Basic Concepts and Equations

The layered Green and Ampt loss method expands upon the previously mentioned Green and Ampt method through the use of two soil layers to account for continuous changes in moisture content.  The method is based on algorithms originally developed for the Guelph All-Weather Sequential-Events Runoff (GAWSER) model (Schroeter and Associates, 1996).

Using the layered Green and Ampt method allows for continuous simulation when used in combination with a canopy method that will extract water from the soil in response to potential ET computed in the meteorologic model.  Between precipitation events, the soil layer will lose moisture as the canopy extracts infiltrated water.  Unless a canopy method is selected, no soil water extraction will occur.  This method may also be used in combination with a surface method that will hold water on the land surface.  The water in surface storage can infiltrate into the soil layer and/or be removed through ET.  The infiltration rate is determined by the capacity of the soil layer to accept water.  When both a canopy and surface method are used in combination with the layered Green and Ampt method, the system can be conceptualized as shown in the following figure.

Conceptual Representation of the Layered Green and Ampt Method

The layered Green Ampt loss method uses two layers to represent the dynamics of water movement in the soil.  Surface water infiltrates into the upper layer, called layer 1.  Layer 1 produces seepage to the lower layer, called layer 2.  Both layers are functionally identical but may have separate and distinct parameters.  Separate parameters can be used to represent layered soil profiles and also allows for better representation of stratified soil drying between storms.  Each layer is described using a bulk depth and water content values for saturation, field capacity, and wilting point.  Soil water in layer 2 can percolate out of the soil profile.  The layered Green and Ampt method is intended to be used in combination with the linear reservoir baseflow method.  When used in this manner, the percolated water can be split between baseflow and deep aquifer recharge.

First, precipitation fills the canopy storage.  Precipitation that exceeds the canopy storage will overflow onto the land surface.  The new precipitation is added to any water already in surface storage.  The infiltration rate from the surface into layer 1 is calculated with the Green and Ampt equation so long as layer 1 is below saturation.  The infiltration rate changes to the current seepage rate when layer 1 reaches saturation.  Infiltration water is added to the storage in layer 1.  Seepage out of layer 1 only occurs when the storage exceeds field capacity.  Maximum seepage occurs when layer 1 is at saturation and declines to zero at field capacity.  The seepage rate changes to the percolation rate when layer 2 is saturated.  Seepage is added to the storage in layer 2.  Percolation out of layer 2 only occurs when the storage exceeds field capacity.  Maximum percolation occurs when layer 2 is at saturation and declines to zero at field capacity.  Most soils observe decreasing hydraulic conductivity rates at greater depths below the surface.  This means that typically the seepage rate is reduced to the percolation rate when layer 2 saturates, and the infiltration rate is reduced to the seepage rate when layer 1 saturates.  The infiltration rate will change to the percolation rate if both layers 1 and 2 are saturated.  Both convergence control and adaptive time stepping are used to accurately resolve the saturation of each layer.

The canopy extracts water from soil storage to meet the potential ET demand.  First, soil water is extracted from layer 1 at the full ET rate.  This extraction from layer 1 continues until half of the available water has been taken to meet the ET demand.  The available water is defined as the saturation content minus the wilting point content, multiplied by the bulk layer thickness.  Second, soil water is extracted from layer 2 at the full ET rate.  This extraction from layer 2 also continues until half of the available water has been taken.  Third, the ET demand is applied equally to both layers until one of them reaches wilting point content.  Finally, the ET demand is applied to the remaining layer until it also reaches wilting point content.  Soil water below the wilting point content is never used for ET.

Since this method allows for the extraction of infiltrated water, this method can be used for both event and continuous simulations.

The layered Green and Ampt loss method can be used for continuous simulation because it is built on a water balance of the two layers.  Infiltrated water is added to the layers and percolated water is removed from the layers.  Potential ET demand also removes water from the layers.  The continuous simulation will include storm events from time to time.  The Green and Ampt equation is used to compute the surface infiltration during each of these storm events.  The initial condition of the Green and Ampt equation for each of these storms must be determined.  The initial content is essentially a water content deficit which can be calculated as the saturated water content minus the current water content.  The water content deficit is automatically calculated at the beginning of each storm based on current soil water storage in layer 1.  The user can control the amount of time that must pass since the last precipitation in order for the initial condition to be recalculated, using the dry duration parameter.  When the time since last precipitation is less than the dry duration, the precipitation is considered a continuation of the last storm event.

Required Parameters

Parameters that are required to utilize the layered Green and Ampt method include those which were previously mentioned for the “standard” Green and Ampt method in addition to: Layer 1 and 2 thicknesses [in or mm], field capacity [in/in or mm/mm], wilting point [in/in or mm/mm], layer 1 maximum seepage rate [in/hr or mm/hr], layer 2 maximum percolation rate [in/hr or mm/hr], and dry duration [days].

The layer 1 and 2 thicknesses define the bulk depth of soil and are typically estimated using soil maps.

The field capacity content specifies the point where the soil naturally stops seeping under gravity while the wilting point content specifies the amount of water remaining in the soil when plants are no longer capable of transpiring infiltrated water.  Both parameters are typically estimated using predominant soil texture and literature values.

The dry duration sets the amount of time that must pass after a storm event in order to recalculate the initial condition.  An initial estimate of 12 hours has been found to be reasonable for the dry duration.  However, this parameter should be calibrated using observed data.

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.