1. Green-Ampt Parameterization for Rainfall Excess Determination
Harry Downing
SWFWMD
April 18, 2008

2. Green-Ampt (GA) Parameter Outline
The District has changed to GA?
GA Process (General)
Data Sources for Developing GA Parameters
Look-Up Table Development (Soils and LU)
ArcHydro Tools for GA
Parameter Generation
Questions

3. Rainfall Excess Process

4. Soil Parameter Sources
NRCS - ‘Soil Survey’ and Soil Survey Geographical (SSURGO) Database - Depth to Water Table and Ksat
IFAS - ‘Site Specific Soil Characterization’ – Soil Moisture Retention Information
Table Information from Other Studies (Text Books)
Site Specific Characteristics from ERP Applications

5. Soil Survey Information

6. Soil Survey Information Continued

7. Soil Moisture for 200 cm Depth to WT
Height Above Water Table Centimeters
Soil Moisture Content %
Soil Moisture
Saturation / Soil Porosity

8. Soil Look-Up Table

9. Land Use Look-up Table

11. Creative Land Use Look-Up Table

12. ESRI ArcHydro Tools

23. ESRI Sub-Catchment Assignment of GA Parameters

25. Questions?

26. Infiltration and Percolation Parameterization
Take You Through Process that the District has Used

27. Parameters Generation
Area Of Unsaturated and Saturated Infiltration
Perimeter Development
Hydraulic Properties of the Soil
Expected Depth to the Water Table (SHGWT or Higher)
Development of Parameters for Input Into ICPR

28. Area of Infiltration

29. Area of Unsaturated Infiltration
First Make a Run with the GA Excess Option Only
Obtain the Maximum Inundation Area and Elevation from the Model Results
Develop a GIS feature class of the Inundation areas (area & perimeter) - units
Use this Information for the Infiltration and Percolation Parameters for ICPR regarding initial area

30. Hydraulic Properties of Soils
SSURGO and IFAS Data
ERP Information
Geotechnical Information Reports
Soil Testing

31. Location of Soil Bores Within Hernando County

32. Vertical Hydraulic Conductivity Results

33. SSURGO Results for Hydrologic ‘A’ Soils
Results from ERP Tests
SSURGO Results for Hydrologic ‘A’ Soils
Minimum Value inches/hr
Mean
Max inches/hr

34. Assumptions Horizontal Hydraulic Conductivity Greater Than Vertical
Use of Vertical for Horizontal Should be Conservative

35. Use SSURGO Potentiometric Surface Map Land Use Map Imagery
Depth to SHGWT
Use SSURGO
Potentiometric Surface Map
Land Use Map
Imagery

39. ICPR Parameterization

40. ICPR Parameterization Continued

41. Questions?

42. Floodplain Result Comparisons

47. Stage Plot Comparison at Node

48. Stage Plot Comparison at Node

49. Stage Plot Comparison at Node

50. Reasonableness of Floodplain

54. ITEMS TO CHECK !!
Double Accounting - Between GA, and Infiltration and Percolation
Area of Influence Around Infiltration Areas

55. Questions?