Application of SWMM for Water Resources Management by Continuous Rainfall-Runoff Modelling in the Rural Watershed of Kalu River Basin of Sri Lanka

Among freely available rainfall-runoff models, SWMM is described as an easy to use, semi-distributed and conceptual hydrodynamic model. However, its application has been limited to event-based analysis, especially for urban drainage management. This study aims to ascertain the applicability of the SWMM for continuous streamflow simulation in the rural watershed. As such, a watershed model with semi-distributed parameters on SWMM had been developed for the Ellagawa watershed (1342 km²) which is a predominantly rural watershed in the Kalu river basin of Sri Lanka. Observed streamflow and observed rainfall data from 2006 to 2014 of relevant gauging stations were collected and divided into calibration data sets (2006-2010) and validation data sets (2010-2014). The model was calibrated and validated combinedly with visual indicators: hydrograph matching, flow duration curve matching, mathematical indicator: Mean Ratio of Absolute Error (MRAE) and annual water balance error. The model had been calibrated with MRAE (overall: 0.39, high flow: 0.36, medium flow: 0.30 and low flow: 0.17) and an average annual water balance error of 35.5%. Similarly, the model had been validated with MRAE (overall: 0.57, high flow: 0.37, medium flow: 0.64 and low flow: 0.40) and an average annual water balance error of 10%. The hydrograph and flow duration curve matching showed that the medium and low flows are fairly estimated whereas the high flows are mostly underestimated by the model. The study concludes that SWMM is well applicable to simulate longterm continuous streamflow even for the rural watershed and can serve as an instrumental tool for water resources management, irrigation water management and environmental flow management projects.