Green and Ampt Loss Model

Basic Concepts and Equations

The Green and Ampt infiltration model included in the program is a conceptual model of infiltration of precipitation in a watershed. The Green and Ampt loss method was originally derived using a simplification of the comprehensive Richard's equation (1931) for unsteady water flow in soil.  Within this method, infiltration proceeds with so-called piston displacement, as shown in the figure below.

According to EM 1110-2-1417

…the transport of infiltrated rainfall through the soil profile and the infiltration capacity of the soil is governed by Richards' equation…[which is] derived by combining an unsaturated flow form of Darcy's law with the requirements of mass conservation.

EM 1110-2-1417 describes in detail how the Green and Ampt model combines and solves these equations. In summary, the model computes the precipitation loss on the pervious area in a time interval as:

in which ft = loss during period t, K = saturated hydraulic conductivity, ($//<![CDATA[ \begin{array}{l}\phi\end{array} //]]>$ - $//<![CDATA[ \begin{array}{l}\theta_i\end{array} //]]>$ ) = volume moisture deficit, Sf = wetting front suction, and Ft = cumulative loss at time t. The precipitation excess on the pervious area is the difference in the MAP during the period and the loss computed with the equation shown above. As implemented, the Green and Ampt model also includes an initial abstraction. This initial condition represents interception in the canopy or surface depressions not otherwise included in the model. This interception is separate from the time to ponding that is an integral part of the model. The solution method used follows that of Li et al. (1976).

Since no means for extracting infiltrated water is included, this method should only be used for event simulation.

Required Parameters

Parameters that are required to utilize this method within HEC-HMS include the initial moisture content or deficit [in/in or mm/mm], wetting front suction head [in or mm], saturated hydraulic conductivity [in/hr or mm/hr], and directly connected impervious area [percent].

A tutorial describing an example application of this loss method, including parameter estimation and calibration, can be found here: Applying the Green and Ampt Loss Method.

A tutorial describing how gSSURGO data can be formatted for use within HEC-HMS can be found here: Formatting gSSURGO Data for Use within HEC-HMS.

The initial moisture content or deficit defines the starting saturation of the soil layer at the start of the simulation. This parameter is a function of the watershed moisture at the beginning of the simulation. It may be estimated in the same manner as the initial abstraction for other loss models.

The wetting front suction head describes the movement of water downwards through the soil column. 

The saturated hydraulic conductivity defines the minimum rate at which precipitation will be infiltrated into the soil layer after the soil column is fully saturated.

Finally, the percentage of the subbasin which is directly connected impervious area can be specified.  Directly connected impervious areas are surfaces where runoff is conveyed directly to a waterway or stormwater collection system.  These surfaces differ from disconnected impervious areas where runoff encounters permeable areas which may infiltrate some (or all) of the runoff prior to reaching a waterway or stormwater collection system.  No loss calculations are carried out on the specified percentage of the subbasin; all precipitation that falls on that portion of the subbasin becomes excess precipitation and subject to direct runoff.

A Note on Parameter Estimation

The values presented here are meant as initial estimates.  This is the same for all sources of similar data including Engineer Manual 1110-2-1417 Flood-Runoff Analysis and the Introduction to Loss Rate Tutorials.  Regardless of the source, these initial estimates must be calibrated and validated.