California's diverse soils — ranging from the expansive clay hardpans of the Central Valley to the sandy loams of the coastal foothills — behave very differently when water is applied. Whether you are designing a compliant irrigated landscape or siting a septic system on a rural parcel, understanding how your soil interacts with water is not optional. Two distinct tests address this: infiltration testing and percolation testing. They are not interchangeable, and confusing them can lead to regulatory non-compliance, failed landscapes, or failed septic systems.

Infiltration vs. Percolation Testing in California: What Each Measures and When You Need It

Infiltration testing and percolation testing both involve measuring how water moves through soil, but they answer fundamentally different engineering questions, serve different regulatory programs, and cannot be substituted for one another. In California, infiltration testing is primarily tied to the Model Water Efficient Landscape Ordinance (WELO), while percolation testing is required for on-site wastewater treatment system (septic) permitting at the county level.

What Is Infiltration Testing?

Soil infiltration is the process by which water at the soil surface enters and moves into the soil profile. The infiltration rate describes how quickly this occurs, typically expressed in inches per hour (in/hr). A high infiltration rate means water enters the soil quickly; a low rate means water moves slowly, increasing runoff risk and reducing the efficiency of surface irrigation.

Infiltration rates are influenced by multiple soil properties, including texture, structure, organic matter content, compaction, and antecedent moisture. Sandy soils tend to have high infiltration rates; compacted clay soils tend to have low rates. The USDA Natural Resources Conservation Service (NRCS) classifies soils into hydrologic soil groups (A through D) that correspond to infiltration potential — a system used widely in landscape and stormwater design across California.

How Infiltration Testing Is Performed

Several standardized methods exist for measuring infiltration rate in the field. The most common for landscape applications in California are the double-ring infiltrometer method and the single-ring method. Both involve driving a metal ring (or rings) into the soil surface, filling it with water, and recording the rate at which the water level drops over time. The double-ring method is preferred because the outer ring buffers lateral flow, producing a more accurate measurement of vertical infiltration.

The USDA NRCS has published technical standards and guidance for conducting infiltration measurements as part of its soil health assessment protocols. Testing should be conducted at field capacity when possible to reflect real-world irrigation conditions, not when the soil is extremely dry, which can produce artificially slow initial readings due to hydrophobicity.

WELO and Why Infiltration Rate Matters for California Landscapes

California's Model Water Efficient Landscape Ordinance (WELO), first adopted under AB 1881 (2006) and significantly revised under AB 1534 and subsequent updates, is administered by the California Department of Water Resources (DWR). It applies to new and rehabilitated landscapes that exceed defined thresholds: 500 square feet of irrigated area for residential projects and 2,500 square feet for commercial, institutional, and other projects, depending on whether a local agency has adopted its own compliant ordinance.

WELO requires landscape designers to submit a Landscape Documentation Package that includes, among other elements, a soil management report based on actual soil data — not assumptions. Critically, this includes soil texture, infiltration rate, and drainage characteristics. These data points directly inform:

• Hydrozone design (grouping plants with similar water needs and soil conditions)
• Irrigation system design and emitter selection
• Maximum Applied Water Allowance (MAWA) calculations, which set a project-level cap on landscape water use
• Estimated Total Water Use (ETWU), which must not exceed the MAWA

Under WELO, the infiltration rate tells the irrigation designer how fast the soil can absorb water without generating runoff. If an irrigation system applies water faster than the soil can absorb it — a mismatch known as exceeding the soil's application rate — runoff and water waste result, which directly violates the ordinance's intent. For example, clay soils in much of Southern California may have infiltration rates as low as 0.05 to 0.20 inches per hour, meaning irrigation systems must use low-flow drip emitters or cycle-and-soak programming to avoid ponding and runoff.

UC Cooperative Extension and UC Agriculture and Natural Resources (UC ANR) have published extensive guidance on matching irrigation application rates to soil infiltration capacity, particularly for Mediterranean-climate conditions common across much of the state.

What Is Percolation Testing?

Percolation testing — commonly called a "perc test" — measures how quickly water drains downward through a pre-saturated soil column in a test hole. The result is expressed in minutes per inch (min/in), meaning the number of minutes it takes for the water level in the hole to drop by one inch. A lower number means faster drainage; a higher number means slower drainage.

Note the inverse relationship: infiltration is reported as a rate (faster = higher number in in/hr), while percolation is reported as a time (faster = lower number in min/in). This distinction causes frequent confusion when professionals from different disciplines discuss soil drainage on the same project.

How Percolation Testing Is Performed

Standard percolation testing for septic system design in California involves digging a test hole (typically 6–12 inches in diameter) to the depth of the proposed absorption field, saturating the soil over a period of hours or overnight, and then measuring the drop in water level at timed intervals. The saturating step is critical — it replicates the conditions the soil will experience once the leach field is in sustained operation and eliminates artificially fast readings from dry, pre-test soil conditions.

California counties generally follow procedures outlined in the California Plumbing Code and local environmental health department regulations. The California State Water Resources Control Board's Division of Drinking Water provides regulatory oversight over systems that may interact with groundwater. Some counties, particularly in the Sierra Nevada foothills and rural Northern California, have developed their own detailed protocols that may require multiple test holes across the project area.

Regulatory Purpose: The Key Distinction

This is where the two tests diverge most sharply in practice:

Infiltration Testing (WELO)

• Regulatory authority: California Department of Water Resources; local water agencies with adopted ordinances
• Applies to: New and rehabilitated landscape projects above threshold area
• What it controls: Irrigation efficiency, water budget compliance, hydrozone design
• Unit of measurement: Inches per hour (in/hr)
• Test location: At the soil surface, within the project landscape area
• Test goal: Ensure irrigation does not exceed the soil's capacity to absorb water without runoff

Percolation Testing (Septic Systems)

• Regulatory authority: County Environmental Health Departments; California State Water Resources Control Board
• Applies to: Parcels proposing on-site wastewater treatment systems (septic tanks and leach fields)
• What it controls: Leach field sizing, setback requirements, system type selection
• Unit of measurement: Minutes per inch (min/in)
• Test location: At the depth of the proposed absorption trench or bed
• Test goal: Determine whether the soil can accept effluent without surfacing or contaminating groundwater

A percolation test result does not substitute for infiltration data under WELO, and infiltration test data does not satisfy county environmental health requirements for septic permitting. These tests are governed by different agencies, use different methods, and answer different engineering questions.

Where California Soils Complicate Both Tests

California's extraordinary soil diversity — shaped by its Mediterranean climate, seismic geology, diverse parent materials, and intensive land use history — creates site-specific challenges for both types of testing.

Hardpan and Claypan Layers

Many California soils, particularly in the Central Valley and foothills, have subsurface hardpan or duripan layers formed by silica or calcium carbonate cementation. These layers can create a deceptive disconnect: surface infiltration rates may appear adequate while subsurface drainage (relevant to perc testing) is severely restricted. The USDA NRCS Soil Survey for California provides mapped data on soil series and their known drainage characteristics, which should be reviewed before any field testing program is designed.

Urban Compaction

In California's developed landscapes, soil compaction from construction equipment and foot traffic dramatically reduces infiltration rates. UC ANR research has documented that compacted urban soils can have infiltration rates 10 to 100 times lower than native reference soils of the same texture class. This has direct implications for WELO compliance: compacted landscapes may require soil amendment and decompaction before an irrigation plan can achieve MAWA compliance.

Seasonal Variation

California's pronounced wet-dry seasonality means that soil moisture and structure vary dramatically across the year. Infiltration rates measured in fall on dry, cracked clay soils may not reflect spring or summer performance. NRCS guidance recommends conducting infiltration measurements when soils are at or near field capacity for landscape design purposes — a condition that typically requires pre-wetting or testing following rain in California's climate.

Practical Guidance: Which Test Do You Need?

If your project involves an irrigated landscape subject to WELO thresholds, you need infiltration testing — not a perc test. If your project involves a new septic system on a rural or semi-rural parcel, you need a percolation test — not an infiltration test. Many rural projects in California require both, for different portions of the same site.

For Landscape and WELO Projects

1. Determine whether your project meets WELO thresholds (check with your local water agency or use DWR's WELO applicability guidance)
2. Commission infiltration rate testing using a standardized double-ring or single-ring infiltrometer method at representative locations within each hydrozone
3. Include infiltration data in the Soil Management Report section of the Landscape Documentation Package
4. Use results to match irrigation application rates and emitter types to soil intake capacity

For Septic System Projects

1. Contact your county environmental health department to obtain the specific percolation test protocol required in your jurisdiction — requirements vary significantly by county
2. Hire a licensed civil engineer, registered geologist, or qualified professional recognized by your county
3. Conduct pre-saturation the day before testing per protocol requirements
4. Submit results with your septic system permit application; local setback and sizing requirements will be applied based on the test outcome

Common Misconceptions

"A fast perc rate means good irrigation drainage."

Not necessarily. Percolation measures subsurface drainage in a vertical hole, not surface infiltration. A fast perc test result in sandy soil could correspond to an infiltration rate so high that drip irrigation is required to avoid wetting unintended areas, or it could indicate excessive leaching of applied nutrients.

"Infiltration testing is only for big commercial projects."

Under California WELO, the 500-square-foot threshold for residential projects means that most new residential landscaping projects requiring a permit — including those associated with new home construction — are subject to WELO and therefore require documented soil data including infiltration rate.

"You only need one test per site."

Soil variability across even a single residential lot can be significant, particularly on sloped sites, sites with cut-and-fill grading, or sites with mixed soil types. Both infiltration and percolation testing protocols recommend multiple test locations to capture the range of conditions present.