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Infiltration is the process by which water on the ground surface enters the
soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
. It is commonly used in both
hydrology Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
and
soil science Soil science is the study of soil as a natural resource on the surface of the Earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to th ...
s. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meters per day but can also be measured in other units of distance over time if necessary.  The infiltration capacity decreases as the soil moisture content of soils surface layers increases. If the
precipitation rate In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...
exceeds the infiltration rate,
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock marke ...
will usually occur unless there is some physical barrier.
Infiltrometer An infiltrometer is a device used to measure the rate of water infiltration into soil or other porous media. Commonly used infiltrometers are single-ring and double-ring infiltrometers, and also disc permeameters. Single ring A single-ring infil ...
s, permeameters and rainfall simulators are all devices that can be used to measure infiltration rates. Infiltration is caused by multiple factors including; gravity, capillary forces, adsorption and osmosis. Many soil characteristics can also play a role in determining the rate at which infiltration occurs.


Factors that affect infiltration


Precipitation

Precipitation can impact infiltration in many ways. The amount, type and duration of precipitation all have an impact. Rainfall leads to faster infiltration rates than any other precipitation events, such as snow or sleet. In terms of amount, the more precipitation that occurs, the more infiltration will occur until the ground reaches saturation, at which point the infiltration capacity is reached. Duration of rainfall impacts the infiltration capacity as well. Initially when the precipitation event first starts the infiltration is occurring rapidly as the soil is unsaturated, but as time continues the infiltration rate slows as the soil becomes more saturated. This relationship between rainfall and infiltration capacity also determines how much
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock marke ...
will occur. If rainfall occurs at a rate faster than the infiltration capacity runoff will occur.


Soil characteristics

The
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
of soils is critical in determining the infiltration capacity. Soils that have smaller pore sizes, such as clay, have lower infiltration capacity and slower infiltration rates than soils that have large pore size, such as sands. One exception to this rule is when clay is present in dry conditions. In this case, the soil can develop large cracks which leads to higher infiltration capacity. Soil compaction also impacts infiltration capacity. Compaction of soils results in decreased porosity within the soils, which decreases infiltration capacity. Hydrophobic soils can develop after wildfires have happened, which can greatly diminish or completely prevent infiltration from occurring.


Soil moisture content

Soil that is already saturated has no more capacity to hold more water, therefore infiltration capacity has been reached and the rate cannot increase past this point. This leads to much more surface runoff. When soil is partially saturated then infiltration can occur at a moderate rate and fully unsaturated soils have the highest infiltration capacity.


Organic materials in soils

Organic materials in the soil (including plants and animals) all increase the infiltration capacity. Vegetation contains roots that extend into the soil which create cracks and fissures in the soil, allowing for more rapid infiltration and increased capacity. Vegetation can also reduce surface compaction of the soil which again allows for increased infiltration. When no vegetation is present infiltration rates can be very low, which can lead to excessive runoff and increased
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
levels. Similarly to vegetation, animals that burrow in the soil also create cracks in the soil structure.


Land cover

If land is covered by impermeable surfaces, such as pavement, infiltration cannot occur as the water cannot infiltrate through an impermeable surface This relationship also leads to increased runoff. Areas that are impermeable often have storm drains which drain directly into water bodies, which means no infiltration occurs. Vegetative cover of the land also impacts the infiltration capacity. Vegetative cover can lead to more interception of precipitation, which can decrease intensity leading to less runoff, and more interception. Increased abundance of vegetation also leads to higher levels of
evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
which can decrease the amount of infiltration rate.  Debris from vegetation such as leaf cover can also increase infiltration rate by protecting the soils from intense precipitation events. In semi-arid savannas and grasslands, the infiltration rate of a particular soil depends on the percentage of the ground covered by litter, and the basal cover of perennial grass tufts. On sandy loam soils the infiltration rate under a litter cover can be nine times higher than on bare surfaces. The low rate of infiltration on bare areas is due mostly to the presence of a soil crust or surface seal. Infiltration through the base of a tuft is rapid and the tufts funnel water towards their own roots.


Slope

When the slope of land is higher runoff occurs more readily which leads to lower infiltration rates.


Process

The process of infiltration can continue only if there is room available for additional water at the soil surface. The available volume for additional water in the soil depends on the porosity of the soil and the rate at which previously infiltrated water can move away from the surface through the soil. The maximum rate that water can enter a soil in a given condition is the infiltration capacity. If the arrival of the water at the soil surface is less than the infiltration capacity, it is sometimes analyzed using
hydrology transport model An hydrological transport model is a mathematical model used to simulate the flow of rivers, streams, groundwater movement or drainage front displacement, and calculate water quality parameters. These models generally came into use in the 1960s a ...
s,
mathematical model A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used in the natural sciences (such as physics, ...
s that consider infiltration, runoff and channel flow to predict river flow rates and stream
water quality Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
.


Research findings

Robert E. Horton Robert Elmer Horton (May 18, 1875 – April 22, 1945) was an American hydrologist, geomorphologist, civil engineer, and soil scientist, considered by many to be the father of modern American hydrology. An eponymous medal is awarded by the Ameri ...
suggested that infiltration capacity rapidly declines during the early part of a storm and then tends towards an approximately constant value after a couple of hours for the remainder of the event. Previously infiltrated water fills the available storage spaces and reduces the capillary forces drawing water into the pores.
Clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
particles in the soil may swell as they become wet and thereby reduce the size of the pores. In areas where the ground is not protected by a layer of forest litter, raindrops can detach soil particles from the surface and wash fine particles into surface pores where they can impede the infiltration process.


Infiltration in wastewater collection

Wastewater Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industr ...
collection systems consist of a set of lines, junctions and lift stations to convey
sewage Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...
to a
wastewater treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environme ...
plant. When these lines are compromised by rupture, cracking or tree root invasion,
infiltration/inflow Infiltration/Inflow (I/I) causes dilution in sanitary sewers. Dilution of sewage decreases the efficiency of treatment, and may cause sewage volumes to exceed design capacity. Although inflow is technically different from infiltration, it may be d ...
of stormwater often occurs. This circumstance can lead to a
sanitary sewer overflow Sanitary sewer overflow (SSO) is a condition in which untreated sewage is discharged from a sanitary sewer into the environment prior to reaching sewage treatment facilities. When caused by rainfall it is also known as wet weather overflow. Cause ...
, or discharge of untreated sewage to the environment.


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 1-D 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 The finite water-content vadose zone flux methodTalbot, C.A., and F. L. Ogden (2008), A method for computing infiltration and redistribution in a discretized moisture content domain, ''Water Resour. Res.'', 44(8), doi: 10.1029/2008WR006815.Ogden, F. ...
solution of the Soil Moisture Velocity Equation. 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. :F=B_I+P-E-T-ET-S-I_A-R-B_O where :''F'' is infiltration, which can be measured as a volume or length; :B_I is the boundary input, which is essentially the output watershed from adjacent, directly connected impervious areas; :B_O is the 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; :''P'' is
precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
; :''E'' is
evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
; :''T'' is
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
; :''ET'' is
evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
; :''S'' is the storage through either retention or detention areas; :I_A is the initial abstraction, which is the short term surface storage such as puddles or even possibly detention ponds depending on size; :''R'' is surface runoff. The only note on this method is one must be wise about which variables to use and which to omit, for doubles can easily be encountered. An easy example of double counting variables is when the evaporation, ''E'', and the transpiration, ''T'', are placed in the equation as well as the evapotranspiration, ''ET''. ''ET'' has included in it ''T'' as well as a portion of ''E''. Interception also needs to be accounted for, not just raw precipitation.


Richards' equation (1931)

The standard rigorous approach for calculating infiltration into soils is Richards' equation, which is a
partial differential equation In mathematics, a partial differential equation (PDE) is an equation which imposes relations between the various partial derivatives of a Multivariable calculus, multivariable function. The function is often thought of as an "unknown" to be sol ...
with very nonlinear coefficients. The Richards equation is computationally expensive, not guaranteed to converge, and sometimes has difficulty with mass conservation.


Finite water-content vadose zone flow method

This method is an approximation of the Richards' (1931) partial differential equation that de-emphasizes soil water diffusion. This was established by comparing the solution of the advection-like term of the Soil Moisture Velocity Equation and comparing against exact analytical solutions of infiltration using special forms of the soil constitutive relations. Results showed that this approximation does not affect the calculated infiltration flux because the diffusive flux is small and that the
finite water-content vadose zone flow method The finite water-content vadose zone flux methodTalbot, C.A., and F. L. Ogden (2008), A method for computing infiltration and redistribution in a discretized moisture content domain, ''Water Resour. Res.'', 44(8), doi: 10.1029/2008WR006815.Ogden, F. ...
is a valid solution of the equation is a set of three ordinary differential equations, is guaranteed to converge and to conserve mass. It requires the assumptions that the flow occurs in the vertical direction only (1-dimensional), and that that soil is uniform within layers.


Green and Ampt

The named was derived from two men; Green and Ampt. The Green-Ampt''Water Resources Engineering'', 2005 Edition, John Wiley & Sons, Inc. method of infiltration estimation accounts for many variables that other methods, such as Darcy's law, do not. It is a function of the soil suction head, porosity, hydraulic conductivity and time. :\int_0^ \, dF = \int_0^t K\,dt where
: is wetting front soil suction head (L); :\theta is
water content Water content or moisture content is the quantity of water contained in a material, such as soil (called soil moisture), rock, ceramics, crops, or wood. Water content is used in a wide range of scientific and technical areas, and is expressed as ...
(-); :K is
hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on ...
(L/T); :F(t) is the cumulative depth of infiltration (L). Once integrated, one can easily choose to solve for either volume of infiltration or instantaneous infiltration rate: :F(t)=Kt+\psi \, \Delta\theta \ln \left +\right Using this model one can find the volume easily by solving for F(t). However the variable being solved for is in the equation itself so when solving for this one must set the variable in question to converge on zero, or another appropriate constant. A good first guess for F is the larger value of either Kt or \sqrt . These values can be obtained by solving the model with log replaced with its Taylor-Expansion around one, of the zeroth and second order respectively. The only note on using this formula is that one must assume that h_0, the water head or the depth of ponded water above the surface, is negligible. Using the infiltration volume from this equation one may then substitute F into the corresponding infiltration rate equation below to find the instantaneous infiltration rate at the time, t, F was measured. :f(t)=K\left 1\right


Horton's equation

Named after the same
Robert E. Horton Robert Elmer Horton (May 18, 1875 – April 22, 1945) was an American hydrologist, geomorphologist, civil engineer, and soil scientist, considered by many to be the father of modern American hydrology. An eponymous medal is awarded by the Ameri ...
mentioned above, Horton's equation is another viable option when measuring ground infiltration rates or volumes. It is an empirical formula that says that infiltration starts at a constant rate, f_0, and is decreasing exponentially with time, t. After some time when the soil saturation level reaches a certain value, the rate of infiltration will level off to the rate f_c. :f_t=f_c+(f_0 - f_c)e^ Where :f_t is the infiltration rate at time ''t''; :f_0 is the initial infiltration rate or maximum infiltration rate; :f_c is the constant or equilibrium infiltration rate after the soil has been saturated or minimum infiltration rate; :k is the decay constant specific to the soil. The other method of using Horton's equation is as below. It can be used to find the total volume of infiltration, ''F'', after time ''t''. :F_t = f_ct+(1-e^)


Kostiakov equation

Named after its founder Kostiakov is an empirical equation which assumes that the intake rate declines over time according to a power function. :f(t) = akt^\! Where a and k are empirical parameters. The major limitation of this expression is its reliance on the zero final intake rate. In most cases the infiltration rate instead approaches a finite steady value, which in some cases may occur after short periods of time. The Kostiakov-Lewis variant, also known as the "Modified Kostiakov" equation corrects for this by adding a steady intake term to the original equation. :f(t) = akt^+f_0\! in integrated form the cumulative volume is expressed as: :F(t) = kt^+f_0t\! Where :f_0 approximates, but does not necessarily equate to the final infiltration rate of the soil.


Darcy's law

This method used for infiltration is using a simplified version of
Darcy's law Darcy's law is an equation that describes the flow of a fluid through a porous medium. The law was formulated by Henry Darcy based on results of experiments on the flow of water through beds of sand, forming the basis of hydrogeology, a branch of ...
. Many would argue that this method is too simple and should not be used. Compare it with the Green and Ampt (1911) solution mentioned previously. This method is similar to Green and Ampt, but missing the cumulative infiltration depth and is therefore incomplete because it assumes that the infiltration gradient occurs over some arbitrary length L. In this model the ponded water is assumed to be equal to h_0 and the head of dry soil that exists below the depth of the wetting front soil suction head is assumed to be equal to -\psi -L. :f=K\left
right Rights are law, legal, social, or ethics, ethical principles of Liberty, freedom or entitlement; that is, rights are the fundamental normative rules about what is allowed of people or owed to people according to some legal system, social convent ...
/math> where : is wetting front soil suction head :h_0 is the depth of ponded water above the ground surface; :K is the
hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on ...
; :L is the vague total depth of subsurface ground in question. This vague definition explains why this method should be avoided. or :f=K\left
right Rights are law, legal, social, or ethics, ethical principles of Liberty, freedom or entitlement; that is, rights are the fundamental normative rules about what is allowed of people or owed to people according to some legal system, social convent ...
/math>Hendriks, Martin R. (2010) ''Introduction to Physical Hydrology'', Oxford University Press : Infiltration rate f (mm hour−1)) :K is the
hydraulic conductivity Hydraulic conductivity, symbolically represented as (unit: m/s), is a property of porous materials, soils and rocks, that describes the ease with which a fluid (usually water) can move through the pore space, or fractures network. It depends on ...
(mm hour−1)); :L is the vague total depth of subsurface ground in question (mm). This vague definition explains why this method should be avoided. : is wetting front soil suction head () = () (mm) :h_0 is the depth of ponded water above the ground surface (mm);


See also

*
Contour trenching Contour trenching is an agricultural technique that can be easily applied in arid sub-Sahara areas to allow for water, and soil conservation, and to increase agricultural production. Between two trenches crops can benefit during the growing se ...
* Discharge (hydrology) *
Drainage basin A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, t ...
*
Drainage system (agriculture) An agricultural drainage system is a system by which water is drained on or in the soil to enhance agricultural production of crops. It may involve any combination of stormwater control, erosion control, and watertable control. Classification W ...
*
Evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
*
Groundwater recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in ...
* Hydrophobic soil *
Interception (water) Interception refers to precipitation that does not reach the soil, but is instead intercepted by the leaves, branches of plants and the forest floor. It occurs in the canopy (i.e. canopy interception), and in the forest floor or litter layer (i.e. ...
* Natural Resources Conservation Service * Permeability (fluid) *
Runoff curve number The runoff curve number (also called a curve number or simply CN) is an empirical parameter used in hydrology for predicting direct runoff or infiltration from rainfall excess. The curve number method was developed by the USDA Natural Resources ...
*
Storm Water Management Model The United States Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) is a dynamic rainfall–runoff– subsurface runoff simulation model used for single-event to long-term (continuous) simulation of the surface/subsurface h ...
*
Sustainable urban drainage systems Sustainable drainage systems (also known as SuDS,The Experimental Hydrology Wiki Infiltration – Hood Infiltrometer
{{Rivers, streams and springs Hydrology Soil physics Aquifers