Hydrological Modelling with the Tank Model for Water Resource Management of Nilwala River Basin

Presently, hydrologic modelling is widely used for the quantitative and qualitative estimation of water resources. Such estimations are the baseline to develop strategies for sustainable water and environmental management. In this study, applicability of a daily lumped conceptual model was evaluated to quantify water resources of Nilwala River at Pitabeddara (291.4 km(2)). A spreadsheet model developed using Sugawara's Tank Model concept was calibrated and verified by using a daily dataset from water year 2008/09 up to 2017/18. The performance of the model with a four-tank structure was evaluated by using an objective function to match daily hydrographs and the flow duration curves while observing the adequacy of annual water balance estimations. Model performed very satisfactorily with Mean Ratio of Absolute Error (MRAE) values of 0.31 and 0.43 during model calibration and validation, respectively. Most importantly, the best results were in the intermediate flow regime. Streamflow estimations at monthly scale had an average accuracy level of 78% with an average water quantity error of 6% per month. The present work by carrying out a systematic development of a four-tank lumped conceptual model, demonstrated the capability to successfully estimate daily and monthly streamflow at the selected watershed. Hence, the model and its parameters can be confidently used to sustainably manage water resources of the selected watershed and similar watersheds.