2:1.1.1 SCS Curve Number

The SCS curve number is a function of the soil’s permeability, land use and antecedent soil water conditions. Typical curve numbers for moisture condition II are listed in tables 2:1-1, 2:1-2 and 2:1-3 for various land covers and soil types (SCS Engineering Division, 1986). These values are appropriate for a 5% slope.

Table 2:1-1: Runoff curve numbers for cultivated agricultural lands (from SCS Engineering Division, 1986)

* Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.

Table 2:1-2: Runoff curve numbers for other agricultural lands (from SCS Engineering Division, 1986)

[2] Poor: < 50% ground cover or heavily grazed with no mulch; Fair: 50 to 75% ground cover and not heavily grazed; Good: > 75% ground cover and lightly or only occasionally grazed. [3] Poor: < 50% ground cover; Fair: 50 to 75% ground cover; Good: > 75% ground cover. [4] Poor: Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning; Fair: Woods are grazed but not burned, and some forest litter covers the soil; Good: Woods are protected from grazing, and litter and brush adequately cover the soil.

Table 2:1-3: Runoff curve numbers for urban areas (from SCS Engineering Division, 1986)

[5] SWAT will automatically adjust curve numbers for impervious areas when IURBAN and URBLU are defined in the .hru file. Curve numbers from Table 6-3 should not be used in this instance. [6] Poor: < 50% grass cover; Fair: 50 to 75% grass cover; Good: > 75% grass cover