Sustainable Water Management in Kalu River Basin of Sri Lanka: Comparison of Watershed Yield Computations for Water Security

Abstract

Irrigation reservoir design associated with ungauged watersheds requires reliable watershed streamflow estimations. In practice, designs are either based on the empirical model of Irrigation Department guideline or based on recent conceptual mathematical models. Use of empirical models require appropriate design levels for the calculation of design streamflow. Popular mathematical models require adequate guidance for the application of safety factors to the model outputs. The two aspects that need attention are, the reliability of streamflow estimations by models and the incorporation of safety factors when converting model outputs as design streamflow. The main gap in the literature is a critical evaluation of the importance of these two factors with real-life applications. The authors, in a review paper, identified that best available method for yield estimation is the Irrigation Department Guideline Method (IGM) but pointed to the necessity of verification with field observations. Accordingly, the present study compared the most commonly used IGM, HEC HMS and the Unit Hydrograph method (UH) models by carrying out an evaluation of yield estimation in Kalu Ganga watershed at Ellagawa. This study, while focusing on the availability of model parameters for streamflow assessments in ungauged watersheds, aimed at the demonstration of a comparison in a watershed which has data and established model parameters. It is expected that this model demonstration would facilitate the selection of an appropriate yield estimation model not only in wet zone but also the dry zone of Sri Lanka. Spreadsheet models were developed for each method while using literature reported model parameters and incorporating best engineering judgment. Comparison of model estimations with observed data was done for annual and seasonal water balance, and for flow duration curves, while using Mean Ratio of Absolute Error (MRAE) as the numerical indicator. Evaluation of outputs from the three methods showed that the IGM estimates of monthly yield estimations closely matched with the observed data. MRAE for Overall Hydrograph, in-case of IGM, HEC HMS, UH methods were 0.35, 0.58 and 0.4, respectively. The necessity of incorporating yield thresholds for modern hydrological models and updating of available guidelines were highlighted. The study revealed the necessity of establishing design guides for selection of model parameters when computing streamflow in ungauged watersheds. IGM with guidance for parameters was recognized as the best model amongst the selected for monthly yield estimation. HEC and UH results create significant ambiguities when compared with IGM because of the unreliability associated with the selection of processes and parameters. This study pointed to the necessity of considering safety factors, as clearly pointed out in Irrigation Department guidelines. This is vital when using modern mathematical models.