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

Hydrologic Design Manual
for Maricopa COunty

where

At =: the computation interval
F = accumulated depth of infiltration at the start of At.

The average infiltration rate is:

Use of the Green and Ampt equation as coded in HEC-1 involves the simulation of
rainfall loss as a two phase process, as illustrated in Figure 4.2. The first phase is
the simulation of the surface retention loss as previously described; this loss is called
the initial loss (lA) in HEC-1. During this first phase, all rainfall is lost (zero rainfall
excess generated) during the period from the start of rainfall up to the time that the
accumulated rainfall equals the value of 1A. It is assumed, for modeling purposes,
that no infiltration of rainfall occurs during this first phase. Initial loss (lA) is
primarily a function of land-use and surface cover, and recommended values of IA
for use with the Green and Ampt equation are presented in Table 4.1. For example,
about 0.35 inches of rainfall will be lost to runoff due to surface retention for desert
and rangelands on relatively flat slopes in Maricopa County.

The second phase of the rainfall loss process is the infiltration of rainfall into the soil
matrix. For modeling purposes, the infiltration begins immediately after the surface
retention loss (lA) is completely satisfied, as illustrated in Figure 4.2. The three
Green and Ampt equation infiltration parameters as coded in HEe-1 are:

.. hydraulic conductivity at natwal saturation CXI<5A TI equal to I<s in Equation 1;

.. wetting front capillary suction (PSIF) equal to 'l' in Equation 1; and

.. volumetric soil moisture deficit at the start of rainfall (DTHET A) equal to
e in Equation 1.

The three infiltration parameters are functions of soil characteristics, ground surface
characteristics, and land management practices. The soil characteristics of interest
are particle size distribution (soil texture), organic matter, and bulk density. The
primary soil surface characteristics are vegetation canopy cover, ground cover, and
soil crusting. The land management practices are identified as various tillages as
they result in changes to soil porosity.

Values of Green and Ampt equation parameters as a function of soil characteristics
alone (bare ground condition) have been obtained from published reports (Rawls
and others, 1983; Rawls and Brakensiek, 1983). Average values ofXKSAT and PSIF
for each of the soil texture classes from Rawls and Brakensiek (1983) are shown in
Columns (2) and (3) of Table 4.2. Values ofXKSA T and PSIF (as a function of percent
of sand and percent of clay for soil with 0.5 percent organic matter and base value
[unaltered) soil porosity) are shown in Figures 4.3 and 4.4, respectively (Rawls and
Brakensiek. 1983). The values of XI<5A T and PSIF from Table 4.2 should be used if

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