Upcoming Changes to the NRCS Curve Number Method

Will Huff

After more than 60 years, potential changes to the NRCS Curve Number method are underway!

The original Curve Number method was developed by the U.S. Department of Agriculture’s (USDA) Natural Resources Conservation Service (NRCS) (Soil Conservation Service at the time). It started in the 1950s as a simple and efficient method for determining the volume of runoff produced from a certain rainfall event that could be applied uniformly across the country. Since then, the method has been widely used; however, there is very little documentation surrounding the original development of the method. It also never went through a critical open review. Needless to say, the civil engineering field has advanced significantly since the inception of the Curve Number method. Several members of the Environmental and Water Resources Institute (EWRI), a technical institute of the American Society of Civil Engineers (ASCE), recognized that an update was warranted to use current knowledge and data to more accurately model the rainfall-runoff relationship while maintaining the current level of simplicity.

In 2015, members from EWRI and the American Society of Agricultural and Biological Engineers (ASABE) teamed up to form the EWRI Curve Number Hydrology Task Committee, which included engineers, researchers and scientists from around the country. The task committee developed a memorandum of understanding and a cooperative agreement with the NRCS to provide updates to the Curve Number method. Specifically, this comprised reorganization and revisions to Chapters 8, 9, 10 and 12 of the USDA National Engineering Handbook Part 630 Hydrology (NEH 630). The task committee met quarterly during the development of the draft NEH chapters and the revisions went through an extensive quality-control process, including an external review of the final draft. In September 2017, the draft NEH 630 chapters were submitted to the NRCS for review and implementation.

The draft chapters can be found at the links below.

Letter to NRCS and Executive Summary
Chapter 8 – Land Use and Treatment Classes 
Chapter 9: Hydrologic Soil-Cover Complexes
Chapter 10: Estimation of Direct Runoff from Storm Rainfall
Chapter 12: Hydrologic Effects of Land Use and Land Treatment

Major changes to the Curve Number method include changes to the calculation of initial abstraction, in which it is assumed that Ia/S = 0.05 instead of the previous assumption of Ia/S = 0.20. Not only does this result in an increase in the amount of direct runoff, but runoff is generated at lower rainfall depths than the previous assumption. This also changes all Curve Number tables and charts developed based on the initial Ia/S = 0.20 assumption. Another major change is the acknowledgement that the Curve Number method is not appropriate for all watersheds. The draft NEH 630 chapters recognize that a watershed’s runoff response to rainfall events is not uniform, but there are actually multiple types of runoff responses. These have been grouped into three types: Standard, Complacent and Violent. It was found that most of the rainfall-runoff data sets (approximately 80 percent) studied were consistent with the Standard type. These are watersheds, which exhibit evidence of overland flow patterns. This condition is frequently observed in most upland rain-fed crop lands. In the Complacent and Violent cases, there is little evidence of overland flow.

The main sources of runoff are channel or impervious interception and subsurface return flow. These types are common in urbanized watersheds and some arid watersheds, as well as mature forests and watersheds with deep pervious soils. It was recommended that the Curve Number method not be used for watersheds exhibiting Complacent or Violent rainfall-runoff patterns, extreme forested watersheds or karst-dominated watersheds. Therefore, the suitability of the Curve Number method for projects should be evaluated on a case-by-case basis.

Once the NRCS review of the draft chapters is complete, they must then decide how to implement these changes. The exact timeframe for the NRCS review and implementation is unknown, so be sure to stay tuned for updates regarding these changes to determine if changes to your community’s criteria might be warranted.