Donald E. Woodward

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.

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.

Computer Program for Project Formulation: Hydrology (TR-20) Revisited

The TR-20 computer program originally developed for the Soil Conservation Service in 1964 has been completely rewritten recently. The TR-20 rewrite involves a system of programs including not only a new watershed scale surface water hydrology model with expanded features but also a data converter for old input data, a HEC-RAS cross section reformatter, an input editor, a graphical post processor and eventually a Geographic Information System (GIS) based input generator. The rewritten TR-20 model has a more physically based input philosophy than previous versions and is based on the approach in the AnnAGNPS ( Ann ualized AG ricultural N on- P oint S ource Pollution) model. Most of the maximum number limitations have been removed from the model as well. Input has been expanded to include the flow path for time of concentration and curve number weighting calculations from Urban Hydrology for Small Watersheds (TR-55) as options. The data converter converts as much of the old input data as practical to the new format. The HEC-RAS reformatter can provide the needed stream cross section rating information in the TR-20 input format. The GIS based input generator can be used to develop input from Digital Elevation Models, soils, and land use information data layers. Converted data or GIS generated input can be used in conjunction with reformatted data as a starting point in the input editor to complete the input data. The post processor provides graphical display of model input/output that can be incorporated into reports or other documents.

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