Water Tension Factor
Overview
WaterTensionFactor is a root-weighted indicator of plant water availability within the root zone. It quantifies the extent to which roots are exposed to readily available soil water by combining:
- The distribution of live root biomass across soil layers.
- The amount of plant available water (PAW) in each layer.
- The plant available water capacity (PAWC) of each layer.
The resulting factor ranges from 0 to 1 and represents the average water supply conditions experienced by the living root system.
| Value | Interpretation |
|---|---|
| 0 | No effective water available to roots, or no roots present. |
| 1 | All roots are in soil layers with plentiful available water. |
| 0–1 | Intermediate levels of water availability weighted by root distribution. |
Calculation
For each soil layer containing live roots, a layer-specific water availability factor is calculated from the ratio of:
\[ \frac{PAW}{PAWC} \]
where:
- PAW = plant available water in the layer.
- PAWC = plant available water capacity of the layer.
This ratio is scaled by a factor of two and then constrained to the range 0–1:
\[ LayerFactor = \min\left(1,\; \max\left(0,\; \frac{2 \times PAW}{PAWC}\right)\right) \]
The overall WaterTensionFactor is then calculated as the weighted mean of all layer factors, where the weighting is based on the proportion of total live root biomass in each layer:
\[ WaterTensionFactor = \sum \left( \frac{RootWeight_{layer}}{TotalRootWeight} \times LayerFactor \right) \]
As a result, layers containing more root biomass have a greater influence on the final value.
Behaviour
- Returns 0 when no plant zone exists.
- Returns 0 when there is no live root biomass.
- Only layers with PAWC > 0 contribute to the calculation.
- Supports multiple soil zones; all rooted zones contribute according to their share of total live root biomass.
- The factor reaches 1 when the available water in a layer is at least half of its PAWC (
PAW ≥ 0.5 × PAWC).
Interpretation
WaterTensionFactor provides an integrated measure of soil water conditions experienced by the root system.
- High values (close to 1) indicate that most roots occupy well-watered soil layers.
- Low values (close to 0) indicate that roots are primarily located in dry soil layers.
- Because the calculation is weighted by live root biomass, drying in heavily rooted layers has a greater effect than drying in sparsely rooted layers.
Example
Consider a root system with:
| Layer | Root biomass (%) | PAW/PAWC | Layer factor |
|---|---|---|---|
| 1 | 60 | 0.50 | 1.0 |
| 2 | 30 | 0.20 | 0.4 |
| 3 | 10 | 0.00 | 0.0 |
The weighted value is:
\[ (0.60 \times 1.0) + (0.30 \times 0.4) + (0.10 \times 0.0) = 0.72 \]
Therefore:
\[ WaterTensionFactor = 0.72 \]
This indicates that although some roots are in dry soil, most of the root system is located in layers with adequate available water.
Dependencies
WaterTensionFactor requires:
- Live root biomass distribution by soil layer.
- Soil plant available water (
PAW). - Soil plant available water capacity (
PAWC). - Defined root zones and associated soil water and physical soil properties.
Notes
- Despite its name, this variable is not a direct measure of soil water tension (matric potential). Instead, it is a root-weighted index of available soil water, derived from the fraction of PAWC currently occupied by PAW.
- The scaling used in the calculation means that water availability is considered non-limiting once a layer contains at least 50% of its PAWC.