NRCS TR-55 Quick Estimation Workbook (Excel)

The TR-55 Quick Estimation Workbook operationalizes the principles outlined in NRCS Technical Release 55 (TR-55), 'Urban Hydrology for Small Watersheds', originally published in 1986 and updated with subsequent technical notes. At its core, the workbook integrates land use, soil type, and watershed morphology data to compute the runoff curve number (CN), which governs the partitioning of rainfall into runoff and infiltration based on antecedent moisture conditions (AMC I, II, or III). It estimates time of concentration (Tc) using empirical equations—such as the Kirpich, FAA, or Manning’s equation variants—depending on flow path components (overland, shallow concentrated, and channel flow), then selects an appropriate rainfall distribution (e.g., Type I, IA, II, or III) from NOAA Atlas 14 or older NRCS rainfall frequency data. Peak discharge is calculated via the Rational Method (Q = CiA) for small, impervious-dominant watersheds or via the TR-55 graphical unit hydrograph method for larger or more natural watersheds, generating a triangular or curvilinear hydrograph with time base and peak timing derived from Tc and watershed area. The workbook automates lookups, interpolation, and unit conversions, reducing manual calculation errors and enabling rapid scenario comparison for land development, conservation planning, and regulatory compliance (e.g., NPDES, local stormwater ordinances).