Variability of the Initial Abstraction Ratio and Curve Number Estimations

Wednesday, May 24, 2023

10:15 AM – 10:30 AM PDT

Room: Del Mar I

Author(s)

John J. Ramirez Avila, Ph.D., Ing., P.H., M.ASCE

Associate Professor
Mississippi State University

Co-Author(s)

MJ

Michael S. Johnson, P.E.

Associate
Buck, Seifert & Jost, Inc.
Glenn E. Moglen, Ph.D, P.E.

Research Hydrologist
Agricultural Research Service
SO

Sandra L. Ortega-Achury, MS, Affl.M.ASCE

Research Associate II
Mississippi State University

Co-Author (not attending)(s)

MM

Michael E. Meadows, PhD, PE

Associate Professor
University of South Carolina

Co-Author(s)

JM

Julianne J. Miller

Research Hydrologist
Desert Research Institute, Las Vegas
Suresh Sharma, Ph.D., P.E.

Associate Professor
YSU

Co-Author (not attending)(s)

ET

Ernest W. Tollner, Ph.D., P.E.

Professor
University of Georgia

The Natural Resources Conservation Service (NRCS) Curve Number (CN) method is widely used and included in many watershed and water quality models to predict runoff from rainfall. In efforts to improve this method’s performance, studies have evaluated the effect of drainage area and slope, storm duration, and the initial abstraction ratio (λ) on the estimation of runoff depth and volume. Because λ also plays an important role in the calculation of the hydrograph peak discharge and the time distribution of runoff, this parameter has recently received more attention. Some research has reported that the standard value of λ=0.2 could provide poor performance in runoff prediction and, in several cases, a value of λ=0.05 has been reported to better predict runoff depths. However, other research has also corroborated the original value of λ=0.2 for specific watershed conditions. Some researchers have reported that neither of these values of λ would predominantly predict runoff more accurately, suggesting caution as λ=0.05 would estimate smaller runoff volumes for storm depths corresponding to larger return periods. Another thought is that a locally observed and verified regional λ parameter might be better because of differences in climate conditions. Members of the Curve Number Hydrology Task Committee are advancing studies to identify the potential effect of CN changes related to λ on the estimation of runoff depth and volumes. Current studies are oriented to validate or refine the existing expressions proposed to modify the CN method from using λ=0.2 to λ=0.05.Preliminary results of these studies will be presented, supported by an extensive literature review and ongoing studies in different settings.