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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #421879
Research Project: Science and Technologies for Improving Soil and Water Resources in Agricultural Watersheds

Location: Watershed Physical Processes Research

Title: The curve number’s initial abstraction and physical hydrology: ASCE-EWRI CN Hydrology Committee synthesis

Author
item Bonta, James
item CHIN, DAVID - University Of Miami
item JOHNSON, MICHAEL - Buck Seifert & Jost Inc
item MILLER, JULIANNE - Desert Research Institute
item MINERVINI, WILLIAM - New Jersey Department Of Environmental Protection
item RAMIEREZ-AVILA, JOHN - Mississippi State University
item MOGLEN, GLENN - University Of North Carolina
item NEELAM, TAHNEEN - Cornell University
item OLIVEIRA, PAULO - Federal University Of Mato Grosso
item SHARMA, SURESH - Youngstown State University
item TALCHABHADEL, ROCKY - Jackson State University
item TOLLNER, ERNEST - University Of Georgia
item WALTER, TODD - Cornell University
item WARD, TIM - Manhattan College
item WOODWARD, DONALD - Natural Resources Conservation Service (NRCS, USDA)

Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Research Notes
Publication Acceptance Date: 12/18/2024
Publication Date: 5/14/2025
Citation: Bonta, J.V., Chin, D.A., Johnson, M., Miller, J., Minervini, W., Ramierez-Avila, J., Moglen, G., Neelam, T., Oliveira, P., Sharma, S., Talchabhadel, R., Tollner, E., Walter, T., Ward, T., Woodward, D. 2025. The curve number’s initial abstraction and physical hydrology: ASCE-EWRI CN Hydrology Committee synthesis. Journal of Irrigation and Drainage Engineering. 151(4):06025002. https://doi.org/https://doi.org/10.1061/JIDEDH.IRENG-10449.
DOI: https://doi.org/10.1061/JIDEDH.IRENG-10449

Interpretive Summary: The USDA-NRCS estimates runoff from agricultural and other unmonitored areas for planning and design purposes. A critical underlying parameter in the methodology, called the "initial abstraction" (Ia), represents the amount of rainfall that typically falls prior to the start of runoff from small watersheds. Ia represents the losses of water due to soil-water infiltration and interception by vegetation. Soil-water losses are due to the condition of the landscape surface conditions, whether wet or dry, prior to the rainfall event. Other factors such as rainfall intensity and other storm characteristics also affect Ia. There are many justifiable interpretations of Ia, but there is no single interpretation that applies to all climate and physiographical locations. The paper reviews the various physical interpretations of Ia and documents the need for additional research to remove uncertainties currently found in the application of the methodology.

Technical Abstract: The curve number method for estimating rainfall runoff has continually evolved since its creation in the mid-20th century. Because it is inherently an empirical model that is broadly applied to many hydrological applications, the rationales for its parameters have become removed from physical interpretation over its evolution. The ASCE EWRI CN-Hydrology Committee, i.e., some of the foremost experts on this methodology, often finds itself internally at odds regarding many aspects of CN-hydrology. Here, the committee has synthesized its member’s perspectives on the physical interpretations of one CN-method parameter, the initial abstraction (Ia). This note provides insights into how the Ia concept has evolved and offers some context into reasons behind its evolution. This note also synthesized different interpretations of Ia within the framework of different hydrological settings, applications, and spatial scales. Broadly speaking, the committee suggests that Ia might need to be interpreted on a case-by-case basis although this note places reasonable boundaries on Ia for most applications.