Basic Concepts and Equations
The Smith Parlange loss method is similar to the Green and Ampt method described previously. However, the Smith Parlange method approximates Richard's equation by assuming the wetting front can be represented with an exponential scaling of the saturated conductivity (Smith & Parlange, 1978). This linearization approach allows the infiltration computations to proceed very quickly while maintaining a reasonable approximation of the wetting front. The Smith Parlange loss method should only be used for event simulation. The governing equations of the Smith Parlange loss method are described in detail within Smith & Parlange (1978) and Smith, et al (2002).
Required Parameters
Parameters that are required to utilize the Smith Parlange method include the initial water content [in/in or mm/mm], the residual water content [in/in or mm/mm], the saturated water content [in/in or mm/mm], bubbling pressure [in or mm], the pore size distribution, the saturated hydraulic conductivity [in/hr or mm/hr], and directly connected impervious area [percent]. An optional temperature time series may be specified. If a temperature time series is selected, a beta zero parameter must also be specified.
The initial water content refers to the initial saturation of the soil at the beginning of a simulation and should be determined through model calibration. The residual water content specifies the amount of water remaining in the soil after all drainage by gravity has ceased. It should be specified in terms of volume ratio and is commonly estimated using the predominant soil texture. The saturated water content specifies the maximum water holding capacity in terms of volume ratio and is often assumed to be equivalent to the total porosity of the soil.
The bubbling pressure is similar to the wetting front suction head, which was previously discussed. The pore size distribution determines how the total pore space is distributed in different size classes. The saturated hydraulic conductivity was also previously described.
The optional temperature time series adjusts the water density, water viscosity, and matric potential based on temperature. If no temperature time series is specified, a default temperature of 25 degrees Celsius (75 deg Fahrenheit) is assumed to prevail. The beta zero parameter is used to correct the matric potential based on temperature. All of the aforementioned parameters are typically estimated using the predominant soil texture and literature values.
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.