommendation. That precise description tells the evaluator which additional evidence could change the decision.
Use Inventory to map sensor types, periods and depth changes, and Due diligence to ask whether a receiving task needs content, tension or a defensible derived balance. Field coordinates and grower practices need their own disclosure approval. Accurate quantity labels do not establish commercial availability or a right to share the underlying farm history.