Measure unsaturated hydraulic conductivity, characterise pore-size structure, and compare soil health across treatments and depths.
1. What This Calculator Does
The Disc Permeameter Infiltration Calculator processes field data from disc permeameter (tension infiltrometer) measurements to calculate unsaturated hydraulic conductivity K(h). It helps you answer: how quickly does water move through different pore sizes in your soil, and how does soil structure vary between treatments, depths, and sites?
The calculator operates in two modes:
| Mode | Description |
| Quick Calculator | Enter reservoir readings from a single tension measurement. The calculator detects steady-state infiltration, applies the Wooding equation, and computes K(h). Includes an option to estimate the sorptive number a* from multi-tension data. |
| Batch Analysis | Upload a CSV file containing field data from multiple sites, tensions, and depths. The tool computes K(h) for every measurement, generates summary tables grouped by treatment and depth, produces pore-class charts, and exports downloadable reports. |
All calculations happen in your browser. No data is sent to a server.
2. Key Concepts
The Wooding Equation
The calculator uses the Wooding (1968) steady-state equation to convert observed infiltration rates into hydraulic conductivity:
K(h) = q_ss / (1 + 4 / (π · a* · r))
where q_ss is the steady-state flux (cm/s), a* is the sorptive number (cm⁻¹) that describes how strongly the soil draws water by capillarity, and r is the disc radius (cm).
Sorptive Number (a*)
a* controls how much of the infiltration is driven by capillarity versus gravity. It can be obtained in two ways:
• Known value: Enter a published or previously determined a* directly. Use 0.05 cm⁻¹ as a reasonable default for most agricultural soils.
• Estimate from data: If you have steady-state rates at multiple tensions, the calculator fits a regression of ln(q) versus h. The slope of that line is a*. The R² value indicates fit quality.
Pore-Size Bands
By running the disc permeameter at multiple tensions (e.g. −15, −6, −3 cm), you introduce progressively larger pores. Subtracting consecutive K(h) values isolates how much water flows through each pore-size band:
| Tension (cm) | Approx. max pore diameter | What it represents |
| −15 | ~0.2 mm | Flow through the finest measurable pores only |
| −6 | ~0.5 mm | Adds medium structural pores |
| −3 | ~1.0 mm | Adds coarse structural pores (inter-aggregate cracks, fine root paths) |
Note: even the coarsest band measured (−3 cm, pores up to ~1 mm) is finer than visible earthworm burrows or large root channels, which only flow under ponded conditions.
Steady-State Detection
The calculator automatically identifies when infiltration has reached steady-state by examining the last N readings (default 6). It checks both the coefficient of variation (CV%) and the slope of the readings over time. You can adjust these thresholds under Advanced Settings in the header.
Units
All results are displayed in mm/hr as the primary unit, with cm/s shown as a secondary value. The CSV input uses cm/min for reservoir drop rate (as read in the field), and the calculator handles all conversions internally.
Statistical Reporting
The calculator reports mean values ± standard error for all summary statistics. This approach is more appropriate than median values for the typical small sample sizes (3–5 replicates) used in infiltration studies, as means utilize all data points and provide better statistical power. Standard errors quantify the precision of mean estimates and enable proper statistical comparisons between treatments.
Quality Control System
The calculator automatically detects physically impossible data where K(h) decreases with less negative heads (e.g., K(-15) > K(-6)), which typically indicates insufficient equilibration time. When violations are detected, users can choose to:
• Remove problematic measurements only (recommended): Excludes only the problematic tensions while retaining valid measurements for partial pore-class analysis
• Exclude entire affected sites: Removes whole sites with any violations
• Keep all data anyway: Proceeds with clear warnings about data quality issues
Partial data analysis allows calculation of pore classes from remaining valid measurements (e.g., if K(-15) is problematic but K(-6) < K(-3) is valid, the -6 to -3 pore class can still be calculated).
All results are displayed in mm/hr as the primary unit, with cm/s shown as a secondary value. The CSV input uses cm/min for reservoir drop rate (as read in the field), and the calculator handles all conversions internally.
3. Configurable Parameters
Four parameters appear in the green header banner and can be edited at any time. Changes take effect immediately for subsequent calculations.
| Parameter | Default | Notes |
| Disc diameter (cm) | 20 | Outer diameter of your disc permeameter base. Common sizes: 20 cm (standard), 25 cm (large). |
| Reservoir diameter (cm) | 5.4 | Inner diameter of the water reservoir tube. |
| Soil depths (cm) | 0, 10 | Comma-separated list of measurement depths. Typically surface (0) and a subsurface depth. |
| Tensions (cm) | −15, −6, −3 | Comma-separated list of applied tensions (most negative first). These drive the dynamic inputs throughout the calculator. |
Advanced Settings
Click “Advanced settings” in the header to reveal tail-detection parameters:
| Parameter | Default | What it controls |
| Tail points (N) | 6 | Number of final readings used to assess steady-state. |
| Tail CV threshold (%) | 20 | Maximum coefficient of variation for the tail to be considered stable. |
| Tail slope threshold | 0.00001 | Maximum absolute slope (cm s⁻¹ min⁻¹) allowed in the tail. A near-zero slope indicates steady flow. |
4. Quick Calculator
Use this tab for processing individual measurements interactively.
Step 1 – Sorptive Number (a*)
Choose one of two modes using the radio cards:
• Use a known value: Enter a* directly. Typical range is 0.01–0.5 cm⁻¹. If unsure, use 0.05.
• Estimate from my data: Enter your own steady-state infiltration rate determined at each configured tension. Click “Estimate a” to run a ln(q) vs h regression. If the fit is acceptable (R² close to 1), click ”Use this a” to transfer it to Step 2.
Step 2 – Enter Reservoir Readings
Enter your time-series of reservoir readings for a single tension run:
- Set the tension h (cm) for this run.
- Enter time (minutes) and reservoir level (cm) readings. New rows are added automatically when you fill the last row.
- Click “Calculate K(h)”.
The calculator converts reservoir drop to disc-surface flux using the conversion factor (reservoir area / disc area), identifies the steady-state tail, and applies the Wooding equation.
Results
A results panel displays:
- K(h) in mm/hr (highlighted) and cm/s
- Steady-state rate (the detected constant infiltration rate)
- Wooding factor = 1 + 4/(π·a·r), showing the capillarity correction
- Quality indicators: tail CV%, slope, and status message (either ✓ Steady-state acceptance checks passed* or warning messages if the CV or slope threshold is exceeded)
More details
Expand “Calculation details” for a step-by-step breakdown of the computation. You can also download the entered data as a CSV.
5. Batch Analysis
Use this tab when you have a complete field dataset to analyse.
CSV Format
Your CSV must contain these columns (header names must match exactly):
| Column | Description | Example |
| Date | Date of measurement | 2025-03-15 |
| Treatment | Treatment or land-use name | Conventional |
| Plot | Plot or site number | 1 |
| Subsample | Subsample or replicate ID | 1 |
| Soil depth (cm) | Measurement depth | 0 |
| Head (cm) | Applied tension | −6 |
| Constant rate (cm/min) | Steady-state reservoir drop rate | 0.0123 |
Click “Download example CSV template” to get a pre-formatted file for your configured depths and tensions.
Building the Summary
- Upload your CSV file. A preview shows the first 3 rows and column headers.
- Click “Build Summary”. The calculator parses every row, groups by treatment/plot/depth, estimates a* from the multi-tension data for each group, and computes K(h) at every tension using the Wooding equation.
Depth and tension matching: CSV values are snapped to your configured depths and tensions with ±0.6 cm tolerance. Values outside this range are dropped and noted in the quality report.
Summary Tables
One table is generated per configured depth. Rows are grouped by treatment and plot, showing mean K(h) ± standard error at each tension (in mm/hr), the mean a* ± standard error, and the number of replicates (n). This provides a direct comparison of hydraulic performance across treatments.
Data Quality Report
A summary of any data issues encountered during processing:
- Rows dropped due to unrecognised depths or tensions
- Rows with missing or non-numeric rates
- Groups with insufficient tensions for a* estimation
Use this to identify data entry errors or field measurement problems before relying on the results.
Downloads
Four export options become available after building the summary:
| Export | Contents | Use for |
| Summary CSV | Mean K(h) ± SE by treatment, plot, and depth | Quick overview, sharing with collaborators |
| Full Data CSV | Every measurement row with all computed K(h) values | Audit trail, custom analysis in R or Excel |
| Porosity Proxies CSV | Pore-class conductance (incremental K between tensions) by treatment | Pore-structure comparison across treatments |
| Pore-Class Charts (PDF) | Box plots, absolute and percentage stacked bars, and executive summary tables | Reports, presentations, printing |
6. Understanding the Charts
Box Plots
One box plot is generated for each tension at each depth. Each box shows the distribution of K(h) values across plots for that treatment. The box spans the interquartile range (Q1 to Q3), the line inside is the median, and the whiskers extend to the minimum and maximum. Use these to see variability within treatments and spot outliers.
Note: Box plots correctly show medians as the center line, which is appropriate for displaying
distributions of individual measurements, even though summary tables use means for statistical analysis.
Absolute Stacked Bars (mm/hr)
Each bar shows the total near-saturated conductivity for a treatment, broken into segments for each pore-size band. The height of each segment represents the incremental flow through that pore-size band:
- Bottom segment (finest pores): K at the most negative tension (e.g. K(−15))
- Middle segments: Incremental K between consecutive tensions
- Top segment (coarsest pores): Incremental K at the least negative tension
How to read: Taller bars mean more total infiltration. If most of the bar is in the bottom segment, the soil lacks structural porosity. Large contributions in the upper segments indicate well-developed inter-aggregate pore networks and fine root paths.
Percentage Stacked Bars (% of total flow)
Every bar is scaled to 100%, showing what proportion of total flow goes through each pore-size band. This makes it easy to compare pore structure between treatments regardless of differences in total infiltration rate. Percentage labels appear inside each segment where space permits.
Executive Summary (Traffic-Light Tables)
One table per pore-size band at each depth. Each cell shows conductance in mm/hr, colour-coded against the dataset quartiles:
| Colour | Meaning | Threshold |
| Green | Higher flow | ≥ Q3 value for that band |
| Amber | Moderate flow | Between Q1 and Q3 |
| Red | Lower flow — may indicate compaction | < Q1 value for that band |
| Grey | No data | Missing measurement |
Note: groups will not always split evenly into quartiles. When multiple sites share similar values they receive the same colour, so ties — common in small datasets — can shift the apparent distribution between colour bands.
7. Quick Start Guide
For a single measurement (Quick Calculator):
- Set your equipment dimensions in the header banner (disc and reservoir diameters).
- Choose your a* method in Step 1. If you have multi-tension rates, use “Estimate from my data”. Otherwise enter a known value (0.05 if unsure).
- Enter your time-series readings in Step 2. Set the tension, then enter time and reservoir level pairs.
- Click “Calculate K(h)” and review the results panel.
For a full field dataset (Batch Analysis):
- Configure depths and tensions in the header to match your field protocol.
- Download the CSV template and enter your data (or format your existing spreadsheet to match).
- Upload the CSV on the Batch Analysis tab. Check the preview.
- Click “Build Summary” and review the tables and quality report.
- Download your exports — summary CSV for quick results, pore-class PDF for presentations.
8. Tips for Best Results
• Run tensions in order from most negative to least negative (−15 → −6 → −3 cm) to avoid wetting hysteresis effects.
• Take frequent early readings (every minute for the first 10 minutes) and space them out as the rate stabilises.
• Ensure good contact between the disc and the soil surface. Use a thin (2–3 mm) layer of fine sand on the prepared surface.
• A minimum of three tensions is needed to estimate a*. Two tensions will give a regression but with no degrees of freedom to assess fit quality.
• If the tail CV exceeds the configured threshold, the run likely did not reach steady-state. Consider extending the measurement or discarding the reading.
• For batch analysis, consistent column naming matters. Use the template CSV to avoid formatting issues.
• If your field data uses different depths or tensions than the defaults, update the header fields before uploading — the calculator will snap CSV values to whatever you configure.
• The calculator is entirely client-side. Your data stays in your browser and is never uploaded to a server. Refresh the page to clear all data.
9. Limitations
• Disc permeameters measure near-saturated conductivity only. They do not characterise micropores (< 0.03 mm) or true saturated conductivity.
• The practical lower tension limit is approximately −15 to −20 cm, constrained by the membrane air-entry value.
• The Wooding equation assumes homogeneous, isotropic soil beneath the disc. Layered or cracked soils may violate this assumption.
• Even at −3 cm tension, pores larger than ~1 mm are excluded. Visible earthworm burrows, root channels, and structural cracks only conduct water under ponded (positive head) conditions.
• Steady-state detection is automated but not infallible. Always review the quality indicators and consider the field context. Make quick checks in the field – if the infiltration rate at a lower tension is less than at a higher tension, it is very likely the higher tension was not run long enough to truly stabilise. You should re-run your measurements to avoid uncalculatable results later.
• The batch analysis uses mean values when summarising across replicates. Standard errors provide appropriate measures of uncertainty around the mean estimates.
10. Quality Control:
The calculator automatically detects physically impossible data (e.g., K(-15) > K(-6)) and offers options to exclude problematic measurements while retaining valid data for partial analysis.
DISCLAIMER
This tool was developed by LandWISE Inc. to support soil health research and monitoring. It is not a commercial product and must not be relied on for critical decision-making without independent verification. We make no warranties and take no responsibility for any errors in the calculator or its outputs. Use it at your own risk. It is made freely available solely on this basis.
Disc Permeameter Infiltration Calculator v1.0.2 — landwise.org.nz — February 2026