Infiltration Overview

Infiltration is the absorption of water by the soil. Water enters the soil through pores, as well as clearly defined cracks in the surface.

The infiltration rate is usually high at the start of a storm, the initial infiltration capacity; and decreases with the passage of time to a fairly constant value, the final infiltration capacity.

The rate at which the infiltration rate decreases, also known as the rate of decay, is a function of the volume of water that has been absorbed.

The infiltration rate is calculated using an implicit form of Horton's equation as proposed by Watson (1981).

The adjusted equation only takes infiltration into account when there is rainfall, as illustrated in the figure below. This results in a more accurate simulation in low precipitation runoff events.

Antecedent Moisture Condition

The infiltration rate is dependant on the moisture content in the soil. There are four categories.

Category	Condition	Total Rainfall During Five Days Preceding Storm Event
1	Completely Dry	0
2	Dry	0.1mm - 12.5mm
3	Rather Wet	12.5mm - 25.0mm
4	Saturated	Over 25.0mm

Soil Type

There are four basic soil types. These are classified according to the following parameters.

Soil Type	Infiltration Rate	Description
A	High	Permeability is rapid. Overall drainage is excessive to well-drained. Typically coarse textured soils i.e. sands and gravels.
B	Moderate	Permeability is slightly restricted. Effective soil depth and drainage. Moderately fine to moderately coarse textured soils.
C	Slow	Rate of infiltration deteriorates rapidly. Permeability is restricted. Soil depth tends to be shallow. Moderately fine to fine textured soils with that impede infiltration.
D	Very Slow	Severely restricted permeability. Very shallow soils. High shrink-swell potential. Typically clay soils with permanently high water tables.

In addition to the four main groups listed, three intermediate soil groups are supported. These intermediate groups are used for soils that have characteristics that place them between the main groups and are called types A/B, B/C and C/D.