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Infiltration Models Descriptions
Description of the SCS (Soil Conservation Service) Model
The SCS Model is an emperically developed approach to the water infiltration process. It has been developed by first finding a mathematical function whoase shape as a function of time matches the observed features of the infiltration rate. This function is then provided a physical explanation of the process (Jury et al., 1991). In semi-emperical models, most physica processes are represented by commonly accepted and simplistic conceptual methodss rather than by equations derived from fundamentally physical principles. The commonly used semi-emperical infiltration model in the fields of soil physics and hydrology is the SCS Model. A scenario was chosen to simulate water infiltration into a soil for conditions with rainfall and surface runoff by using the SCS model. Input parameters and simulations results are discussed in Volume II of the project report available for downloading on the main page of the Infiltration Site.
Description of the Philip's Two-Term Model
The Philip's Two-Term model (PHILIP2T) is a truncated power series solution developed by Philips (1957). During the initial stages of infiltration, i.e., when t is very small, the first term of the model/equation dominates the process. In this stage, the vertical infiltration proceeds at almost the same rate as abvsorption, or horizontal infiltration. In this stage of infiltration the gravity component, represented by the second term of the model/equation, is negligible. As infiltration continues, the second term becomes progressively more important until it dominates the infiltration process. Philips (1957) suggested the use of the two-term model in applied hydrology when t is not too large. A scenario was chosen to simulate the water infiltration into a sandy soil by using the PHILIP2T model. Input parameters and simulation results are discussed in Volume II of the project report.
Description of the Layered Green-Ampt Model
The Green-Ampt Model has been modified in this application to calculate water infiltration into non-uniform soils by several researchers (Bouwer, 1969; Fok, 1970; Moore, 1981; Ahuja and Ross, 1983). The implementation for layered systems (GALAYER) utilized for this project was that developed by Flerchinger et al. (1989). Specifically, the model could be utilized for the determination of water infiltration over time in vertically heterogenous soils. Two simulation scenarios were selected for inclusion in the applcations worksheet. The first scenario was to estimate water infiltration into a soil with two layers (sand over a loam), and the second scenario was designed to estimate the water infiltration into a soil with three layers (sand over loam, over clay). Comparisons and results are presented and discussed in Volume II of the project report.
Description of the Explicit Green-Ampt Model
The initial Green-Ampt model was the first physically-based model/equation describing the infiltration of water into soil. It has been the subject of considerable developments in soil physics and hydrology owing to its'simplicity and satisfactory performance for a great variety of water infiltration problems. This model yields cumulative infiltration and the infiltration rate as an implicit function of time (i.e., given a value of time (t), values of the cumulative infiltration (I) and the infiltration rate (q) can be directly obtaind. Thus, the model functions are q(t) and I(t), rather than of t(q) and t(I). The Explicit Green-Ampt model as defined and utilized for this project's application was developed by Salvucci and Entekhabi (1994), which provides a straightforward and accurate estimation of infiltration for any given time. This formulation supposedly yields an error of less than 2% at all times when compared to the exact values resulting from the Implicit Green-Ampt Model.
Description of the Constant Flux Green-Ampt Model
For the constant flux Green-Ampt model, two formulatins are required, one for the condition that the application rate (r) is less than the saturated hydraulic conductivity (Ks), and one for the condition that the application rate is greater than the saturated hydraulic conductivity. When r<Ks, the infiltration rate (q) is always equal to the surace application rate (r), and the surface never becomes saturated. When r>Ks , the surface becomes saturated at the time of the initial application (t0).
Description of the Infiltration/Exfiltration Model
The vertical movement of water in the soil profile from the surface to water table is a dynamic condition, and can be conceptualized as being composed of basically two predominant processes: 1) infiltration and 2) exfiltration. Exfiltration can be envisioned as the processes dominating during drying periods, and water rleased during this period can be thought of as being released through evaporation to the atmosphere. The model (INFEXF) selected for this project is a formulation of the Philips model developed by Eagleson (1978) to account for water infiltration during the wetting season and exfiltration during the drying season. Infiltration and exfiltration as described in this application assumes the soil medium to be effectively semi-infinite and the internal soil water content at the beginning of each storm event and inter-storm period is assumed to be uniform at its' long-term and space-time average. The exfiltration equation is modified for the presence of natural vegetation through the approximate introduction of a distributed sink rpresenting the moisture extraction by plant roots. Two scenarios are presented in the accompanying worksheet applications: 1) demonstrates water infiltration during the rainy season and 2) exfiltration during the drying season.