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Open AccessArticle

A Pragmatic Slope-Adjusted Curve Number Model to Reduce Uncertainty in Predicting Flood Runoff from Steep Watersheds

1
Department of Agricultural Engineering, University of Engineering and Technology, Peshawar 25120, Pakistan
2
Center of Excellence in Water Resources, University of Engineering and Technology, Lahore 54890, Pakistan
3
Department of Civil and Environmental System Engineering, Hanyang University, Seoul 04763, Korea
4
Department of Civil and Environmental Engineering, Hanyang University, Ansan 15588, Korea
*
Author to whom correspondence should be addressed.
Water 2020, 12(5), 1469; https://doi.org/10.3390/w12051469
Received: 26 April 2020 / Revised: 19 May 2020 / Accepted: 19 May 2020 / Published: 21 May 2020
The applicability of the curve number (CN) model to estimate runoff has been a conundrum for years, among other reasons, because it presumes an uncertain fixed initial abstraction coefficient (λ = 0.2), and because choosing the most suitable watershed CN values is still debated across the globe. Furthermore, the model is widely applied beyond its originally intended purpose. Accordingly, there is a need for more case-specific adjustments of the CN values, especially in steep-slope watersheds with diverse natural environments. This study scrutinized the λ and watershed slope factor effect in estimating runoff. Our proposed slope-adjusted CN (CNIIα) model used data from 1779 rainstorm–runoff events from 39 watersheds on the Korean Peninsula (1402 for calibration and 377 for validation), with an average slope varying between 7.50% and 53.53%. To capture the agreement between the observed and estimated runoff, the original CN model and its seven variants were evaluated using the root mean square error (RMSE), Nash–Sutcliffe efficiency (NSE), percent bias (PB), and 1:1 plot. The overall lower RMSE, higher NSE, better PB values, and encouraging 1:1 plot demonstrated good agreement between the observed and estimated runoff by one of the proposed variants of the CN model. This plausible goodness-of-fit was possibly due to setting λ = 0.01 instead of 0.2 or 0.05 and practically sound slope-adjusted CN values to our proposed modifications. For more realistic results, the effects of rainfall and other runoff-producing factors must be incorporated in CN value estimation to accurately reflect the watershed conditions.
Keywords: initial abstraction coefficient; slope-adjusted curve number; rainfall; precise runoff; model accuracy initial abstraction coefficient; slope-adjusted curve number; rainfall; precise runoff; model accuracy
MDPI and ACS Style

Ajmal, M.; Waseem, M.; Kim, D.; Kim, T.-W. A Pragmatic Slope-Adjusted Curve Number Model to Reduce Uncertainty in Predicting Flood Runoff from Steep Watersheds. Water 2020, 12, 1469.

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