Estimation of runoff coefficient and curve number based on observed rainfall-runoff events from contrasting catchments in the urban environment

In most hydrological analyses, it is common practice to select values for the runoff coefficient (RC) and curve number (CN) from standard lookup tables available in literature-based handbooks or engineering manuals. However, these generic values may not adequately account for the distinct characteristics of a given catchment, particularly in urban environments where the heterogeneity of land use/land cover creates a diverse range of hydrological responses. This study focuses on estimating and analyzing the RC and CN based on observed rainfall-runoff events in two contrasting catchments in the city of Ljubljana, Slovenia, namely the urban mixed forest and highly impervious urban area. The analysis of 86 rainfall events that occurred between August 2021 and August 2023 revealed that the two studied catchments demonstrated contrasting runoff responses to rainfall, which could be attributed to their distinct land use/land cover patterns. The urban mixed forest generated an order of magnitude less runoff per unit of rainfall than the urban area. A mean RC of 0.1 was observed in the urban mixed forest, approximately 5 times less than those in the urban area (0.6). These computed mean RC values are lower compared to the tabulated RC values from the American Society of Civil Engineers (ASCE) manual for the given soil type and slope of the specific land use being compared. Similarly, the mean and median CN values in the urban mixed forest are 82.7 and 83.9, respectively, which are lower than the values recorded in the urban area (mean = 95.5, median = 96.8). Additionally, a standard behavior response with asymptotic CN_∞ of 71.7 and 90.7 was observed in the urban mixed forest and urban area, respectively. Thus, the CN values based on the central tendency method appear to be higher than the CN_∞ estimated from the standard asymptotic fit and the tabulated CN values of the Natural Resources Conservation Service National Engineering Handbook (NRCS-NEH). Furthermore, we observed an absence of statistically significant seasonal differences in RC and CN between the growing and dormant seasons in both catchments. However, a bi-monthly analysis revealed a temporal variation in both parameters, with RC peaking in autumn and CN being highest in winter. High-intensity storms in summer and long-duration heavy rainfall events in autumn may have potentially overwhelmed the dry antecedent soil conditions. Hence, examining specific rainfall-runoff events in the urban mixed forest revealed that the initial soil moisture and antecedent rainfall have a contributing role in the observed variations in RC and CN.

 

Acknowledgments: Results are part of the ongoing research entitled “Microscale influence on runoff” supported by the Slovenian Research and Innovation Agency (N2-0313) and National Research, Development, and Innovation Office (OTKA project grant number SNN143972). The study was also carried out within the scope of the CELSA project entitled “Interception experimentation and modeling for enhanced impact analysis of nature-based solutions”.