General Questions
What is a disc permeameter?
A disc permeameter (also called a tension infiltrometer) is a device that measures how water infiltrates into soil at controlled negative pressure (tension). Unlike ring infiltrometers that flood the soil surface, disc permeameters control which pore sizes water can flow through, giving you detailed information about soil structure.
Why would I use this instead of a simple ring infiltrometer?
Disc permeameters provide much more information:
- Pore-size distribution: See how much flow occurs through cracks and large pores vs. fine matrix pores
- Soil structure assessment: Detect compaction, poor structure, or excellent drainage capacity
- Preferential flow: Identify bypass flow pathways
- Multiple hydraulic properties: Get α, Ks, and macroporosity from multi-tension measurements
Ring infiltrometers only give you saturated flow – they can’t distinguish between structural pores and matrix flow.
What equipment do I need?
- Disc permeameter with tension control system (mariotte or bubble tower)
- Contact material (fine sand or glass beads, 0.1-0.5 mm)
- Circular template matching disc diameter
- Timer or stopwatch
- Level
- Data sheets and pencils
- This calculator (on your tablet or computer)!
Before You Measure
When should I measure?
Best conditions:
- Soil at or near field capacity (1-2 days after rain or irrigation)
- Moderate temperatures (not extreme heat or cold)
- During growing season if comparing year-to-year
Avoid:
- Frozen or nearly frozen soil
- Extremely dry soil (infiltration will be very slow)
- Immediately after heavy rain (soil too wet)
How many locations should I measure?
- Single field assessment: 3-5 representative spots
- Comparing management treatments: 5-10 replicates per treatment
- Spatial mapping: Grid sampling based on field size and variability
More measurements = more confidence in results, especially in variable soils.
Where exactly should I place the disc?
- Avoid: obvious cracks, animal burrows, fresh disturbances, rocks, wheel tracks (unless specifically assessing compaction)
- Choose: representative areas that reflect typical field conditions
- For compaction assessment: deliberately measure both trafficked and non-trafficked areas to compare
Taking Measurements
What tensions should I use?
Standard three-tension protocol:
- -15 cm: Measures pores > 0.2 mm (fine macropores and large mesopores)
- -6 cm: Measures pores > 0.5 mm (medium macropores)
- -3 cm: Measures pores > 1.0 mm (large macropores)
Measure in this order: -15 → -6 → -3 cm
For quick assessment: Use -6 cm (most common) or -3 cm (most conservative)
Note on pore sizes: Calculated using capillary equation: diameter (mm) ≈ 3 / tension (cm). Disc permeameters measure macropores and larger mesopores only. They cannot assess micropores (< 0.03 mm), which require tensions more negative than -100 cm.
How long does each measurement take?
Typically 30-60 minutes per tension to reach steady-state, but it varies:
- Sandy soils: May reach steady-state in 20-30 minutes
- Clay soils: May take 60-90 minutes
- Very dry soils: Can take 2+ hours
Don’t rush! Steady-state is essential for accurate results.
How do I know when I’ve reached steady-state?
You’ve reached steady-state when:
- Three consecutive readings show less than 5% variation in rate
- The rate of water decline in the reservoir becomes constant
- Typically occurs after the cumulative infiltration curve becomes linear
In the calculator: If your CV% is < 30%, you likely had good steady-state data.
What if my soil is really dry and infiltration is very slow?
Options:
- Pre-wet the soil: Apply water at -1 cm tension for 10-15 minutes, then proceed with your measurement tensions
- Use less negative tensions: Start with -3 cm instead of -15 cm
- Be patient: Accept that measurements will take longer
- Measure at better timing: Come back when soil is closer to field capacity
Can I measure on sloping ground?
Yes, but:
- Level the measurement area as much as possible
- Ensure the disc sits flat and level
- Note the slope in your records
- Be aware that results may be affected by gravity-driven lateral flow
- Consider measuring on more level areas if slope is steep (> 10%)
Can disc permeameters measure micropores?
No. Disc permeameters have practical limits:
What they CAN measure:
- Macropores and larger mesopores (pores > 0.2 mm)
- Tensions from 0 to about -15 or -20 cm
What they CANNOT measure:
- Micropores (< 0.03 mm)
- Require tensions more negative than -100 cm
- Extremely slow infiltration (hours to days)
- Air entry problems at high tensions
For micropore assessment, use:
- Pressure plate apparatus (up to -1500 kPa / -15 bar)
- Water retention curves
- Field capacity measurements
- Laboratory methods (mercury intrusion, gas adsorption)
Understanding the limitation: Micropores hold plant-available water, while macropores provide drainage. Disc permeameters characterize drainage pores, not water-holding pores.
Using the Calculator
Do I enter all my measurements or just steady-state data?
Enter only steady-state measurements. Skip the early transient phase where infiltration is rapidly changing.
Look at your raw data – identify where the rate stabilizes, then enter those time points and their corresponding cumulative infiltration values.
What units should I use?
- Time: minutes (from start of measurement)
- Water level: cm (reading from reservoir scale)
- Disc diameter: cm (measure across the disc)
- Reservoir diameter: cm (measure inner diameter of tower)
- Tension: cm (as negative value, e.g., -6)
The calculator automatically converts water level to cumulative infiltration and displays results in multiple units (cm/s, cm/hr, mm/hr).
What is “sorptivity parameter (b)” and what value should I use?
Sorptivity parameter accounts for capillary effects at the disc edge.
Use these values:
- 0.55: Default for most soils (clay loam to loam)
- 0.45-0.50: Sandy soils
- 0.60-0.70: Heavy clay soils
Unless you have site-specific data, use 0.55 – it’s appropriate for most agricultural soils and the variation has minimal impact on results.
The calculator shows rates between my data points – what are those?
These are instantaneous infiltration rates calculated from consecutive measurements:
Rate = (Infiltration₂ – Infiltration₁) / (Time₂ – Time₁)
They help you visually confirm steady-state – these values should be relatively constant during the steady-state period.
Should I use single tension or multi-tension analysis?
Use single tension when:
- You only measured at one tension
- You want quick results for a specific pore-size range
- You’re doing routine monitoring
Use multi-tension when:
- You measured at 2-3 different tensions
- You want to understand pore-size distribution
- You’re characterizing soil structure in detail
- You want to estimate saturated conductivity
Both are valuable – multi-tension gives more complete information but requires more field time.
Interpreting Results
What’s a “good” K(h) value?
It depends on your soil type and land use:
For agricultural soils:
- 5-60 mm/hr: Generally adequate range
- < 5 mm/hr: May indicate compaction or drainage problems
- > 60 mm/hr: Very good drainage, but watch for nutrient leaching
Context matters! A clay soil with K = 8 mm/hr might be excellent, while sandy loam with the same value might indicate problems.
My K(h) is “Very Low” – is that bad?
Not necessarily. Consider:
Naturally low K:
- Heavy clay soils naturally have low K
- Subsoils typically have lower K than topsoil
- Fine-textured soils without good structure
Problematic low K:
- Compacted soil (especially if previously higher)
- Traffic pans or plow pans
- Surface sealing or crusting
- Recent management causing structure degradation
Compare your result to:
- Expected values for your soil texture
- Previous measurements from same site
- Non-compacted reference areas
What does CV% tell me and why does it matter?
CV (coefficient of variation) measures consistency:
CV < 20%: Excellent – your steady-state measurements were very stable
CV 20-30%: Good – acceptable variation, results are reliable
CV 30-50%: Fair – some instability, interpret with caution
CV > 50%: Poor – steady-state questionable or highly variable soil
High CV can mean:
- You didn’t wait long enough for steady-state
- Soil is naturally variable (macropores are heterogeneous)
- Measurement problems (air locks, poor contact)
What is macroporosity and why should I care?
Macroporosity shows what percentage of flow occurs through large structural pores (cracks, root channels, worm burrows) versus fine-textured matrix pores.
Why it matters:
High macroporosity (> 50%):
- ✓ Excellent structure and drainage
- ✓ Good root penetration
- ⚠️ Potential for preferential flow (bypass of nutrients/pesticides)
- ⚠️ Rapid pesticide/nutrient movement
Low macroporosity (< 20%):
- ⚠️ May indicate compaction
- ⚠️ Limited large pore pathways
- ⚠️ Restricted drainage
- ✓ Better filtration of water
Moderate (20-50%): Ideal balance for most purposes
What does α (alpha) tell me?
Alpha describes pore-size distribution:
- Large α (> 0.1): Soil has uniform, similar-sized pores – common in compacted or structureless soils
- Small α (< 0.05): Wide range of pore sizes from very fine to very large – indicates good structure
- Medium α (0.05-0.1): Normal distribution for agricultural soils
Think of α as a “structure quality” indicator – smaller values generally indicate better structure.
The R² for my Gardner model is only 0.78 – what does that mean?
R² measures how well the Gardner exponential model fits your data:
R² < 0.85 suggests:
- Your soil doesn’t follow the simple Gardner model
- Possible bimodal pore distribution (two distinct pore populations)
- Potential measurement errors at one tension
- Complex soil structure
What to do:
- Review your individual tension measurements for quality
- Use the single-tension results rather than relying on the Gardner fit
- Note that not all soils follow simple models – this is okay!
My K values increased from -15 to -3 cm. What went wrong?
K(h) should decrease or stay similar as tension becomes less negative. If it increases:
Check these:
- Did you achieve steady-state at each tension?
- Did you measure in the correct order (-15 → -6 → -3)?
- Could the disc have moved between measurements?
- Could swelling clays be blocking smaller pores?
Most commonly, this indicates the more negative tension didn’t fully reach steady-state. Review those measurements carefully.
Specific Applications
How do I use this to assess compaction?
Compare measurements from:
- Wheel tracks vs. inter-row areas
- Surface (0-5 cm) vs. subsurface (10-20 cm)
- Before and after traffic events
Compaction indicators:
- K(h) < 5 mm/hr (especially if previously higher)
- Macroporosity < 20%
- Large decrease in K compared to non-trafficked areas
Can this help me design drainage systems?
Yes! K(h) data is essential for drainage design:
- Use K at -3 cm (conservative, represents matrix flow)
- Or use Ks from Gardner model for maximum drainage conditions
- Lower K values = need closer drain spacing
- High variability in K = need more flexible design
Note: Consult a drainage specialist for actual system design.
How do I use results for irrigation management?
Application rate guideline: Keep irrigation rates below K(h) at field tension (typically -3 to -6 cm) to prevent:
- Surface ponding
- Runoff
- Surface sealing
Example:
- K at -3 cm = 15 mm/hr
- Keep sprinkler application rate < 15 mm/hr
Can I compare measurements from different seasons?
Yes, but note that K(h) varies seasonally:
Higher K in:
- Late spring/summer (biological activity peak)
- After freeze-thaw cycles (structure development)
- During active root growth
Lower K in:
- Winter (reduced biological activity)
- After heavy rainfall (some structure breakdown)
- During/after cultivation
Best practice: Measure at same time of year for year-to-year comparisons.
Troubleshooting Problems
My infiltration is extremely slow (< 0.1 cm in 30 minutes)
Possible causes:
- Soil is very dry (wetting front advancing slowly)
- Severe compaction or clay pan
- Air lock in system
- Membrane clogged
- Poor contact between disc and soil
Solutions:
- Pre-wet the soil with -1 cm tension
- Check system for air bubbles
- Verify membrane is clean and functioning
- Improve contact layer quality
Water is pooling around the edge of the disc
Causes:
- Positive pressure head (check tension setting)
- Surface not level
- Contact layer too uneven
- Soil has preferential flow pathway just outside disc
Solutions:
- Verify tension control is working
- Re-level the surface and disc
- Remake contact layer more carefully
- Consider relocating measurement
My readings are erratic – they jump up and down
Likely causes:
- Air entering/leaving system
- Temperature fluctuations affecting water volume
- Macropore intercepted (worm hole, crack) causing intermittent flow
- Reading water level incorrectly
Solutions:
- Check for air leaks in system
- Shade reservoir from direct sun
- For macropore: note it, consider relocating, or just report high variability
- Take more careful readings
The contact material keeps washing away
Causes:
- Applying too much water initially
- Setting tension too close to zero
- Layer too thick
- Material too coarse
Solutions:
- Start with a more negative tension
- Use thinner layer (2-3 mm maximum)
- Use finer material (0.1-0.3 mm)
- Apply initial water very slowly
Data Quality and Confidence
How many measurements do I really need per site?
Absolute minimum: 3 steady-state points per tension
Recommended: 5-8 points per tension
Better: 10+ points per tension
More points = lower CV = more reliable average rate calculation.
Should I throw out outlier measurements?
Be cautious about discarding data!
Consider removing if:
- Clear measurement error (misread scale, recording mistake)
- Equipment malfunction at that point
- Obvious disturbance (accidentally bumped equipment)
Keep if:
- Just seems “different” but no clear error
- Soil variability could explain it
- Part of steady-state period even if slightly off trend
When in doubt, include it and let the CV tell you about data variability.
My two replicate measurements at the same site differ by 50%. Is that normal?
It can be, especially for:
- Macropore flow: Highly variable by nature
- Structured soils: Spatial variability is real
- Stratified soils: Small differences in depth sampled = big K differences
What to do:
- Report mean and range of replicate measurements
- Take more replicates if possible
- Note high spatial variability in interpretation
- Consider if sampling strategy needs adjustment
Advanced Questions
Can I use this method on rocky soils?
Challenging but possible:
- Choose locations between rocks
- Use small stones to level around larger rocks
- Accept that results represent the fine fraction between rocks
- Consider that bulk soil K may be higher due to rock voids
- Take extra replicates due to high variability
What about cracking clay soils?
Interesting case:
- Cracks dominate flow when open (very high K, high macroporosity)
- Matrix dominates when cracks swell closed (low K)
- Timing matters – measure when you want to understand flow conditions
- Multi-tension analysis shows dramatic difference between tensions
- Results highly dependent on moisture content
Can I convert K(h) to saturated K?
From single tension: No reliable conversion – too many assumptions required
From multi-tension Gardner model: Yes! The calculator extrapolates to estimate Ks
Caution: Extrapolated Ks is an estimate. For true saturated K, measure at zero or positive head.
How do seasonal changes in biology affect results?
Significantly:
- Earthworm activity creates macropores → higher K, higher macroporosity
- Root growth creates channels → higher K
- Freeze-thaw cycles improve structure → higher K
- Tillage disrupts structure → temporarily changes K
This is why timing measurements consistently is important for monitoring.
Getting Help
When should I consult a professional?
Consider getting expert help if:
- Results consistently contradict field observations
- Designing major infrastructure (drainage, irrigation systems)
- Making significant management investments based on data
- Measurements show concerning trends you don’t understand
- You’re working with unusual or problem soils
- Legal or regulatory requirements involved
Where can I learn more?
Scientific literature:
- Search for “tension infiltrometer” or “disc permeameter” in Soil Science Society of America Journal
- Look for Reynolds, Elrick, Wooding, and Perroux in author searches
Extension resources:
- USDA-NRCS Soil Quality resources
- State university extension soil science pages
- FAO soil management guides
Equipment manufacturers:
- Often provide detailed instruction manuals
- May offer training courses or videos
Final Tips
✓ Quality over quantity – one good steady-state measurement beats five poor ones
✓ Document everything – photos, conditions, anomalies help interpret results later
✓ Be patient – rushing to steady-state gives unreliable results
✓ Context is king – numbers mean nothing without site knowledge
✓ Track changes – baseline measurements enable monitoring improvement over time
✓ When in doubt, measure again – if results seem questionable, a repeat measurement gives clarity
Still have questions? The comprehensive guide provides detailed methodology and interpretation. The calculator’s “About” tab includes scientific references and background information.