Review of Curve Number Hydrology: State of the Practice by R. H. Hawkins, T. J. Ward, D. E. Woodward, and J. A. Van Mullem: ASCE, Reston, VA 20191-4400; ISBN 978-0-7844-1004-2.

 

Vic Mockus followed Dr. Einstein’s advice when he developed the runoff curve number (CN). He provided a simple-to-use method that was not overly simplified. I imagine that Vic was trying to solve an immediate problem when he donned his innovation cap and put together the CN method. At that time, I doubt that he realized the extent in both time and space that his creativity would impact the discipline of hydrology. The CN method continues to be used a half-century later and all over the world. We should all be so lucky to have such a legacy. The simplicity of the CN method is certainly a factor in its widespread use, but the CN was designed in a way that it was sufficiently flexible and could be used for a wide variety of problems in planning, design, and evaluation.

This book by Hawkins, Ward, Woodward, and Van Mullem is a must read for every person who deals with hydrology. The book would be useful to a student because (1) it provides a basic background on the CN method, and (2) it demonstrates that structural simplicity and conceptual complexity can be achieved simultaneously. It is important for students of hydrology to recognize the implications of CN use even though it is computationally simple. The book is not intended as a primary text for a university course, but it would be valuable as a supplemental text. The book contains important knowledge for the hydrologic designer as it describes the constraints and limitations of the method, which should help in deciding whether or not to apply the method or use a more complex procedure. Fundamental issues such as return period equivalency, drainage area limitations, and nonhomogeneity are discussed and would be relevant when using the method in design. The book will be of value to those in research because the book identifies limits of the method, all of which offer the opportunity to conduct research that could lead to improvements in a method that is a centerpiece of hydrology. Just a few CN issues mentioned where research is needed include the adaptation of the CN model to continuous moisture accounting models, identifying and evaluating reasons for storm-to-storm variation of CN, and remote sensing assessment of CNs. The book also acts as a comprehensive literature review of the subject, which will help provide a good starting point for research. The reference list is very comprehensive, with entries to 2007. Those who are responsible for developing drainage manuals, design practices, or storm-water policies will benefit from the book as the content can lead to guidelines for proper practice with curve number hydrology, such as on the issues of antecedent moisture or the initial abstraction coefficient. In summary, the book contains something for everyone.

The 73-page book (plus 33 pages of appendices, including 27 pages of references and bibliography) is divided into four chapters. The chapters after the introductory chapter focus on the curve number method, findings, and development, and then the concluding chapter, which is far more knowledge-meaty than would be suggested by its title or length. Chapter 1 (“Introduction”) is a potpourri of well-written thoughts, including some historical notes, clarifying points about just what constitutes the curve number method, and its roles in applications.

In Chapter 2 (“Curve Number Method”), the basic method is described and then followed by important points about its application. The content initiates discussions of controversial topics such as the assumption of equivalent return periods of rainfall and runoff, the questionable drainage area limitation on the CN method, and whether or not the CN method implies an infiltration model. Definitive answers to these issues are not given, nor should they be expected, as these are unanswered questions throughout the domain of hydrologic modeling. Many of these application concerns are relevant to other hydrologic methods such as unit hydrograph modeling. The chapter discusses many other issues (e.g., soils, units) that every reader has needed to consider. It is important that these points are raised in the book, as they are central to its proper application and, in some way, serve as red flags about the use of the CN method.

Chapter 3 (“Findings and Developments”) includes discussions of many aspects of the CN method from its sensitivity to the calibration of CNs, from a component like the initial abstraction to the issue of antecedent soil moisture, and from GIS and remote sensing applications to current practices. With such a diverse array of subjects, it is difficult to summarize the content of the chapter. Let it suffice to say that those who are intending to use the CN method should be familiar with all of these issues. The book provides a concise, well-written summary of each topic. The discussion demonstrates that the CN method is much more than a simple estimation method that uses a rainfall depth and a number taken from a table. The model has implications of watershed losses and their time distribution when the CN equation is applied with a time distribution of rainfall. While the discussions are brief, they are sufficient to provide a reasonably full understanding of the issues. If the references cited are consulted, the coverage is fully adequate.

Chapter 4 (“Summary, Conclusion, Discussion, and Recommendations”) is a “must read” for the researcher. The content of this eight-page chapter is rich in research needs. The current status of the CN method is discussed with respect to issues such as continuous modeling, its universality, the $I_a$ versus $S$ coefficient, and calibration to local conditions. These are just a few of the issues discussed, but it will generate considerable thought to anyone who has ever used CN hydrology. The curious researcher can use the content of the chapter to conduct research on the CN method.

A book review should identify both the strong points and weak points of the book. The breadth of coverage in Curve Number Hydrology is certainly a strong point, but increased depth of coverage on a few topics would have been helpful. The extensive list of references and bibliography is a definite positive. Also, the authors avoided drawing conclusions that were unsupported by analyses; therefore, the discussions are unbiased. A more complete CN table would have been helpful. A more comprehensive index would also be helpful to the reader. All in all, the positives overwhelm the negatives. I suspect that this book will be widely read and widely referenced, as it should be. In some ways, the book is a history book that focuses on one narrow but important part of the growth of the field of hydrologic analysis and design. Many of the CN topics discussed are relevant to other hydrologic methods, such as the calibration of the coefficients, the return period issue, and the effects of soils. If a book was written about the rational method or continuous simulation, the structure used by the authors in writing Curve Number Hydrology: State of the Practice would be good model.

In the Hawkins et al. book, many of the concerns about the basis for the CN methods are discussed. These issues should be of concern to some users, especially those using CNs in design. However, the decades of successful use of the CN method suggests that it is a piece of hydrologic machinery that works and works very well. Experience has shown that the CN “machinery” does what it is supposed to do in spite of the questions about the underlying theory. I expect that the CN method will continue to serve the hydrologic community for decades to come.