Geoscience Reference
In-Depth Information
rain event. The CN ranges from 0 to 100, where larger CN represents greater proportion of
surface runoff.
Basically, four steps are necessary to evaluate runoff from a rainfall by the SCSCN model: (i)
to determine the hydrologic soil group (Table 1); (ii) to determine the five-day antecedent
moisture condition of the soil from the precipitation record; (iii) to determine the runoff CN
(on the basis of land cover, soil treatment, plus hydrologic condition and hydrologic soil
group of the soil); and (iv) to calculate the runoff volume for one rain event. Concepts
related to these four main steps are given below.
HSG
Characteristics
A
Soils with high infiltration rates
B
Soils with moderate infiltration rates
C
Soils with low infiltration rates
D
Soils with very low infiltration rates
Table 1. Hydrologic soil group (HSG) according to SCSCN model
In this model, the soils are classified to one of four HSG (A, B, C or D) defined by the SCS.
This classification was accomplished by the analysis of the infiltration capacity of the soil.
The description of each group, according to SCS (1972) and Rawls et al. (1992), is listed in
Table 1.
In the SCSCN model, the watershed surface setting is assessed as a function of land cover,
type of soil treatment and soil hydrologic condition. Land cover varies with landuse and can
include key categories such as forests, swamps, pasture, bare soil, impermeable areas, etc.
The soil treatment is related to automated farming practices (plantation along topographic
contour lines and terraces) and management practices (pasture control, crop rotation and
reduction). The association between landuse and the type of soil treatment is named class.
Some examples of classes are: cereal plantations on topographic contour lines; dense forests;
dense pasture, flat bare soil, paved highways, etc.
The association between specific HSG, land cover and type of soil treatment is referred to as
soil-cover hydrologic complex, for which the CN attribute can be derived from the
specialized literature (SCS, 1972; Rawls et al., 1992; Pilgrim & Cordery, 1993).
Antecedent Moisture Conditions (AMC) are related to the soil moisture due to accumulated
rain, but considering the five last days that precede a particular rain event. There are three
types of AMC: AMC I = soil is dry; AMC II = soil moisture is medium; and AMC III = soil is
saturated in water.
The CNs were firstly obtained by measures made in a great number of watersheds for AMC
II. The CN derived for AMC II can be converted to AMC I or AMC III through a transfer
table provided by SCS (SCS, 1972).
The runoff begins when the portion of lost rain by infiltration, evapotranspiration,
interception and depression storage, denominated initial abstractions, is less than the total
precipitation. The runoff equation defined by SCS and detailed on the National Engineering
Handbook (SCS, 1972) is the following:
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