Updating the Curve Number: Task Group Report
Richard H. Hawkins, Glenn E. Moglen, Tim J. Ward, Donald E. Woodward
Abstract
The USDA Natural Resources Conservation Service (NRCS, formerly Soil Conservation Service) curve number (CN) method for estimating runoff depth from rainfall depth is a ubiquitous technique that is used worldwide in a variety of applications. Since its creation over 60 years ago, it has undergone numerous analyses on both practical and theoretical grounds. Although originally intended to model runoff depth from rainfall design storms on small agricultural and rangeland watersheds, it has been applied to a wide variety of conditions including green roofs, forests, continental scale river basin runoff, and daily time-step models, in applications other than those originally intended. Numerous problems, limitations, needs, and inappropriate extensions have become evident. In late 2015, a volunteer joint work group of the American Society of Civil Engineers (ASCE) and the American Society Agricultural and Biological Engineers (ASABE) members, began a review of the state of knowledge and experiences about CN methodology. The task group of 16 members delivered a set of suggested revisions and updates to the NRCS in late 2017. This presentation will summarize the task group’s activities, efforts, and the suggested revisions and updates. Included in the presentation are how to determine for which watersheds the CN method is applicable, techniques for estimating CN values from rainfall and runoff data, and how to apply a revised (from the historical CN method version) relationship between initial abstraction and CN. The presentation is intended as a report on the task group’s recommendations and to inform and solicit feedback from the audience on the suggested revisions and updates to this widely used methodology. At this time (January 2020), the report’s recommendations have not been formally incorporated in NRCS institutional hydrology design guides.