Consequences of Changes to the NRCS Rainfall-Runoff Relations on Hydrologic Design

A proposed quantification of the fundamental concepts in the Natural Resources Conservation Service (NRCS) rainfall-runoff relation is examined to determine changes relevant to peak discharge estimation and drainage design. Changes to the NRCS curve number, storage, and initial abstraction relations result in different estimates of the runoff volume, timing of runoff, and peak discharge produced by long-established empirical equations that quantify event-based runoff. The ratio of existing-to-proposed runoff depth estimates is determined across a range of curve numbers and storm depths. Similarly, the ratio of existing to proposed peak discharge is determined as a function of curve number, storm depth, and design storm within the NRCS TR-55 model. Using an infrastructure design based on the existing equations as the baseline, that infrastructure is underdesigned (overdesigned) if the proposed NRCS equations produce larger (smaller) estimates of volumes or discharges than the existing equations for the same watershed conditions. This study shows that the proposed NRCS equations more likely identify existing infrastructure as being underdesigned for smaller storm events and lower curve numbers. Conditions and design storm produce large variation in the design ratios. Storms with return periods on the order of 2–10 years generally result in peak discharge underdesign ratios that span from 1 to as high as 2 or 3 depending on the watershed conditions, storm size, curve number, and design element in question. Existing infrastructure overdesign is more likely the case when return periods approach 25, 50, or 100 years, with typical overdesign ratios ranging downward from 1.0 to as small as 0.8. In most cases, required storage volumes, as prescribed by the NRCS TR-55 approach, are found to be smaller using the proposed equations, leading to both cost savings and reduced pond residence times. Three common design problems are provided only to illustrate typical underdesign and overdesign findings. The central focus is on the potentially negative consequences of proposed changes to a limited aspect of curve number hydrology.