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 β D=clay/poorly-drained). The composite CN is area-weighted. Maximum retention S = 1000/CN β 10 (inches), initial abstraction Iβ = 0.2S, and runoff depth Q = (Pβ0.2S)Β²/(P+0.8S)for P > Iβ.
This calculator implements the NRCS (formerly SCS) Curve Number method from Technical Release 55 (TR-55) to compute direct runoff depth from a design storm, based on composite land use and hydrologic soil group β the foundational stormwater hydrology tool required in virtually every site development project in the United States.
The potential maximum retention S = 1000/CN β 10 (in inches) measures how much rainfall the watershed can absorb before runoff begins. Initial abstraction Ia = 0.2S represents the water captured by interception, depression storage, and initial infiltration before runoff commences. Runoff depth Q = (P β 0.2S)Β² / (P + 0.8S) for P > Ia; no runoff occurs for P β€ Ia.
Curve Number CN ranges from 30 (excellent infiltration: dense forest on HSG A sandy soil) to 98 (fully impervious: pavement or roof). It is determined by the combination of land use and hydrologic soil group (HSG). Hydrologic Soil Groups are: A (low runoff potential, deep well-drained sands), B (moderately low runoff, sandy loams), C (moderately high runoff, sandy clays), and D (high runoff potential, clays, poorly drained). For mixed land cover, the composite CN is the area-weighted average: CN_composite = Ξ£(CN_i Β· A_i) / Ξ£A_i.
The Curve Number method is documented in NRCS National Engineering Handbook Part 630, Chapter 9 and USDA Technical Release 55 (TR-55, 1986 revised). It is required or preferred by most state stormwater management programs and is integrated into HEC-HMS, TR-20, WMS, and SWMM as the primary runoff generation method. FEMA accepts TR-55-based analyses for Flood Insurance Studies.
Hydrologic Soil Groups for specific map units are available from the NRCS Web Soil Survey (websoilsurvey.sc.egov.usda.gov). Many state environmental agencies (e.g., Virginia DCR, Georgia EPD) publish stormwater design manuals that prescribe CN tables for regional land use classifications, sometimes more conservative than the NRCS national tables.
The 0.2S initial abstraction assumption (Ia = 0.2S) is a national average that may not be appropriate in all regions. Some research suggests Ia = 0.05S gives better predictions for forested watersheds. Several state stormwater manuals (e.g., Virginia, Maryland) use modified Ia values and updated CN tables based on regional calibration studies.
Antecedent moisture condition (AMC) significantly affects CN: AMC-I (dry) reduces CN by 5β20 points, AMC-III (wet) increases CN by 5β20 points. Standard design practice uses AMC-II (average), but detention pond design for consecutive storms may warrant AMC-III to produce a conservative (higher) peak. TR-55 provides conversion tables for AMC-I and AMC-III from AMC-II values.
Enter the 24-hour design storm rainfall depth P (inches) from NOAA Atlas 14 for your location and design return period. Add sub-areas by selecting land use and hydrologic soil group for each polygon in your drainage basin. The calculator computes the area-weighted composite CN, maximum retention S, initial abstraction Ia, and runoff depth Q. Use the runoff depth and drainage area as inputs to the Runoff Hydrograph Simulator to generate peak flow and hydrograph shape for detention sizing.
The NRCS Web Soil Survey at websoilsurvey.sc.egov.usda.gov provides HSG data for all US soil map units. Navigate to your site, define an area of interest, and download the soil report which includes HSG in the tabular data. Alternatively, your geotechnical engineer can classify HSG from boring logs based on USCS soil classifications and percolation test data.
Fully impervious surfaces (asphalt pavement, concrete, rooftops) always have CN = 98 regardless of soil group, because the soil is completely covered. This is consistent across all TR-55 land use categories for impervious cover. Only areas with infiltration potential (pervious surfaces, green spaces) show variation by HSG.
Development dramatically increases CN: converting an HSG B forest (CN = 55) to 1/4-acre residential (CN = 75) raises S from 8.2 in to 3.3 in and approximately doubles or triples the runoff depth for a typical 3β4 inch design storm. This is why post-development stormwater management (detention ponds, bioretention) is required to offset the increased CN.
TR-55 is a simplified method for small watersheds up to 2000 acres using the triangular unit hydrograph and tabular hydraulic routing. TR-20 is the more rigorous full computerized hydrology model that uses the NRCS dimensionless S-graph unit hydrograph and detailed routing through reaches and reservoirs. Both use CN for runoff computation but TR-20 is more accurate for complex watersheds with multiple subbasins and storage facilities.
Yes. A parking lot (impervious) has CN = 98. The initial abstraction is only Ia = 0.2 Γ (1000/98 β 10) = 0.04 in, so nearly all rainfall becomes runoff. For a 2-inch storm: Q = (2 β 0.04)Β² / (2 + 0.8 Γ 0.204) = 3.84 / 2.16 = 1.78 in β 89% runoff. This high runoff fraction drives the need for inlet and storm drain capacity analysis using the Rational Method for small parking lot catchments.