Investigation of nutrient concentration changes in reservoirs with simple budget models

The increasing nutrient concentrations in artificial and natural lakes are one of the main causes behind eutrophication. As a result of the enrichment of organic matter and nutrients such as phosphorus and/or nitrogen, the uncontrolled growth of algae and macrophytes in water bodies, undesired changes occur in their structure, function and stability, with the degradation of water quality. For the last two decades, the two main modelling approaches have been developed to quantify changes in nutrients for the artificial and natural lakes: (a) simple and single empirical equations of budgets (Vollenweider, 1968), and (b) 3-D detailed process-based models. This thesis focuses on the exploration of simple budgetary models and nutrient dynamics in dam reservoirs. The fundamental hypothesis of the present thesis is based on the ability of accurately predicting nutrients using the simple budgetary models. In this context, this hypothesis was tested using in-situ measurements carried out between 2010 and 2014 for dam reservoir Yuvacık. The in-situ data were acquired from the General Directory of Kocaeli Water and Sewage Management. TP concentration varied between 0.014 mg/L and 0.065 mg/L, while TN concentration changed from 0.67 mg/L to 1.450 mg/L in the reservoir. In the thesis, the trophic level of the reservoir was determined according to the trophic level index (TLI), and TP and TN concentrations were estimated using a total of nine Vollenweider budgetary models. R2 values of the budgetary models ranged from 20.02% to 29.7% for TN and from 11.20% to 57.20% for TP. The coefficients used in the models were made site-specific for the reservoir. After the calibration of site-specific coefficients, the R2 values of the models have risen to the range of 23.72% to 34.96% for TN and of 37.06% to 57.79% for TP. The predictive models were developed for the predictions of TP and TN in the reservoir. Cross-validation predictive power of these models was 31.47% for TN and 60.71% for TP.