Incorporating Green-Ampt Infiltration Model into CRWR-prepro

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

2. Overview • Introduction • Background • Methodology • Conclusion • Future Work

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

4. 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)

5. 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.

6. 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?

7. 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.

8. 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.

9. 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)

10. 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

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

12. 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.

13. 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.