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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Hydrologic Design Manual
for Maricopa County

Rainfall Losses

general soil texture classification of the drainage area is available. The values of
XKSA T and PSIF from Figures 4.3 and 4.4 can be used if more specific soil texture
classification is available from a detailed soil survey for which the percentages of
sand and clay have been determined by an appropriate field soil survey. The use
of the information in Figures 4.3 and 4.4 will require an extensive study of the soil
for the drainage area, and for most drainage studies only general soil texture
classification will be known so the values from Table 4.2 should be used.

The soil moisture deficit (OTHET A) isa volumetric measure of the soil moisture storage
capacity that is available at the start of the rainfall. DTHET A is a function of the effective
porosity of the soil. The range of DTHETA is 0.0 to the effective porosity. If the soil is
effectively saturated at the start of rainfall then DTHETA equals 0.0; if the soil is devoid
of moisture at the start of rainfall th4'TIiET A equals the effective porosity of the soil.
The porosity of soil as a function of soil texture (percent of sand and percent of clay) is
shown in Figure 45 (Brakensiek and othen;, 1984).

. Under natural conditions, soil seldom reaches a slate of soil moisture less than the
wilting point of vegetation. Figure 4.6 is a graph of volumetric soil moisture at
wilting point as a function of soil texture. Due to the rapid drainage capacity of
most soils in Maricopa County, at the start of a design storm the soil would not be
expected to be in a state of soil moisture greater than the field capacity, Figure 4.7
is a graph of volumetric soil moisture at field capacity as a function of soil texture.

Table 4.2
Green and Ampt Loss Rate Parameter Values for Bare Ground

Soli Texture XKSAT PSIF DTHETA1
Classification Inches/hour Inches Dry Normal Saturated
(1) (2) (3) (4) (5) (6)
sand 4.6 1.9 0.35 0.30 0
loamy sand 1.2 2.4 0.35 0.30 0
sandy loam 0.40 3.5 0.35 0.25 0
loam 0.25 4.3 0.35 0.25 0
silty loam 0.15 6.6 0.40 0.25 0
silt 0.10 7.5 0.35 0.15 0
sandy clay loam 0.06 8.6 0.25 0.15 0
clay loam 0.04 8.2 0.25 0.15 0
sillY clay loam 0.04 10.8 0.30 0.15 0
sandy clay 0.02 9.4 0.20 0.10 0
silty clay 0.02 11,5 0.20 0.10 0
clay 0.01 12.4 0.15 0.05 0

1 Selection of OTHET A:
Dry = Nonirrlgated lands, such as desert and rangeland;
Normal = Irrigated lawn, turf, and permanent pasture;
Saturated = Irrigated agricultural land.

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