Incorporating Green-Ampt Infiltration Model into CRWR-prepro Jerry Perales University of Texas

Overview Introduction Background Methodology Conclusion Future Work

Introduction Green & Ampt proposed the simplified picture of infiltration shown here.

The Green & Ampt method: Introduction The Green & Ampt method: Models infiltration from a ponded surface (h0) into a deep soil column (L) Applies Darcy’s law assuming water enters soil as a slug (q = -K ¶h/¶z) Assumes a sharp wetting front separating wetted zone from unwetted zone (qi)

Introduction From these assumptions, it can be shown that the infiltration rate f(t) can be found from the expression: Where K is the hydraulic conductivity, y is the wetting front suction head, Dq is the change in moisture content and F(t) is the cumulative infiltration depth.

Background Currently CRWR-prepro uses two methods to calculate abstraction parameters SCS method Initial loss plus a constant loss rate Why do we need another method to calculate abstraction parameters?

Background The Green & Ampt model better represents the physical parameters associated with soil textures. The SCS method arbitrarily assigns curve numbers to represent soil type and land use. The initial loss plus constant loss rate is again arbitrary and relies upon experience.

Methodology To calculate Green & Ampt infiltration parameters for each subwatershed in a region to be analyzed, the STATSGO map was first clipped to match the delineated watershed.

Methodology From this point an Avenue Script uses the attributes of this shape file, and data base files supplied by STATSGO to assign USDA soil textures from a conversion table shown here. (719 STATSGO soils to 12 USDA soils and one other category)

Methodology The Avenue Script creates a table of information compiled from the STATSGO database files and the conversion table. This table is known as g&a.dbf

Methodology Next, a look-up table is used to assign the parameters necessary to calculate abstractions with the Green & Ampt model.

Methodology Once the parameters have been assigned to each mapunit in the g&a.dbf table the change in the moisture content (Dq) can be calculated from a user input initial moisture content. The values will be averaged for each subwatershed and supplied to the basin file created for HEC-HMS.

Conclusion Using the Green-Ampt method for calculating abstractions will potentially allow for a more accurate calculation for infiltration because this approach is developed from a more physical theory.