ACCURACY AND PRECISION OF NRCS MODELS FOR SMALL WATERSHEDS¹

Abstract

In the last 30 years, the National Resource Conservation Service's TR‐55 and TR‐20 models have seen a dramatic increase in use for stormwater management purposes. This paper reviews some of the data that were originally used to develop these models and tests how well the models estimate annual series peak runoff rates for the same watersheds using longer historical data record lengths. The paper also explores differences between TR‐55 and TR‐20 peak runoff rate estimates and time of concentration methods. It was found that of the 37 watersheds tested, 25 were either over‐ or under‐predicting the actual historical watershed runoff rates by more than 30 percent. The results of this study indicate that these NRCS models should not be used to model small wooded watersheds less than 20 acres. This would be especially true if the watershed consisted of an area without a clearly defined outlet channel. This study also supports the need for regulators to allow educated hydrologists to alter pre‐packaged model parameters or results more easily than is currently permitted.

How efficiently do corn‐ and soybean‐based cropping systems use water? A systems modeling analysis

“…Surface runoff is simulated using the USDA‐NRCS runoff curve number technique (Rallison, ; Fennessey et al ., ). The runoff parameter, Cn2bare, was initially estimated from soil hydrological group and field slope information and subsequently adjusted further during calibration to a value of 65 (Table S2).…”

“…Plant water uptake from different soil layers is calculated using approaches from Campbell (1985) and Cowan (1965), as described in detail by Huth et al (2012). Surface runoff is simulated using the USDA-NRCS runoff curve number technique (Rallison, 1980;Fennessey et al, 2001). The runoff parameter, Cn2bare, was initially estimated from soil hydrological group and field slope information and subsequently adjusted further during calibration to a value of 65 (Table S2).…”

Sources of uncertainty in the NRCS CN model: Recognition and solutions

“…Generally, CN is selected from well‐known handbook tables, which have been developed by the Soil Conservation Service (NRCS, ). However, differences between tabulated CN obtained from watershed characteristics and fitted values from rainfall–run‐off data have been highlighted (Banasik, Krajewski, Sikorska, & Hejduk, ; D'Asaro, Grillone, & Hawkins, ; Fennessey, Miller, & Hamlett, ; Hawkins, ), which suggests that a local calibration of the CN value is recommended. Uncertainty on CN determination is based on three main sources: the procedure for the selection of the most representative rainfall–run‐off events, the best methodology to calibrate the parameters of the model (curve number CN and initial abstraction I a ), and the relationship between the mentioned parameters (λ = I a /S).…”

Impact of Residential Soil Disturbance on Infiltration Rate and Stormwater Runoff1

“…The Natural Resources Conservation Service (NRCS) curve number (CN) method is the mathematical basis for the rainfall‐runoff models that are most widely applied to estimate urban runoff, such as TR‐55 (USDA‐NRCS, 1986). In a recent survey, 49 of 50 municipalities in Pennsylvania recommend or require TR‐55 for stormwater management applications (Fennessey et al. , 2001).…”