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.

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Ž. The Natural Resources Conservation Service NRCS of the US Department of Agriculture provides assistance for land management planning and the use of conservation measures on private farmland in the United States. The NRCS must concern itself with a broad range of issues with regard to models and their application to support the assessments and decision making associated with these activities. These issues include the basic science for the description of physical processes, user issues in the practical application of the model, and software maintenance. In recent years, a significant amount of effort has gone into implementing existing agricultural hydrologyrerosionrwater quality models. There are, however, some important areas of model development that need to be addressed, including: reconciling the strengths and weaknesses of existing models; accounting for spatial variability of precipitation over the catchment; rectifying weaknesses in the stochastic climate generators currently included in some erosion models; improving the representation of runoff generating processes and water flow paths; Ž . and improving our understanding of ephemeral gully thalweg erosion and including algorithms to describe it. New model development also needs to follow modern standards of software engineering to ensure code reusability and maintainability. Although the NRCS is primarily a model user agency, it must be involved in all aspects of model development as well as model application to ensure satisfactory results. q