Richard H. Hawkins

Runoff Curve Number Method: Examination of the Initial Abstraction Ratio

The Initial Abstraction ratio (Ia/S, or λ) in the Curve Number (CN) method was assumed in its original development to have a value of 0.20. Using event rainfall-runoff data from several hundred plots this assumption is investigated, and λ, values determined by two different methods. Results indicate a λ value of about 0.05 gives a better fit to the data and would be more appropriate for use in runoff calculations. The effects of this change are shown in terms of calculated runoff depth and hydrograph peaks, CN definition, and in soil moisture accounting. The effect of using λ=0.05 in place of the customary 0.20 is felt mainly in calculations that involve either lower rainfall depths or lower CNs.

Curve Number Hydrology : State of the Practice

An appendix provides solutions to the curve number equation. This book will be valuable to water and environmental engineers involved in hydrology, especially the analysis of rainwater runoff problems.

Runoff Curve Numbers for 10 Small Forested Watersheds in the Mountains of the Eastern United States

AbstractEngineers and hydrologists use the curve number method to estimate runoff from rainfall for different land use and soil conditions; however, large uncertainties occur for estimates from forested watersheds. This investigation evaluates the accuracy and consistency of the method using rainfall-runoff series from 10 small forested-mountainous watersheds in the eastern United States, eight annual maximum series from New Hampshire, West Virginia, and North Carolina, and two partial duration series from Georgia. These series are the basis to compare tabulated curve numbers with values estimated using five methods. For nine of 10 watersheds, tabulated curve numbers do not accurately estimate runoff. One source of the large uncertainty is a consistent decrease in storm-event curve numbers with increasing rainfall. A calibrated constant curve number is suitable for only two of 10 watersheds; the others require a variable watershed curve number associated with different magnitude rainfalls or probabilities...

Curve Number Determination Methods and Uncertainty in Hydrologic Soil Groups from Semiarid Watershed Data

AbstractFour curve number (CN) determination methods are evaluated from 16 watersheds in the southwestern U.S. using 1,284 events that satisfy rainfall and runoff criteria. The use of ordered pairs versus natural pairs of rainfall and runoff data has a larger effect on the CN, whereas the difference from using a partial duration series versus an annual series was not significant. The best-available USDA soil series data were obtained for 20 Arizona watersheds and 10 groups of New Mexico natural runoff plots. The hydrologic soil groups (HSG) were determined from either direct USDA assignment or textural properties and compared with the HSGs required by the CNs and the cover condition. This study showed a standard error of about one HSG, resulting in an error in CN of approximately seven units when using the best-available data. Compared with USDA handbook table values, the CNs found from rainfall and runoff data were higher for 21 of the 30 semiarid watersheds.

Curve Number and Peakflow Responses Following the Cerro Grande Fire on a Small Watershed.

The Curve Number (CN) method is routinely used to estimate runoff and peakflows following forest fires, but there has been essentially no literature on the estimated value and temporal variation of CNs following wildland fires. In May 2000, the Cerro Grande Fire burned the headwaters of the major watersheds that cross Los Alamos National Laboratory, and a stream gauging network presented an opportunity to assess CNs following the fire. Analysis of rainfall-runoff events indicated that the prefire watershed response was complacent or limited watershed area contributed to runoff. The post-fire response indicated that the complacent behavior continued so the watershed response was not dramatically changed. Peakflows did increase by 2 orders of magnitude following the fire, and this was hypothesized to be a function of increase in runoff volume and changes in watershed network allowing more efficient delivery of runoff. More observations and analyses following fires are needed to support definition of CNs for post-fire response and mitigation efforts.

Curve Number Method: Time to Think Anew?

As most hydrologists know, the curve number method is popular, enduring, ubiquitous, versatile, and widely used to calculate event rainfall-runoff volumes. To many, it is quite comforting. It is also badly in need of some updating. Originally created on short notice in the mid 1950s for the ad hoc needs of Public Law 566, the curve number method was targeted at and developed for agricultural uplands and overland flow. With its origins in the USDA, PL566 (a USDA program) was preordained for acceptance, and its handbook-guided use in planning and design led to millions of dollars of cost sharing and grants. Thus it was accepted without much question. Happily, it also fit nicely into a waiting technologic niche in the emerging science and profession of hydrology. It had an assuring aura of cutting edge. In this setting, it was too big to fail. As a working man’s hydrology, it is simple, transparent, and appealing, and it is true to its USDA origins in that soils and land condition play major roles. At that time, an alternative model with the benefits of the curve number method was not available, and there still is not. Within its genus, it is monotypic. It serves as the poster boy and hypothesis for rainfall-runoff, and its terms and concepts (however approximate) serve a vocabulary role for the general hydrology case. The curve number concept was developed in the “quiet past” of Lincoln’s statement and may be “inadequate for the stormy present” of modern hydrologic engineering. It is overdue for an overhaul for several compelling reasons. Experiences with the procedure and comparisons with other rainfall runoff studies over the past 50 years have been both eyeopening and unsettling. Many tables and assumptions posted in the foundation documents [NEH4/630; USDA Soil Conservation Service (1954); USDA Natural Resource Conservation Service (2003)] are not matched by on-the-ground data or supported by critical analysis. Curve numbers (CN) tables based on soils and cover are often wide of the mark, a condition exacerbated by the model’s demonstrated primary sensitivity to the choice of the CN. Furthermore, some watersheds were found to perform quite differently from the basic CN runoff response patterns, leading to great differences between the model and reality. The inferred internal infiltration sequence is questionable. The hydrologic soils classifications, a supposed strong point considering the source agency, seem internally inconsistent and vague. These observations, and a host of others like them, become more common as applications depart from the rain-fed, agricultural upland, large-storm settings that spawned the original development. The expository literature continues to mount (Hawkins et al. 2009). At the same time, temptations to apply it well beyond its simple upland agriculture origins have grown in response to modern needs. It has been an easy off-the-shelf filler hydrology for rainfall-runoff and other targets, such as urban and river basin hydrology and agricultural and water quality models. Many of the latter apply creative extensions of the CN method to daily time-step continuous models. It is now comfortably embedded in flood control, environmental impact, and sediment-erosion methodologies world-wide. Finally, times and expectations have changed. There are now five decades more data upon which to base methodologies, much better means of analyses, and a greater variety of recognized lands and land uses. Equally important, as a profession, more is expected now than was expected in the mid 1950s. In its birth years, the CN method did not experience professional peer review, and most of the foundation data and calculations have since been lost. Alas, it was established by administrative fiat. This is now an age that values open communications, scientific cross-pollination, freedom of information, consilience, stakeholder participation, data sharing, and intellectual honesty. Is it not time to nudge quality and credibility up a notch? How and under what auspices might such needed renovations occur? Considering its development, origins, and history, and in a spirit of noblesse oblige, an unavoidable onus of leadership rests with the USDA. Via the image and authority of the USDA, the user world has followed its lead for over 50 years, but in the 21st century the user community should be an active contributor, too, perhaps through professional societies such as ASCE, American Society of Agriculture and Biological Engineers (ASABE), and American Water Resources Association (AWRA). It is no small task and promises to create discomfort. These issues are raised here to provide food for thought, grist for discussion, and, hopefully, reader feedback in these pages.

Storm Water Management in Exurbia

In urban and suburban settings, storm water management and design is a well-developed and widely practiced profession, but the situation in exurban areas is far different. Beyond the metropolitan fringe, lower residential densities and smaller capital budgets limit storm water improvements, which heighten the need for sound storm water management by planners and private property owners. This chapter describes the impact that dissection of landscapes by infrastructure can have on storm water and stream stability in exurbia. It considers how, and why, roads and utilities and the siting of homes and exurban subdivisions impact drainage networks and contribute to flood hazards. Recommendations for dealing with storm water management in exurbia are also discussed.

Progress Report: ASCE Task Committee on Curve Number Hydrology

The preliminary findings of the ASCE/EWRI Task Committee on Runoff Curve Numbers are presented, and review comments, contributions, and critique are solicited prior to final submission. The major findings are given in condensed form in the major topic areas of: A review and restatement of the method; Development, analysis, and study results from field data; Current use and professional applications; Critique, and Summary.

The Complacent-Violent Runoff: A Departure from Traditional Behavior

The Complacent-Violent rainfall-runoff response pattern is presented, documented, and illustrated. The recognition of this rainfall-runoff behavior is urged in investigations, design, and analysis. Contrasts are made with the traditional upward-open and losslimited curvilinear response as represented by the CN method. These naturallyoccurring alternate forms of substantially different structure and scale are described. The Complacent form exhibits a linear and low-response relationship between rainfall and runoff. The Violent behavior springs from an ongoing Complacent event and is further defined by an unpredictable higher response at some higher rainfall depth threshold. The Complacent-Violent response is a pattern described by the by the Complacent fraction C, the threshold critical rainfall Pt, and the Violent response fraction b2. Illustrations are presented for Complacent-Violent rainfall-runoff responses based on data from numerous small watersheds. Typically, but not universally, these watersheds are in forest or wild-land cover and have a base-flow component feeding a perennial (non-ephemeral) stream. Conceptual models are offered for such rainfallrunoff behavior, and its generality is presented. BACKGROUND Setting. Portrayal of the runoff response to rainstorms is basic to hydrology and environmental science. With direct applications such as drainage design or flood plain delineation, or in post-event analyses, the Curve Number (CN) method for estimate runoff depth form rainfall depth is also critical in several comprehensive hydrologic models as the rainfall-runoff component. In some environments, rainfall generated runoff is the entire surface water supply. Further, the CN method reflects the upland soils and vegetation, linking upstream land management and downstream hydrology. Considering event direct runoff as a function of event rainfall, a unit input-output