Runoff Curve Number

Runoff curve number, abbreviated as RCN, a dimensionless number, is a hydrological parameter used in hydrology and civil engineering to estimate runoff from a rainfall event in a particular area.  It is commonly employed in the field of watershed modeling and design of stormwater management systems.  The RCN is part of the Soil Conservation Service (SCS) or Natural Resources Conservation Service (NRCS) hydrologic method, which is widely used in the United States.

Key Points about runoff curve number

• Land Use and Cover  -  This factor takes into account the type of land cover (urban, forested, agricultural) and land use practices within the watershed.  Different land uses have varying degrees of runoff potential.
• Hydrologic Soil Group  -  Soils are classified into different groups (A, B, C, and D) based on their ability to infiltrate water.
• Antecedent Moisture Condition  -  This factor accounts for the moisture content of the soil before a rainfall event.  Soil that is already saturated from previous rainfall will contribute more to runoff.

The RCN is calculated using these factors, and the formula yields a numerical value typically ranging from 30 to 100.  A higher RCN indicates a greater potential for runoff, while a lower RCN suggests less runoff.  The formula takes into account the relationships between land use, soil type, and antecedent moisture conditions to estimate the runoff volume and timing for a given storm event.

Engineers and hydrologists use the RCN in hydrologic modeling to predict how rainfall will behave in a specific watershed and to design stormwater management systems, such as detention ponds, to control runoff and reduce the risk of flooding and erosion.  It is a valuable tool for managing and mitigating the impact of stormwater in various land development and land management projects.