Infiltration capacity is the maximum rate at which soil can absorb and transmit water from the ground surface into the subsurface under given conditions, typically expressed as a depth per unit time, such as inches per hour or millimeters per hour. It represents the upper limit of how quickly water can enter the soil, regardless of how much precipitation is available.
In stormwater management, infiltration capacity is a critical factor in determining whether precipitation will infiltrate or become runoff. When rainfall intensity is less than the infiltration capacity, water is absorbed into the soil with little to no surface runoff. However, when rainfall intensity exceeds the infiltration capacity, excess water accumulates on the surface and begins to flow as runoff. This condition is commonly referred to as infiltration-excess runoff or Hortonian flow.
Infiltration capacity is not constant and typically decreases over time during a storm event. At the beginning of rainfall, dry soil can absorb water rapidly due to capillary forces and available pore space. As the soil becomes saturated, the rate of infiltration declines and approaches a steady-state value controlled primarily by the soil’s permeability.
Several factors influence infiltration capacity, including soil type, soil structure, compaction, organic content, vegetation cover, antecedent moisture conditions, and land use. Coarse-grained soils such as sands and gravels generally have high infiltration capacities, while fine-grained soils such as clays have much lower rates. Compacted or disturbed soils, which are common in developed areas, significantly reduce infiltration capacity and increase runoff potential.
In stormwater design, infiltration capacity is a key parameter for evaluating and sizing practices such as infiltration basins, trenches, and permeable pavements. It is often determined through field testing, such as double-ring infiltrometer tests, or estimated from soil classifications.
Understanding infiltration capacity is essential for predicting hydrologic response, managing runoff, and designing systems that promote groundwater recharge and reduce surface water impacts.