The Curve Number (CN) method, developed by the USDA Soil Conservation Service (now NRCS), is a widely recognized hydrological tool used to estimate direct runoff from a rainfall event. This empirical, dimensionless parameter reflects the runoff generating characteristics of a specific land area. The standard CN value ranges from 30 (high infiltration) to 100 (completely impervious surfaces). Selecting this number involves classifying a watershed based on two primary physical characteristics: the underlying soil and the surface cover.
Defining Hydrologic Soil Groups (HSGs)
The foundation of the Curve Number method rests on classifying soil into one of four Hydrologic Soil Groups (HSGs), labeled A, B, C, and D, based on the soil’s inherent infiltration rate.
Group A soils, such as deep, well-drained sands, have the highest infiltration rates and the lowest potential for generating runoff. Water moves freely through these coarse textures even when the soil is thoroughly wet.
Group B soils exhibit a moderate infiltration rate, typically consisting of moderately deep loams or silt loams that are moderately well-drained. Water transmission through these soils is slower than Group A but still substantial.
Group C soils have a restricted infiltration rate, often containing a layer that impedes water movement or consisting of moderately fine-textured soils like sandy clay loam.
Group D soils represent the highest runoff potential due to their very slow infiltration rates, even when saturated. These soils are often heavy clays, silty clays, or shallow soils situated over an impervious layer. The high clay content and low permeability ensure most rainfall becomes surface runoff.
Accounting for Land Cover and Treatment
Beyond the soil type, surface characteristics, known as the soil-cover complex, are the second variable used to determine the standard Curve Number. Land cover refers to the material covering the ground, such as pavement, forest, or an agricultural field. The presence of vegetation or concrete dramatically alters the amount of water that can infiltrate.
The “treatment” and “condition” of the land cover further refine the runoff potential, particularly in vegetated and agricultural areas. For example, a cultivated field managed with straight-row tillage generates more runoff than the same field managed with contouring, which slows water flow. A forest or pasture in “good” condition (dense cover) allows for greater infiltration than one in “poor” condition.
Land use changes, like converting a meadow to a residential development, increase the CN because impervious surfaces reduce the area available for infiltration.
Selecting the Standard Curve Number from the Table
Finding the initial Curve Number involves locating the intersection of the two major site characteristics within the standard CN table. This table is structured as a matrix where the Hydrologic Soil Groups (A, B, C, D) form the columns, and the land cover, treatment, and condition classifications form the rows. The value selected is designated as the Antecedent Moisture Condition (AMC) II number, representing an average soil moisture state.
To select the CN, one identifies the HSG (e.g., Group B) and then finds the corresponding land cover row (e.g., “Residential area with 1/2-acre lots, good lawn condition”). Following the row and column to their intersection yields the initial CN value. The table also accounts for the density of development; for example, areas with larger lot sizes (more pervious area) have a lower CN than those with smaller lot sizes. For highly developed areas like concrete roads or rooftops, the CN approaches 98 or 100, as these surfaces permit virtually no infiltration. This standard CN is then used in the NRCS runoff equation to calculate the estimated volume of runoff for a given rainfall depth.
Adjusting the Curve Number for Site Conditions
The standard CN value (AMC II) derived from the table assumes average moisture conditions and is often not the final number used for hydrological modeling. The Antecedent Moisture Condition (AMC) accounts for the actual wetness of the soil in the five days preceding the rainfall event, which significantly impacts the soil’s capacity to absorb water.
The three AMC classes are AMC I (dry soil), AMC II (average conditions), and AMC III (wet or saturated conditions). If the soil is very dry (AMC I), the CN is adjusted downward, resulting in less predicted runoff. Conversely, if the soil is nearly saturated (AMC III), the CN must be adjusted upward, predicting a greater volume of runoff. Conversion formulas or separate adjustment tables are used to convert the AMC II number into its corresponding AMC I or AMC III value.
Accounting for Urban Impervious Surfaces
A separate consideration in urban areas is whether impervious surfaces are connected to the drainage system. Runoff from surfaces like rooftops or driveways that flow directly into a storm sewer are considered “connected” and are highly efficient at generating runoff, typically assigned a CN close to 98. If the impervious area is “unconnected”—meaning runoff flows onto a pervious surface like a lawn before reaching the drain—some infiltration occurs, and the composite CN for the area is slightly reduced.