Exploring an Approach to Estimate Runoff in an Ungauged Mixed Urban Micro Catchment - A Case Study, Pune, India

Sustainable and integrated water resource management needs an hour, and achieving accurate estimation of runoff is key. The decision-making on urban landscaping planning for low-impact development techniques depends largely on the accuracy of rainfall. The haphazardly developed cities in India are encountering flooding crises due to the unexpected expansion. These mixed urban catchments comprise a muddle of residential, commercial, urban-rural, and industrial zones in any combination. Due to this change in urban catchments, the hydrological cycle gets affected and results in elevated runoff volume. The solutions to these are therefore necessary to be planned at a micro catchment level. This paper aims to explore an approach to calculate the runoff of such a micro mixed urban catchment. The geographical scope of this study is the fringe boundary of Pune city. For this ungauged basin, the basic mass balance equation was used to estimate runoff values compared with the runoff values calculated from empirical equations previously developed. From this comparison, it is observed that runoff values obtained from empirical equations were underestimated, which may be due to rapid land-use caused by urbanization. Hence, a need was felt to re-evaluate the coefficients of these empirical models, which take into cognizance the current scenario and its allied changes over the years. An attempt is made to modify the coefficients of empirical equations considering precipitation as the primary parameter. These modified coefficients fetched better runoff results than the runoff results obtained from the coefficients of previously established empirical equations. However, even with these modified coefficients, the runoff results were underestimated, which may be because of not considering the physical characteristics of the catchment in these equations. Therefore, to increase the accuracy of these results, a numerical model that considers these catchment characteristics was chosen. In the present study, a dynamic rainfall-runoff model - stormwater management models (SWMM) is used and compared to assess runoff for an ungauged micro-catchment. The runoff results achieved from these SWMM models better reproduced the hydrologic and hydraulic behavior of the study area (with RMSE of 2.51) by considering detailed catchment characteristics compared to those obtained from all the other empirical models.