How to perform an infiltration test? A Comprehensive Guide

September 27, 2025 •

4 min read

A startling statistic from the USDA is that highly compacted soils can decrease water infiltration rates by 90% or more, leading to runoff and erosion. To combat this and understand your landscape's health, you must know how to perform an infiltration test? This guide offers authoritative, step-by-step instructions for accurate field testing.

Quick Summary

An infiltration test measures the rate at which water penetrates the soil, using methods like a simple hole or specialized ring infiltrometers. These tests require prepping a site, pre-soaking the soil, and then timing the water level drop over a period to calculate the infiltration rate.

Key Points

Pre-soak is Essential: Always presoak the soil before testing to achieve a more accurate, stable infiltration rate, as initial rates on dry soil are misleadingly high.
Double-Ring Method is Most Accurate: The double-ring infiltrometer is the most reliable field method as the outer ring prevents lateral water flow, ensuring you measure true vertical infiltration.
Consider Your Project's Needs: The right test method depends on your project. Use a simple perc test for quick estimates, or the double-ring method for detailed, engineering-grade data.
Test Multiple Areas: Soil composition varies, so perform infiltration tests in several locations across your site to get a representative average rate.
Calculation is Key: The final infiltration rate is calculated by dividing the change in water depth by the elapsed time, ensuring you use consistent units like inches or cm per hour.
Minimize Disturbance: During the test, avoid compacting the soil or disturbing the water flow by pouring gently and using a diffuser.

Understanding Soil Infiltration

Soil infiltration is the process by which water on the ground surface enters the soil. This rate is a critical indicator for environmental engineering, agriculture, and landscape planning. It affects everything from stormwater management for rain gardens to designing proper drainage and irrigation systems. Several factors influence this rate, including soil texture, compaction, moisture content, and organic matter. Performing an infiltration test allows you to quantify your soil's capacity to absorb water, providing vital data for informed decision-making.

Different Types of Infiltration Tests

There are several methods for measuring soil infiltration, ranging from simple do-it-yourself options to more complex, standardized procedures. The choice depends on the project's scale and the required accuracy. Three common methods include:

Simple Percolation Test: Often called a "perc" test, this involves digging a hole and monitoring the water level drop. It's a quick and easy method suitable for small residential projects.
Single-Ring Infiltrometer: This uses a single metal cylinder pounded into the soil. It provides a more controlled measurement than a simple hole but can still be influenced by lateral water flow.
Double-Ring Infiltrometer: Considered the most accurate field method, it uses two concentric rings. The outer ring is filled with water to create a buffer zone, forcing the water in the inner ring to flow vertically.

Materials Needed for a Double-Ring Infiltrometer Test

To ensure accurate results with the double-ring method, gather the following materials:

Double-ring infiltrometer (concentric metal rings, e.g., 12-inch and 24-inch diameters)
Sledgehammer or mallet and a wooden block
Water source (bucket, hose, or Mariotte tubes)
Ruler or hook gauge for measuring water levels
Stopwatch or timer
Data sheet and pen
Level
Hand trowel
Clearance of a 10x10 ft area

Step-by-Step: Double-Ring Infiltrometer Procedure

This method is the gold standard for measuring true vertical infiltration rates.

Select and Prepare the Site: Choose a representative and level area for your test. Clear the site of any vegetation, loose debris, or rocks. If testing at a specific depth, dig a pit to that level.
Drive the Rings: Place the outer ring first, and then the inner ring, centered within it. Use a wooden block and sledgehammer to drive both rings into the soil to an even depth (e.g., 6-8 inches). Use a level to ensure they are vertical. Firm the soil around the outer edge to prevent leaks.
Presoak the Soil: Add water to both the inner and outer rings simultaneously, filling to a predetermined mark. Allow the water to fully drain. This process saturates the surrounding soil, ensuring more stable measurements. For some soils, a 30-60 minute presoak may be sufficient, while heavier clays may require several hours or even overnight.
Perform the Test: Refill both rings to the mark. Start your timer immediately. As water infiltrates, the level will drop. Refill both rings to the same mark at regular intervals (e.g., 10, 15, or 30 minutes, depending on the presoak results) and record the volume of water added to the inner ring.
Achieve a Stabilized Rate: Continue readings until the infiltration rate in the inner ring stabilizes, meaning four consecutive measurements are within a certain tolerance (e.g., 1/4 inch difference).
Calculate the Infiltration Rate: The final steady-state infiltration rate, or the average of the stabilized readings, represents your soil's infiltration rate in inches per hour.

Comparison of Infiltration Test Methods


Feature	Simple Perc Test	Single-Ring Infiltrometer	Double-Ring Infiltrometer
Cost	Very low (basic tools)	Low to moderate	Moderate to high
Accuracy	Low (highly variable)	Moderate (prone to lateral flow)	High (isolates vertical flow)
Effort	Easy, minimal setup	Moderate	High (requires careful installation)
Best For	Preliminary site checks, small projects	Initial testing, relative comparisons	Detailed engineering, critical applications
Equipment	Shovel, bucket, ruler, timer	Metal ring, water, ruler, timer	Two concentric rings, water, hammer, level

Calculating the Infiltration Rate

Regardless of the method used, the basic calculation involves dividing the change in water depth by the time elapsed to get a rate.

$Infiltration Rate (I) = \frac{Change in Water Depth (ΔH)}{Time Elapsed (Δt)}$

For a simple perc test, you might measure the drop over 15 minutes and multiply by four to get an hourly rate. For infiltrometer tests, you convert the volume of water added during each interval to a water depth based on the ring's area before calculating the rate. The final units are typically inches or centimeters per hour.

Tips for Accurate Testing

Test Multiple Locations: Soil can vary significantly across a site. Perform tests in several spots to get a representative average.
Pre-saturate: Always presoak the soil. A dry, cracked soil will absorb water much faster initially, leading to inaccurate results. Testing on wet soil gives a more consistent, long-term infiltration rate.
Minimize Disturbance: Avoid compacting the soil around the test area and be careful not to disturb the soil surface when adding water. Using a diffuser or pouring gently is key.
Record Conditions: Note all relevant site conditions, including weather, soil type, and any recent rainfall. This helps interpret your results.

Conclusion

Conducting a proper infiltration test is a fundamental step in evaluating soil performance for various applications. While a simple perc test offers a quick estimate, the double-ring infiltrometer provides the most reliable data for engineering-level projects. Understanding your soil's capacity for water absorption is essential for effective stormwater management and sustainable landscape design. For further reading on soil science and conservation, the Natural Resources Conservation Service offers a wealth of information and guides.

Frequently Asked Questions

A percolation test is a specific, often simplified, type of infiltration test that measures the time it takes for a specific amount of water to drain away. An infiltration test is the broader, more scientific process of determining the rate at which water enters the soil.

The frequency depends on your project and the purpose. For new construction or landscape projects, you only need to test once. For agricultural or soil health monitoring, periodic testing every few years can help assess long-term changes, especially after significant land disturbances.

It is best to wait for moderate weather conditions. Heavy rain can saturate the soil and skew results, while freezing temperatures will prevent water from infiltrating at all. Ideal conditions include dry or moderately moist soil and stable temperatures.

A slow rate often points to poor drainage, which can be caused by soil compaction, high clay content, or a restrictive soil layer. This may indicate the need for soil amendments or a different approach for projects like rain gardens or septic systems.

A fast rate, typically found in sandy or well-aerated soils, indicates excellent drainage. This is often good for many applications but can also mean water moves through the soil so quickly that it doesn't have time to benefit plants or be filtered properly.

The outer ring of a double-ring infiltrometer acts as a buffer zone, ensuring that the water from the inner ring moves vertically into the soil. This prevents lateral (sideways) water movement, which can cause significant measurement errors in single-ring tests, providing a more accurate result.

For a simple perc test, you will need basic tools such as a shovel or post-hole digger, a measuring tape or ruler, a bucket for water, and a stopwatch. The process is less equipment-intensive than professional methods like using a double-ring infiltrometer.