Stormwater Infiltration: A Beginner’s Overview
Stormwater infiltration is the downward movement of rainfall from the land surface into the soil profile. It is a fundamental hydrologic process that reduces runoff, recharges groundwater, and supports ecosystems.
In this article, we’ll explore:
What infiltration is and why it matters.
The factors that affect infiltration.
How engineers represent it in models such as EPA SWMM.
Three major infiltration methods used in practice.
Real-world applications, challenges, and limitations.
What Is Infiltration?
When rain falls, some of it runs off directly into gutters, ditches, and streams, but a portion infiltrates into the soil.
This process:
Reduces surface runoff, lowering flood risk.
Replenishes aquifers, helping sustain groundwater supply.
Improves water quality by filtering sediments and pollutants.
Supports ecosystems, supplying moisture for vegetation and contributing to stream baseflow.
Without infiltration, cities would face higher flood risks, degraded water quality, and unstable streamflows.
Why Infiltration Matters
Infiltration plays multiple roles in stormwater management:
Reduces peak flows, lowering flood and erosion risks.
Improves water quality through soil filtration.
Recharges aquifers that feed streams and wells.
Supports Low Impact Development (LID) and green infrastructure, where infiltration is a cornerstone practice.
Factors Affecting Infiltration
The amount of infiltration at a site depends on local conditions:
Soil type: sandy soils allow rapid infiltration, clay soils restrict it.
Vegetation: protects soil structure and promotes infiltration.
Compaction: construction activities reduce infiltration capacity.
Initial soil moisture: dry soils absorb more at first, but capacity declines as saturation occurs.
Slope: steep slopes promote runoff, while flatter ground allows more infiltration.
Infiltration Rates Over Time
Infiltration is not constant during a storm:
High at the beginning when soils are dry.
Declines as soils saturate, approaching a minimum capacity.
These changes directly affect the runoff hydrograph, shaping how much and how quickly stormwater reaches drains and streams.
Continuous simulation methods in models like EPA SWMM do a better job of reflecting these changing conditions compared to single-event methods such as the Rational Method or NRCS Runoff Method.
Common Infiltration Methods
Horton Method
Proposed in 1933.
Models infiltration as an exponential decline from an initial maximum rate toward a constant minimum.
Empirical (based on observed data, not soil physics).
Useful when detailed soil data is unavailable.
Green-Ampt Method
Physically based.
Considers soil suction, porosity, and hydraulic conductivity.
Assumes a sharp wetting front between saturated and unsaturated zones.
Provides more realistic results with proper soil data.
NRCS Curve Number Method
Developed by the USDA Soil Conservation Service (now NRCS).
Estimates runoff based on land cover, hydrologic soil group, and antecedent moisture.
Widely used in design reports where agencies require it.
Directly computes how much rainfall becomes runoff.
EPA SWMM supports all three approaches, allowing engineers to choose based on project needs and available data. Calibration against field data is recommended for critical projects.
Real-World Applications
Infiltration modeling guides the design of many stormwater control practices, including:
Rain gardens and bioretention areas.
Bioswales and vegetated strips.
Permeable pavements that allow rainfall to soak through.
Detention and retention ponds, where infiltration is part of the water balance.
Groundwater recharge projects, designed specifically to enhance infiltration.
Many LID regulations explicitly require engineers to document infiltration performance in their designs.
Challenges and Limitations
While essential, infiltration modeling comes with challenges:
Soil properties vary across a site.
Compaction during construction often reduces infiltration.
Systems like permeable pavements may clog without maintenance.
Seasonal conditions (frozen ground, dry soils) affect performance.
Reliance on estimated parameters without field testing can reduce accuracy.
That’s why calibration, field testing, and ongoing maintenance are critical to infiltration-based designs.
Conclusion
Stormwater infiltration is one of the most important processes in hydrology and stormwater management. By understanding infiltration factors, modeling methods, and practical applications, engineers can design systems that reduce flood risks, improve water quality, and support resilient urban development.
At Clear Creek Solutions, we provide resources to help you build this knowledge, including a free 24-video EPA SWMM course that walks you through modeling basics step by step.
