ARS | Publication request: No-till and curve numbers a closer look

Submitted to: Georgia Water Resources Conference
Publication Type: Proceedings
Publication Acceptance Date: March 10, 2011
Publication Date: April 11, 2011
Citation: Endale, D.M., Schomberg, H.H., Fisher, D.S., Jenkins, M. 2011. No-till and curve numbers a closer look. Georgia Water Resources Conference. CDROM.

Interpretive Summary: The USDA’s Curve Number (CN) model, a simple empirical model developed in the 1950s for estimating direct runoff from a rainfall event, continues to be used across the world, and is a vital component of many popular and more complex hydrologic models. Consensus is developing from research across many locations that this model needs adjustment to provide more realistic prediction of runoff. Researchers at the USDA-ARS J. Phil Campbell Senior Natural Resource Conservation Center used rainfall and runoff data collected from 1976 to 2009 (127 events) on a 6.5-acre watershed that had been managed as a crop-field under a no-till system to test the CN model to calculate runoff from rainfall. Using the model standard abstraction ratio of 0.2, the team found that the average CN value that gave an estimate of mean runoff value that matched the measured mean runoff value was 57, which is 16 less than the average of the range of CN values (73) given in standard handbook tables for the catchment. The derived median value of the abstraction ratio was 0.04 (compared to 0.2, the standard value) which is very close to that found by many other researchers of 0.05. Use of standard CN model coefficients and values for fields managed in no-till, and possibly other conservation tillage systems, would likely lead to overestimation of runoff. The commonly used model was not well adjusted to the location soil managed under no-till for long periods. This is not surprising since the original model coefficients were developed from limited data in comparison to the data that have been gathered since its development. Such long-term data are essential for improving models. The Curve Number model is popular throughout the world and these observations should be of interest to a wide variety of users such as teachers, researchers, farmers, environmental groups, regulators, engineers, and water resource managers.

Technical Abstract: Since its inception in the 1950s, worldwide adoption and use of the Curve Number (CN) methodology for estimating runoff has highlighted some inconsistencies, limitations and problems. Analysis of curve numbers derived from 34 years of rainfall-runoff data from a 2.7 ha Georgia Piedmont catchment managed under no-till, showed that the average CN (57) used to estimate mean runoff to match mean measured runoff was 16 less than the average of the range of CN values (73) given in standard handbook tables for the catchment. The derived median value of the initial abstraction ratio (INR) was 0.04, compared to 0.2, the standard value. Many researchers recommend 0.05 for INR. Use of standard CN coefficients and values for fields managed in no-till, and possibly other conservation tillage systems, would likely lead to overestimation of runoff.