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 Soil Group Classification. Most of the soils in the United States are classified by a
hydrologic soil group letter A through D. The hydrologic soil group is determined by drainage class or
infiltration rate. A list of U.S. Soils and their hydrologic group is given in SCS (1986). Local soil
conservationists or soil scientists should be consulted if difficulty arises in determining soil groups.
Antecedent Moisture Condition. Once the hydrologic condition is compiled for a source
area, the antecedent moisture condition (AMC) of the soil may be determined by summing the total
precipitation in the 5 days prior to a storm. Soil moisture will affect infiltration capacity and the resultant
runoff.
In most applications, the choice of average antecedent soil moisture condition (AMC II) is
appropriate. AMC II CNs are used for input into the loading function model. During winter months,
where soils are frozen AMC III may be appropriate for many areas. During dry conditions, for
example, mid to late summer, AMC I may be appropriate. Careful consideration should be given in
choosing an AMC for site and temporal conditions.
Curve numbers for AMC II are given in Table 1.1.1. The CNs for the other two antecedent
conditions, AMC I and AMC III, are determined by conversion equations (Hawkins 1978). Direct
runoff may be determined from Figure (1.1.2), as well as from equation (1.1.1).
1.1.1.5 Precipitation and Snowmelt
The design storm may be used to drive single event loading models. Precipitation events are highly
variable infrequency, duration and intensity and are best characterized by statistical analysis of long-
term precipitation records. From historical data analysis, precipitation intensity-duration curves are
constructed. From the resulting probabilistic curves, the rainfall depth for a specified duration and
recurrence interval for the design storm may be determined.
The design storm is defined as an estimate of rainfall depth for a specified duration (10 min-24
hours) for a specified return period (2-100 years). A probability distribution derived from the historic
rainfall record is used to statistically estimate the design storm rainfall depth. The relationship between
return period and probability of design storm event occurrence is defined by the following relationship.
1
T=
(1.1.3)
P
Where:
T = return period in years
P = probability (0-1) an annual maximum event of any
year will equal or exceed some given value
1.1-5
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