FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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Rainfall Losses

Hydrolowlc Design Manual
for Maricopa County

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parameter (PSIF) is relatively insensitive in comparison with the hydraulic conduc-
tivity parameter (XKSAT); therefore only the hydraulic conductivity parameter is
adjusted for the influences of cover over bare ground.

Procedures have been developed (Rawls and others, 1988) for incorporating the
effects of soil crusting, ground cover, and canopy cover into the estimation of
hydraulic conductivity for the Green and Ampt equation; however, those proce-
dures are not recommended for use in Maricopa County at this time. A simplified
procedure to adjust the bare ground hydraulic conductivity for vegetation cover is
shown in Figure 4.10. This figure is based on the documented increase in hydraulic
conductivity due to various soil covers as reported by investigators using rainfall
simulators on native western rangelands (Kincaid and others, 1964; Sabol and
others, 1982a; Sabol and others, 1982b; Bach, 1984; Ward, 1986; Lane and others,
1987; Ward and Bolin, 1989). This correction factor can be used based on an estimate
of vegetation coveras used by the Soil Conservation Service in soil surveys; that is,
vegetation cover is evaluated on basal area for grasses and forbs, and is evaluated
on canopy cover for trees and shrubs. Note that this correction can be applied only
to soilS other than sand and,uAdj' Iga~
10.",," ~tT ,
The influence of tilJage results in a change in total porosity and therefore a need to
modify the three Green and Ampt equation infiltration parameters. The effect of
tillage systems on soil porosity and the corresponding changes to hydraulic con-
ductivity, wetting front capillary suction, and water retention is available (Rawls
and Brakensiek. 1983). Although this information is available, it is not presented in
this manual. nor is it recommended that these adjustments be made to the infiltra-
tion parameters for design purpose use in Maricopa County, because for most flood
precliee8R purposes it cannot be assumed that the soil will be in any particular state
of tilJage at the time of storm occurrence and therefor~e base.,S9ndition infiltration
parameters, as presented, should be used for flood prelL~ti\ purposes. However,
appropriate adjustments to the infiltration parameters can be made, as necessary,
for special flood studies such as reconstitution of storm events.

The necessary soils information may not be available for all areas of Maricopa
County for the purpose of estimating the Green and Ampt equation parameters
based on soil texture, but the SCS CN method has been used extensively in Maricopa
County to estimate rainfall losses. Estimates of CN can be obtained by comparison
of watersheds for which no general soilS reports are available to watersheds for
which soils data are available. Brakensiek and Rawls (1983) have grouped soil
according to texture into the four hydrologic soil groups as shown below:

56

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