The effect of thermal discharges from power plants on river temperature - a model based approach : case study - The Danube river

Water temperature, as one of the most important physical characteristics of a river, strongly affects physical, chemical and biological processes in the river system. Natural fluctuations in thermal conditionsof rivers, over the time and space, are influenced mainly by atmospheric conditions, river discharge, topography, and streambed characteristics. However, very often, intensive human activities may strongly modify the pattern in natural behaviour of a river temperature. Many investigations on long-term trends intemperature behaviourof rivers from different parts of Europe have been done so far, however studies about the effects of thermal effluents on river temperature are very scarce. Therefore, the main objective of this research is to definethermal regime of one of the main freshwater resources in Europe, the Danube River. Moreover, this study aims to estimate the effect of thermal effluents from power plants on the river temperature. Data on relevant meteorological conditions (air temperature, air pressure, relative air humidity, wind speed, global radiation, and cloud cover) and estimates of heat discharges for year 2000 were used in modelling the river water temperature. Results of this study showed that, considering spatial scale, the highest water temperature was recorded in the lower part of the Danube, close to the river's outlet. On the other hand, the lowest value was found in the middle part of the Danube. Seasonal variations in Danube's temperatures, as it was expected, were as such that increased values were observed going from winter to summer. However, clear differences among the water temperatures at the beginning and the end of the summer were evident in all parts of the river. In regard to impacts of thermal effluents on the river temperature, results of this study indicate a general trend of smaller impact during summer then in winter. Beside this, along the total watercourse of the Danube relatively low (approximately 0.5oC) contribution of the surplus temperature was observed. However, the results of the Danube's main tributaries indicate that the effect of thermal effluents from power plants located on the Sava was most significant. Additionally, this effect on the water temperature was followed with the low river's discharge in autumn. Performance of 1-D temperature model usedin this study was estimated with the root mean squared error method. Estimated error value for model's performance in modelling river temperature in the study equals 1.54. However, poorer performance of the model was observed incases of rivers' parts with large impoundment areas. Therefore, use of 3-D temperature models may be an advantage in modelling rivers withpossible thermal stratification