Infiltration Rate: Field Measurement of Soil Intake
Infiltration rate describes the speed at which water enters the soil surface under specific boundary conditions. Expressed as millimetres per hour or centimetres per hour, it captures the combined effects of soil texture, structure, antecedent moisture, and surface condition. Engineers and agronomists use infiltration rate measurements to size stormwater controls, schedule irrigation, and assess land degradation. This article defines infiltration rate, reviews its historical measurement methods, elaborates key concepts, and surveys field applications.
Read this article in tandem with the sorptivity guide to interpret the early-time behaviour of infiltration tests, and consult the rain garden sizing calculator when translating measured rates into green infrastructure dimensions.
Definition and Governing Concepts
Infiltration rate versus capacity
Instantaneous infiltration rate f(t) measures how quickly water is entering at time t, while infiltration capacity fc represents the maximum rate the soil can sustain under given conditions. When rainfall intensity exceeds fc, ponding and runoff occur. Field tests typically report either steady-state infiltration rate (approach to fc) or a curve of f(t) over time, often modelled with Horton or Philip equations.
Measurement techniques
Double-ring infiltrometers isolate a central test area to minimise lateral flow, maintaining constant head while the outer ring buffers edge effects. Single-ring and tension infiltrometers, rainfall simulators, and infiltration basins offer alternative methods depending on scale. Measurements must account for initial moisture content, which strongly influences early infiltration rates.
Units and conversion
Infiltration rate typically uses millimetres per hour in hydrology, but irrigation practitioners may prefer inches per hour. Conversion to metres per second allows direct comparison with saturated hydraulic conductivity (m·s-1). Consistent units ensure compatibility with design rainfall intensities and infiltration basin sizing formulas.
Historical Development
Early agronomic experiments
Late nineteenth-century agronomists such as E.W. Hilgard examined infiltration to improve irrigation efficiency on western US farms. Their manual ring tests established relationships between soil texture and infiltration, revealing the impacts of crusting and compaction.
Horton’s infiltration equation
Robert E. Horton formalised infiltration capacity in the 1930s, proposing an exponential decay function describing how f(t) decreases from an initial value f0 to a constant minimum fc. Horton’s work linked infiltration measurement to urban hydrology, influencing stormwater design standards across North America.
Modern instrumentation
Contemporary practice uses automated infiltrometers with dataloggers, rainfall simulators controlled by peristaltic pumps, and geospatial mapping via satellite soil moisture. Data assimilation integrates infiltration rate observations into distributed hydrologic models.
Applications and Importance
Stormwater management
Designing infiltration basins, bioretention cells, or permeable pavements requires field-verified infiltration rates to ensure regulatory compliance. Pair rate measurements with the stormwater runoff volume calculator to balance inflow and storage capacity.
Irrigation scheduling
Farmers adjust application rates and cycle-soak strategies according to infiltration rate to prevent runoff and soil erosion. Combining rate data with the irrigation run time calculator helps convert infiltration measurements into practical schedules.
Land restoration and erosion control
Rehabilitating degraded rangelands or wildfire scars often targets improved infiltration to reduce overland flow. Soil amendments, mulching, and vegetation establishment are evaluated by repeating infiltration rate tests over time, providing quantitative evidence of recovery.