Mathematical Modelling of Watershed Runoff Coefficient for Reliable Estimations to meet the Future Challenges of Water Resources Development in Sri Lanka

Major purpose of water resource development is to enhance water availability. These activities which faces a huge competition requires careful assessment of water reaching a particular point of interest. In this regard runoff coefficient is a quick and popular tool among water engineers and planners. Although the event runoff coefficient provides a simple and clear indication, only limited work could be found on event based or watershed characteristic based runoff coefficient estimates. Present study used event and base flow separations to capture runoff coefficients for selected events and then to develop a regression model for estimation of watershed runoff coefficients. Incorporating the influence of event rainfall, antecedent wetness and average rainfall intensity as independent variables, the model was calibrated and verified the model using 54 events of Attanagalu Oya watershed at Karasnagala (52.6km²). Model performance was very good with Mean Ratio of Absolute Errors showing values of 0.3942 and 0.3567 during calibration and verification respectively. Model coefficients indicated U1at the influence of antecedent conditions was approximately one order of magnitude lower than that of the other two. This model provides water resources developer with opportunity to make better estimates, identify changes to a watershed by observing changes to runoff coefficients, and also to arrive at future resea ch needs. Present runoff coefficient modelling incorporating a systematic calibration and verification process demonstrates the applicability of the method to any gauged watershed irrespective of its location.