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SCS / TR-55 Runoff Curve Number

Composite CN · Q = (P−0.2S)² / (P+0.8S)

When to use: The SCS (NRCS) Curve Number method estimates direct runoff depth from a design storm. Each sub-area gets a CN from its land use and hydrologic soil group (A=sandy/well-drained → D=clay/poorly-drained). The composite CN is area-weighted. Maximum retention S = 1000/CN − 10 (inches), initial abstraction Iₐ = 0.2S, and runoff depth Q = (P−0.2S)²/(P+0.8S)for P > Iₐ.

Design Storm
24-hr storm
in
Sub-Areas
ac
CN = 98
ac
CN = 74
Runoff Depth
2.20
inches (55% of rainfall)
Results
Composite CN82.0
Total Area6.00 ac
Max Retention S2.20 in
Initial Abstraction Iₐ0.44 in
Runoff Depth Q2.20 in
Runoff Volume1.10 ac-ft
References
NRCS TR-55 / NEH-4 CN method
S = 1000/CN − 10 (inches)
Iₐ = 0.2S, runoff = 0 if P ≤ Iₐ
Q = (P−0.2S)²/(P+0.8S)