Infiltration Parameters

It is important to parameterize infiltration in Vflo™, as runoff is generated only once rainfall rates exceed infiltration rates. Vflo™ utilizes the Green and Ampt infiltration routine to model infiltration as piston flow, or the progression of a wetting front through a specified soil depth. Green and Ampt and other parameters related to infiltration are accessed in Vflo™ through the Infiltration panel. Parameters featured in the Infiltration panel include: saturated hydraulic conductivity (called just “hydraulic conductivity” in Vflo™), wetting front suction (called “wetting front” in Vflo™), effective porosity, soil depth, initial saturation, abstraction, and impervious area (called “impervious” in Vflo™). By including these parameters, the Vflo™ infiltration routine accounts for variable infiltration rates affected by soil properties and soil depth, antecedent conditions, urbanization, and other factors.

Two potential runoff processes exist in Vflo™: infiltration excess and saturation excess. Modeling both saturation excess and infiltration excess serves a wide variety of applications. Both may operate simultaneously in any given grid cell and spatially across the watershed. Before saturation, infiltration excess dominates. Once the available porosity in the soil profile is filled, then saturation excess, or runoff, begins. The Green and Ampt equation models infiltration as an abrupt wetting front, rather than a range of saturation. A single-layer soil model is used.

If gridded soil data is available for the modeled area, then infiltration parameter maps should be developed using GIS software, then imported to Vflo™ as ESRI ASCII grids (see instructions here). Green and Ampt parameters can be estimated from soil properties such as bulk density and percentages of sand, silt, and clay. If, however, only hydraulic conductivity is known or can be estimated, then the remaining infiltration parameters may be left blank, and Vflo™ will calculate infiltration based on hydraulic conductivity alone, treating it as a constant rate. Infiltration parameters may be entered and edited at any time using the Infiltration panel.

Hydraulic conductivity

The hydraulic conductivity parameter is actually saturated hydraulic conductivity, represented as Ks in the Green and Ampt infiltration routine. Saturated hydraulic conductivity may especially control the infiltration process when rainfall occurs over already saturated soil. As mentioned previously, hydraulic conductivity may be used alone as a constant rate to model infiltration, if no other infiltration parameters can be estimated. Hydraulic conductivity should be specified for a single-layer soil profile and entered in units of centimeters/hour (metric) or inches/hour (US customary).

Wetting front

The wetting front parameter is actually the wetting front suction head, or average capillary potential, of the Green and Ampt infiltration routine. The wetting front parameter can be especially important in calculating infiltration under unsaturated conditions. In the Green and Ampt routine, the wetting front value is independent of soil moisture at any particular time. Wetting front should be entered for a single-layer profile in units of centimeters (metric) or inches (US customary)

Effective porosity

In the Green and Ampt routine, effective porosity is the difference between total porosity and residual soil moisture content. Effective porosity is considered a soil property and is independent of soil moisture at any particular time. Effective porosity should be entered as a unit-less decimal fraction, with complete porosity being a value of one, and zero porosity being a value of zero.

Soil depth

Soil depth demonstrates the depth to which infiltration may occur in the soil. Assessment of the soil depth is dependent upon the limiting factor in the infiltration process. If the wetting front is obstructed by a perched water table, then the depth to the water table is the limiting depth. If the soil profile is limited by an impermeable layer, then the depth to that layer is the limiting depth.

Soil maps should be used to estimate the soil depth. It is important to ensure that the soil depth read from a soil map is actually the limiting layer. Often, sources will report the depth to which they have probed or depth to rock, whichever is less, as the maximum soil depth; however, this depth is not necessarily the limiting layer and permeable soil may continue below this depth.

If soil depth is not known, then a reasonable value may be selected based on the geologic material, slope, and elevation of the area. Subsequently, assumed soil depth can be modified through the calibration of simulations to observed streamflow (see the Calibration page).

Soil depth should be entered in units of centimeters (metric) or inches (US customary).

Initial saturation

The Vflo™ model is sensitive to antecedent moisture conditions through the initial saturation parameter. Initial saturation values represent the degree, or percentage of initial saturation preceding a storm event, and should be entered as a unit-less decimal fraction with complete saturation being a value of one, and no initial saturation being a value of zero.

Abstraction

Abstraction is equivalent to the amount of rainfall that must occur before any runoff may be generated for a cell. Like initial saturation, abstraction is configured on an event basis, considered as a single accumulated value at the beginning of each solve period. As such, abstraction is not used as a parameter for Continuous Vflo™ simulations. Abstraction should be entered in units of centimeters (metric) or inches (US customary), with a mimimum value of zero.

Impervious

The impervious parameter represents the percentage of a cell area that is impervious to infiltration. The impervious property is especially important in modeling urban basins. The impervious fraction in a cell overrides the infiltration routine as follows:

  1. If rainfall rate is less than infiltration rate, then runoff is the product of rainfall rate and the impervious fraction.
  2. If rainfall rate is greater than infiltration rate, then runoff is the difference between the rainfall and the infiltration rate, not affected by the impervious fraction.

Impervious values should be entered as unit-less decimal fractions, with zero being a total lack of impervious areas within a cell, and one being one-hundred percent impervious areas.

Evapotranspiration

For information on modeling evapotranspiration in Vflo™, see Evapotranspiration.