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

Drainage Design Manual for Maricopa County
Hydrology: Rainfall Losses
Procedure for Areally Averaging Green and Ampt Parameter Values
Most drainage areas or modeling subbasins will be composed of several subareas containing
soils of different textures. Therefore, a composite value for the Green and Ampt parameters that
are to be applied to the drainage areas for modeling subbasins needs to be determined. The
procedure for determining the composite value is to average the area-weighted logarithms of the
XKSAT values and to select the PSIF and DTHETA values from a graph.
The XKSAT value (and naturally occurring rock outcrop percentage) for each map unit as identified
by the National Resources Conservation Service (NRCS) is provided in APPENDIX C. The
data contained in this appendix covers the majority of the northern portion of Maricopa County.
The values for XKSAT listed in the appendix are weighted based on the percentage of each
unique soil texture present in the map unit. The weighted values take into consideration the horizon
depth of the soil textures in regard to the expected depth of infiltration during the design
storm duration. An example of the weighting procedure along with other assumptions and criteria
used in developing the XKSAT values are provided at the front of APPENDIX C. The composite
XKSAT is calculated by Equation (4.4):
ΣA i log
XKSAT alog 10
XKSAT i
=
10
------------------------------------------
A T
(4.4)
where:
XKSAT = composite subarea hydraulic conductivity, inches/hour
XKSAT i = hydraulic conductivity of a map unit, inches/hour
(from APPENDIX C)
A i = size of subarea
A T = size of the watershed or modeling subbasin
After composite XKSAT is calculated, the values of PSIF and DTHETA (normal or dry) are
selected from Figure 4.3, at the corresponding value of XKSAT.
Procedures for Adjusting XKSAT for Vegetation Cover
The hydraulic conductivity (XKSAT) can be affected by several factors besides soil texture. For
example, hydraulic conductivity is reduced by soil crusting, increased by tillage, and increased by
the influence of ground cover and canopy cover. The values of XKSAT that are presented for
bare ground as a function of soil texture alone should be adjusted under certain soil cover conditions.
Ground cover, such as grass, litter, and gravel, will generally increase the infiltration rate over
that of bare ground conditions. Similarly, canopy cover – such as from trees, brush, and tall
grasses – can also increase the bare ground infiltration rate. The procedures and data that are
August 15, 2013 4-13