Infiltration_(hydrology)

# Infiltration (hydrology) **Infiltration** is the process by which [[Water|water]] on the ground surface enters the soil. Soil can take in water only so fast, and its infiltration capacity falls as it wets: Robert Horton described the decline as f(t) = f_c + (f₀ − f_c)e^{−kt}.[^horton1933] When rain arrives faster than the soil can take it, the excess ponds and runs off — the start of every storm [[Hydrograph|hydrograph]]. The amount and intensity of precipitation, the soil's type and saturation, land cover and slope all control it.[^usgs-infil] <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Infiltration_(hydrology).html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" allow="accelerometer; gyroscope" title="Infiltration (hydrology) — p5.js microsim"></iframe> </div> *Microsim (THY-109): rain on sand, loam and clay; the capacity curve falls as the soil wets, and runoff begins when the rain outruns it. ILLUSTRATIVE: typical Horton parameters per soil.* Videos: [16:9](https://wikitube-3d-microsims.netlify.app/media/Infiltration_(hydrology)_16x9.mp4) · [9:16](https://wikitube-3d-microsims.netlify.app/media/Infiltration_(hydrology)_9x16.mp4) ## Factors that affect infiltration The U.S. Geological Survey lists precipitation, soil type, soil saturation, land cover, slope and evapotranspiration as the main controls.[^usgs-infil] ### Precipitation The greatest factor controlling infiltration is the amount and character of precipitation — its intensity and duration.[^usgs-infil] ### Soil characteristics Clay-rich soils absorb water more slowly than sandy soils, so more of the rain runs off.[^usgs-infil] ### Soil moisture content Saturated soil behaves like a wet sponge: it cannot take more water, so more runs off.[^usgs-infil] ### Land cover Vegetation slows runoff and allows more infiltration; impervious surfaces act as a fast lane for rain into storm drains and streams.[^usgs-infil] Plant cover and organic matter also keep soil open to water, while bare, compacted soil limits infiltration.[^theis-water] ### Slope On steep slopes water runs off faster and has less time to soak in.[^usgs-infil] ## Process Water enters the soil at the surface and moves down behind a wetting front. At first dry soil draws water in rapidly by capillary suction; as the wetted layer deepens, the suction gradient weakens and the rate falls toward a steady value set by the soil's saturated conductivity. Water not held by the soil percolates down toward the water table, recharging [[Groundwater|groundwater]].[^theis-water] ## Research findings Field and laboratory studies show that infiltration rates vary by orders of magnitude between soils and with land management, and that crusting, compaction and frozen ground can cut them sharply. ## Infiltration in wastewater collection In sewer engineering, infiltration also names groundwater that leaks into sewer pipes through cracks and joints, adding flow that treatment plants must handle. ## Infiltration calculation methods Infiltration is estimated from water budgets, from physically based equations for unsaturated flow, or from empirical equations fitted to measurements. ### General hydrologic budget Over a catchment, infiltration can be estimated as precipitation minus runoff, evaporation and changes in surface storage. ### Green and Ampt The Green–Ampt model treats infiltration as a sharp wetting front moving down into the soil, driven by gravity and by the suction at the front. ### Horton's equation Horton's empirical equation gives the capacity as f(t) = f_c + (f₀ − f_c)e^{−kt}, decaying from an initial rate f₀ toward a final rate f_c with a constant k.[^horton1933] ### Darcy's law At the scale of a soil column, the flux is the hydraulic conductivity times the gradient of hydraulic head, which in unsaturated soil includes the capillary suction.[^zeidouni] ## See also - [[Hydrograph]] - [[Groundwater]] - [[Water_cycle]] - [[Porous_medium]] ## References [^horton1933]: Horton, Robert E. (1933). "The role of infiltration in the hydrologic cycle." *Transactions, American Geophysical Union* 14: 446–460. https://doi.org/10.1029/TR014i001p00446 [^usgs-infil]: U.S. Geological Survey, Water Science School. "Infiltration and the Water Cycle." https://www.usgs.gov/water-science-school/science/infiltration-and-water-cycle [^theis-water]: Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation*, ch. 7, module "Water Cycle and Fresh Water Supply." https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation [^zeidouni]: Zeidouni, Mehdi (2025). *Petroleum Reservoir Dynamics*, ch. 2 (Darcy equation), §3.3 (radial flow), ch. 8 (transient radial flow and drawdown). LSU Scholarly Repository. https://open.umn.edu/opentextbooks/textbooks/petroleum-reservoir-dynamics ### Portal Books - Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation); on the [[PORTAL_Energy]] shelf. - Zeidouni, Mehdi (2025). *Petroleum Reservoir Dynamics* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/petroleum-reservoir-dynamics); on the [[PORTAL_Energy]] shelf. ## External links - [Infiltration and the Water Cycle](https://www.usgs.gov/water-science-school/science/infiltration-and-water-cycle), USGS Water Science School  *Connected to the Apex Spine:* Infiltration (hydrology) → [[Hydrology|Hydrology]] — [[WT!Thury_Hydrodynamics_Compendium|Compendium]] section 19, *Hydrology*.  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Infiltration_(hydrology)) : [Wikitube](https://en.wikitube.io/wiki/Infiltration_(hydrology)) · pinned revision [1318947043](https://en.wikipedia.org/w/index.php?oldid=1318947043) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Stock_and_flow]]. --- *Thury station wave · 2026-09-10 · drafted · microsim THY-109 (p5.js) · parent [[Hydrology]].*