Evaluating the Options for Streamflow Modelling in Ungauged Watersheds for Sustainable Engineering Designs - A Case Study at Attanagalu River Basin, SriLanka

Water management and infrastructure designs all over the world are mostly associated with ungauged watersheds. The critical task is to estimate the streamflow at a desired location and then to convert the streamflow as design streamflow by incorporating safety factors. In the current setting, practicing engineers find a deficiency in guidelines to select a model and also find out ways to incorporate safety factors ensuring sustainable design of water infrastructure. The guideline of Irrigation Department, Sri Lanka, provides guidance on the use of an empirical model while the Snyder’s synthetic unit hydrograph method and HEC-HMS model are two other popular process-based models opted by most recent modelers to estimate watershed streamflow. The authors carried out a critical literature review and a case study in the Kalu river basin to investigate the issue of model selection faced by ungauged watershed modelers. A streamflow modeler of ungauged watersheds requires to responsibly select a streamflow model, model parameters and safety factors. In this context, the major challenge in the Sri Lankan practice is the unavailability of catchment studies with comparative evaluations of model studies with observed streamflow. The objective of the present work was to first determine the rationale for a design engineer to select a watershed model and parameters when computing design streamflow from an ungauged watershed and then to verify the selection using observed streamflow. Accordingly, the Irrigation Department Empirical Model (IDEM), Snyder’s UH Model (SUHM) and HEC-HMS model were selected and developed for Dunamale watershed of the Attanagalu river basin using methods and parameters from responsible publications. Then the model estimates were critically evaluated by comparing the observed streamflow. IDEM estimates of monthly yield estimations closely matched with the observed data. In the case of overall hydrographs, the IDEM indicated a MRAE value of 0.43 for design rainfall estimation, while the same for HEC-HMS model and SUHM method were 0.63 and 0.61, respectively. The qualitative review based on the uncertainties associated with the computations using IDEM, HEC-HMS and SUHM models classified them into three, as Moderately uncertain, Uncertainty between moderate and high, and Highly uncertain, respectively. The study concluded that, amongst the three selected models, the IDEM is still the best option available for an ungauged watershed modeler to compute design streamflow but pointed to the urgent need for upgrading the ID guidelines with focused research. The study pointed to the importance of identifying appropriate models and parameters for each watershed for meaningful use of process based streamflow models for design of water infrastructure. It was identified that explicit safety factors should be established to incorporate the uncertainties associated with the models and their parameters.