Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Application
9.3 RAINFALL LOSSES
9.3.1 Procedures for the Green and Ampt Method
A. When soils data are available:
1. Prepare a base map of the drainage area delineating subbasins, if used.
2. Determine the location of the study area in regard to the limits of the soil surveys provided
in APPENDIX C.
a. If the study area is completely contained within these limits:
i. Overlay the watershed limits on the soil survey maps from the appropriate
soil survey report(s) and tabulate the map units present within
the watershed.
ii.
Cross reference the map units with those listed in APPENDIX C and
tabulate the weighted value of XKSAT for each map unit and the corresponding
percent imperviousness.
iii. Proceed to item (3) or (4).
b. If the study area is partly or entirely outside the limits of the soils surveys provided
in APPENDIX C:
i. Refer to the figure showing the status of soil surveys in Arizona (at the
front of APPENDIX C) for other sources of soils data. Other sources of
soils data are:
• General soils surveys by county prepared by the NRCS.
• Other detailed soil surveys.
• US Forest Service Terrestrial Ecosystem Reports.
ii.
Using the data contained in the alternative source, follow the example
procedure for determination of the weighted XKSAT value for each
unique map unit that is included at the front of APPENDIX C.
iii. Proceed to item (3) or (4).
3. If the watershed or subbasin contains only one soil texture, then determine XKSAT,
PSIF and DTHETA from Table 4.1.
August 15, 2013 9-25