A Diagnostic Application of HEC– HMS Model to Evaluate the Potential for Water Management in the Ratnapura Watershed of Kalu Ganga Sri Lanka

Practicing water managers and engineers require watershed models that could be applied with confidence. This requires already calibrated and verified mathematical watershed models with transparent and rational guidance on the selection of model components representing watershed processes, identification of the objective function, evaluation criteria, data and data resolution etc. This work demonstrates the diagnostic development of a popular and free HEC HMS model for Ratnapura watershed (635 km²) while evaluating the potential for water management with respect to floods, water resources and dry weather flows, using rainfall of four stations, evaporation at one station and streamflow corresponding to the period from 2006-2014. Activities, such as model development, parameter optimisation, evaluation etc., were determined after a rational evaluation. Flood peak magnitude and time of occurrence matched well while water yield estimation, except for questionable data periods, was very good. This reflects that the developed HEC- HMS model with its optimised parameters can be used for flood management with confidence. Most critical flow components for water resources management showed a very good agreement between the observed and modelled streamflow values. High errors in low flow estimation indicated that this model cannot be recommended for environmental flow management.