Water quality modelling for Sardar Sarovar Reservior located in Narmada River basin

Abstract

A water quality model is a mathematical representation of water quality processes that occur within a water body. Water quality models are developed for assessing the water quality changes that can occur as a result of the stress (Natural or man-made) imposed on the system. Water quality models for rivers, lakes and marine areas have been used in water quality planning and decision-making. The verified model is used for forecasts of expected water quality under a variety of potential scenarios. Sardar Sarovar is a terminal dam in the series of dams so far constructed in Narmada basin. The projects on the upstream are Maheshwar, Omkareshwar and Indira sagar. The downstream impacts are directly related to the manner in which upstream dams are operated and all types of developments, including water resources, in the upstream area that can affect the water quality. The present mathematical model study is to know the distribution of water quality parameters viz. Biochemical Oxygen Demand (BOD), Dissolved Oxygen (DO), Nitrate (N) and Phosphate (P), within the reservoir during normal flow year (2004-2005). The water quality data was collected during field investigations, covering three seasons. The data for summer and winter represented the non-monsoon scenario. Hence, the respective data sets were used for calibration and verification of the model. In order to simulate the characteristics of hydrodynamic field and mass transport processes in the Sardar Sarovar reservoir a 2-D coupled model of hydrodynamics and water quality (MIKE 21) was used. The hydrodynamic model was driven by employing observed data of winds, temperature and daily measured flow to simulate the seasonal water cycle of the reservoir. The water quality related state variables in this model included BOD, DO, Nitrate and Phosphate. The simulation of the mass transport and transformation processes of BOD, DO, Nitrate and Phosphate was based on the unsteady diffusion equations, driven by observed meteorological forcing and external loadings. The model was calibrated and verified by using the data observed from Sardar Sarovar Reservoir (Non-monsoon and Monsoon). The field-observed data about water quality showed the variation of average DO, BOD, Nitrate-Phosphate through the reservoir over one year as BOD : 0.611 - 1.73; DO : 6.519 - 7.83; Nitrate : 0.19 - 0.857; Phosphate : 0.006 - 0.35 mg/l. The model parameters were calibrated for one month (in Non-monsoon) and 5 days (in Monsoon) depending on the flushing time. Using the field-data of different periods for same time intervals further validated the model. Model variables like BOD-decay constant, Concentration of BOD from upstream boundary, Half Saturation and Temperature were found to be sensitive in the given order. The ranges of standard errors for simulated and measured BOD, DO, Nitrate and Phosphate concentrations, as percentages of the measured values, were 0.47. 12.56; 0.18 - 2.67; 1.28 - 6.87 and 0.09 - 3.80 respectively, hence, there was a good agreement between the simulated and measured values. The simulation also showed that, in Sardar Sarovar, the quality of inflow-river influences much the quality of reservoir. This calibrated and verified model can be used for prediction of expected water quality under a variety of potential scenarios. Results indicates that both the macrophytes are capable in improving water quality by reducing nutrient concentration and uptake heavy metals too. Purification of water through macrophytes is a good example of natural means.