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# Infiltration

## Description

Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes saturated. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier. It is related to the saturated hydraulic conductivity of the near-surface soil. The rate of infiltration can be measured using an infiltrometer.

Infiltration calculation methods

Infiltration is a component of the general mass balance hydrologic budget. There are several ways to estimate the volume and/or the rate of infiltration of water into a soil. The rigorous standard that fully couples groundwater to surface water through a non-homogeneous soil is the numerical solution of Richards’ equation. A newer method that allows full groundwater and surface water coupling in homogeneous soil layers, and that is related to the Richards equation is the Finite water-content vadose zone flow method. In the case of uniform initial soil water content and a deep well-drained soil, there are some excellent approximate methods to solve for the infiltration flux for a single rainfall event. Among these are the Green and Ampt (1911) method, Parlange et al. (1982). Beyond these methods there are a host of empirical methods such as, SCS method, Horton’s method, etc., that are little more than curve fitting exercises.

General hydrologic budget

The general hydrologic budget, with all the components, with respect to infiltration F. Given all the other variables and infiltration is the only unknown, simple algebra solves the infiltration question.

Related formulas

## Variables

 F infiltration, which can be measured as a volume or length (dimensionless) BI boundary input, which is essentially the output watershed from adjacent, directly connected impervious areas (dimensionless) P precipitation (dimensionless) E evaporation (dimensionless) T transpiration (dimensionless) ET evapotranspiration (dimensionless) S storage through either retention or detention areas (dimensionless) IA initial abstraction, which is the short term surface storage such as puddles or even possibly detention ponds depending on size (dimensionless) R surface runoff (dimensionless) BO boundary output, which is also related to surface runoff, R, depending on where one chooses to define the exit point or points for the boundary output (dimensionless)