Excess Stormwater Quantification in Ungauged Watersheds Using an Event-Based Modified NRCS Model

Abstract

Quantifying runoff from a storm event is a crucial part of rainfall-runoff model development. The objective of this study is to illustrate inconsistencies in the initial abstraction (I (a)) and curve number (CN) in the Natural Resources Conservation Service (NRCS) model for ungauged steep slope watersheds. Five alternatives to the NRCS model were employed to estimate stormwater runoff in 39 forest-dominated mountainous watersheds. The change to the parameterization (slope-adjusted CN and I (a)) leads to more efficient modified NRCS models. The model evaluations based on root mean square error (RMSE), Nash-Sutcliffe coefficient E, coefficient of determination (R (2) ), and percent bias (PB) indicated that our proposed model with modified I (a,) consistently performed better than the other four models and the original NRCS model, in reproducing the runoff. In addition to the quantitative statistical accuracy measures, the proposed I (a) modification in the NRCS model showed very encouraging results in the scatter plots of the combined 1799 storm events, compared to other alternatives. This study's findings support modifications to the CN and the I (a) in the NRCS model for steep slope ungauged watersheds and suggest additional changes for more accurate runoff estimations.