CN Infiltration

Soil Conservation Service Curve Numbersti

One of the advantages of use of the curve number (CN) method for loss estimates (Section 3.3) is the highly structured method for CN estimates provided by the NRCS (SCS, 1972,1988; McCuen, 1998, Bedient and Huber, 2002 and virtually every hydrology text).  Such estimates are embedded in engineering practice through Table 3-18, in which CN values are given as function of land use and soil Hydrologic Soil Group (A through D).  Hydrologic Soil Group has the meaning indicated in Table 3-17, ranging from well-drained (type A) to poorly drained (type D).  Every soil in the United States has an A-D classification, or sometimes a dual classification, such as B/D, meaning drained (artificially) and undrained (natural) condition. 

Table 3-17. Hydrologic soil group meanings (SCS, 1972, Chapter 7).

            Hydrologic Soil Group is provided on the Soil Survey Interpretation sheets discussed in Section 3.8.8.2.  For instance, the Conestoga Silt Loam of Figure 3-37a is in hydrologic soil group B.  A more concise listing is provided in the Soil Survey for each county, published by the NRCS or the local Soil and Water Conservation District. 

            Hence, parameter estimation for SWMM (and any use of curve numbers) is straightforward and relatively unambiguous.  Curve numbers may also be used to back-calculate estimates of initial moisture deficit, as discussed earlier in conjunction with Equation 3-135, since the CN and storage, S, are related through

                                                                                                             (3-136)

where it is essential to remember that units of S are in inches.  And early discussion of Horton infiltration parameters has also been related to Hydrologic Soil Group. 

Table 3-18. Runoff curve numbers for selected agricultural, suburban, and urban land use (SCS, 1986).  (Antecedent moisture condition II; I_a = 0.2S)

1. For a more detailed description of agricultural land use curve numbers, refer to the SCS (1972) National Engineering Handbook, Section 4, “Hydrology.”

2. Good cover is protected from grazing and litter and brush cover soil.

3. Curve numbers are computed assuming that the runoff from the house and driveway is directed toward the street with a minimum of roof water directed to lawns where additional infiltration could occur.

4. The remaining pervious areas (lawn) are considered to be in good pasture condition for these curve numbers.

5. In some warmer climates of the country a curve number of 95 may be used.