Calculate plant available water in the root zone from soil texture or your own field capacity and wilting point, in mm or inches, plus capacity in %.
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Plant Available Water Formula
The calculator uses volumetric field capacity and permanent wilting point to estimate plant available water capacity as a percentage of soil volume. Use the same volumetric basis for both inputs, not gravimetric percentages.
Pick a soil texture (or enter your own values) and a root zone depth, and the calculator estimates the depth of water available in that root zone.
- PAW = plant available water, as a percent
- FC = field capacity, as a percent
- PWP = permanent wilting point, as a percent
- AWD = available water depth in the root zone
- D = root zone depth
- D_cm = root zone depth in centimeters
- D_in = root zone depth in inches
The percent result shows the difference between field capacity and permanent wilting point. The calculator converts that percentage into a water depth using the root zone depth. For centimeter depth inputs, it reports millimeters and inches. For inch depth inputs, it converts internally and still reports both units.
Typical Plant Available Water by Soil Texture
These are approximate volumetric estimates for soils with 2.5% organic matter, adapted from SDSU Extension, Table 1 (field capacity at โ33 kPa and wilting point at โ1500 kPa). Actual site values vary; use measured local values when available.
| Soil texture | Typical field capacity | Typical permanent wilting point | Typical PAW |
|---|---|---|---|
| Sand | 10% | 5% | 5% |
| Sandy loam | 18% | 8% | 10% |
| Loam | 28% | 14% | 14% |
| Clay loam | 36% | 22% | 14% |
| Clay | 42% | 30% | 12% |
Example Calculations
Example 1: Plant available water percent
Field capacity is 32%. Permanent wilting point is 14%.
The plant available water is 18%.
Example 2: Available water in a 45 cm root zone
Field capacity is 32%, permanent wilting point is 14%, and root zone depth is 45 cm.
The available water in the root zone is 81 mm, which is about 3.19 in.
FAQs
What is plant available water?
Plant available water is the water held in soil between field capacity and permanent wilting point. Field capacity is the amount of water left after excess water drains away. Permanent wilting point is the point where plants can no longer extract enough water to recover.
Why must field capacity be greater than permanent wilting point?
Plant available water is calculated by subtracting permanent wilting point from field capacity. If field capacity is lower, the result would be negative, which is not physically useful for this calculation.
Is plant available water the same as current soil moisture?
No. Plant available water describes the storage capacity between two soil water limits. Current soil moisture tells you how much water is in the soil right now. To estimate current available water, you also need a current soil moisture measurement.
