Welcome to the U.S. EPA Infiltration Model Web Site

Infiltration
Models:

SCS Model:
Description
MathCad Code

Philip's
Two-Term
Model:
Description
MathCad Code

Layered
Green Ampt
Model:
Description
MathCad Code

Explicit
Green Amp
Model:
Description
MathCad Code

Constant
Flux Green
Ampt Model:
Description
MathCad Code

Infiltration-
Exfiltration
Model:
Description
MathCad Code

Descriptions
of All
Infiltration
Models:

Descriptions

You will need the free Adobe Reader to view some of the files on this page. See EPA's PDF page to learn more about PDF, and for a link to the free Acrobat Reader.

Infiltration models presented on this web site were developed using Mathcad Plus 6.0 (Mathcad is a registered trademark of Mathsoft, Inc., of Cambridge, Massachusetts. Mention and use of this package, or any other commercial product does not constitute endorsement or recommendation by the U.S. Environmental Protection Agency.) worksheets to facilitate their ease of use. The worksheets have been tested and found to be compatible with MathCad 7.0. To utilize the models, you will need to either have your browser recognize and launch the MathCad application, or save the worksheet to a local storage media by right clicking on the link and selecting "Save Link As" or "Save Target As", and open at a later time. The models are accessible through the side panel options to the left.

Note: A description of each model is accessible through the link provided under each model located in the side panel. These descriptions are in HTML format. A description of all of the infiltration models included in this site is located at the end of the side panel.

When using the worksheets, parameter values can be easily changed, and the resulting output can be quickly observed to evaluate any resulting changes. By utilizing the tiling feature available within MathCad, the same worksheet can be loaded several times to allow comparisons of results for various model scenarios. Any changes made and saved back to the same file name will result in the original information being lost. Therefore, it is recommended that a copy of the original files be archived for future use.

Phenomena of Water Infiltration in the Unsaturated Zone

Water applied to the soil surface through rainfall and irrigation events subsequently enters the soil through the process of infiltration. If the supply rate of water to the soil surface is greater than the soil's ability to allow the water to enter, excess water will either accumulate on the soil's surface or become runoff. Infiltrability is a term generally used in the disciplines of soil physics and hydrology to define the maximum rate at which rain or irrigation water can be absorbed by a soil under a given condition. Indirectly, infiltrability determines how much of the water will flow over the ground surface (i.e., runoff or overland flow), terminating in lakes, streams or rivers, and how much will enter the soil. This term can be utilized in the estimation of water available for downward percolation through drainage, runoff, or returned to the atmosphere by the process of evapotranspiration.

Figure 1. Zone of the infiltration process for the
water content profile under ponded conditions.

The distribution of water during the infiltration process under ponded conditions is illustrated in Figure 1. In this idealized profile for soil water distribution for a homogeneous soil, five zones are illustrated for the infiltration process.

Saturated zone:

The pore space in this zone is filled with water, or saturated. Depending on the length of time elapsed from the initial application of the water, this zone will generally extend only to a depth of a few millimeters.

Transition zone:

This zone is characterized by a rapid decrease in water content with depth, and will extend approximately a few centimeters.

Transmission zone:

This zone is characterized by a small change in water content with depth. In general, the transmission zone is a lengthening unsaturated zone with uniform water content. The hydraulic gradient in this zone is primarily driven by gravitational forces.

Wetting zone:

In this zone, the water content sharply decreases with depth from the water content of the transmission zone to near the initial water content of the soil.

Wetting front:

This zone is characterized by a steep hydraulic gradient and forms a sharp boundary between the wet and dry soil. The hydraulic gradient is characterized primarily by metric potentials.

Beyond the wetting front, there is no visible penetration of water. Comprehensive reviews of the principles governing the infiltration process have been published by Philip (1969) and Hillel (1982).

Figure2. Infiltration rate will generally be high
in the first stages, and will decrease with time.

Soil water infiltration is controlled by the rate and duration of water application, soil physical properties, slope, vegetation, and surface roughness. Generally, whenever water is ponded over the soil surface, the rate of infiltration exceeds the soil infiltrability. On the other hand, if water is applied slowly, the infiltration rate may be smaller than the soil infiltrability, and the supply rate becomes a determining factor for the infiltration rate. This type of infiltration process is termed supply controlled (Hillel, 1982). However, once the infiltration rate exceeds the soil infiltrability it is the latter which determines the actual infiltration rate, and thus the process becomes profile controlled. Generally, soil water infiltration has a high rate in the beginning, decreasing rapidly, and then slowly decreasing until it approaches a constant rate. As shown in Figure 2, the infiltration rate will eventually become steady and approach the value of the saturated hydraulic conductivity.

As might be expected, the slope of the land can also indirectly impact the infiltration rate. A steep slope will result in runoff, which will impact the amount of time the water will be available for infiltration. In contrast, gentle slopes will have less of an impact on the infiltration process due to decreased runoff. When compared to the bare soil surface, vegetation cover tends to increase infiltration by retarding surface flow, allowing time for water infiltration. Plant roots may also increase infiltration by increasing the hydraulic conductivity of the soil surface through the creation of additional pore space. Due to these impacts, infiltration may vary widely under different types of vegetation.

This page is has been developed by Joe R. Williams of the U.S.EPA's Subsurface Protection and Remediation Division of the National Risk Managment Research Laboratory in Ada, Oklahoma.