FLOOD08379

• County: Statewide
• Community: State
• Stream Name: All
• Basin: Statewide
• Title: Procedures for Determining Peak Flows in Colorado
• Date: 3/1/1984
• Prepared For: State of Colorado
• Prepared By: USDA Soil Conservation Service
• Floodplain - Doc Type: Educational/Technical/Reference Information

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SECTION 4

GENERAL

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The soil series, antecedent soil moisture, vegetative cover, and condition
of cover affect the volume of runoff from precipitation.

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WATERSHED COVER AND TREATMENT

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Crop residues tilled into the soil and the residual root system from grasses
that have been in crop rotations improve the hydrologic condition, thereby
reducing the volume of runoff.

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Vegetative Cover

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Vegetation affects runoff in several ways. The foliage and its litter
maintain the soil's infiltration potential by preventing the sealing of the
soil surface from the impact of the raindrops. Some of the raindrops are
retained on the surface of the foliage, increasing their change of being
evaporated back to the atmosphere. Some of the intercepted moisture is so
long in draining from the plant down to the soil that it is withheld from
the initial period of runoff. Vegetation, including its ground litter,
forms numerous barriers along the path of the water flowing over the surface
of the land. This lengthens the time-of-concentration (See Section 5) and
reduces the peak discharge rate.

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Conservation Practices

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Conservation practices, in general, reduce sheet erosion and thereby
maintain an open structure of the soil surface. This reduces the volume of
runoff but the effect diminishes rapidly with increase in storm magnitude.

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Contouring and terracing reduce sheet erosion and increase the amount of
rainfall withheld from runoff by the small reservoirs they form. Gradient
terraces increase the distance water must travel and thereby increase the
time-af-concentration. This, in turn, reduces the peak rate of discharge.

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HYDROLOGIC SOIL COVER COMPLEX (CN)

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A combination of a hydrologic soil group and a land use and treatment class
is a hydrologic soil cover complex (CN). This complex is represented by an
index number which reflects the runoff potential, the higher the CN the
higher the potential. Figures S-3 and S-4 show CN values for most soil
cover complexes. These eN's are for an antecedent moisture condition II
(see the following discussion of Antecedent Moisture Condition).

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ANTECEDENT MOISTURE CONDITION

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Curve number data was developed from a number of research watersheds with
each having a single soil group and vegetation complex. It was found that
measured runoff from a given watershed, for similar rainfall events, varied
through the years of data collection. The major reason was determined to be
different antecedent soil moisture conditions. Rainfall vs runoff data for
each watershed was plotted on a graph and a median line drawn through the

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