simulation of water quality in dam’s basins (case study – torogh dam basin)

abstract in this research the thermal stratification of the water in the torogh dam basin and the resulted changes in its quality with respect to temperature, salinity and dissolved oxygen was investigated using the dynamic reservoir simulation model. the results indicated that in wet years such as 1998, when the average annual precipitation and the resulted inflows to the basin is higher than its long-term average, the water level in the basin reaches above 50 m. under such conditions, thermal stratification starts from mid-spring and lasts to the end of summer, and establishes through the total depth of the basin. consequently, considerable changes in water quality occur through the depth. however in dry years such as year 2000, the water level in the basin diminishes considerably (16 to 20m in depth) and thermal stratification either does not establish or if establishes, it is incomplete, starts much earlier and persists for much shorter periods of time. on the basis of the model’s results, the differences observed in the average temperatures of the surface and deep layers of the basin during the spring and summer seasons of 1998 were 10◦c and 11◦c, respectively while in the spring of 2002, the observed difference was only 1.5◦c. the results of the study also showed concurrent occurrence of salinity stratification with thermal stratification which results in increase in salinity with increase in depth. the difference between salt concentration of the surface and deep water layers of the basin was observed to be 43 mg/lit in the spring of 1998 and 10 mg/lit for the summer of the same year. in regard to the dissolved oxygen concentration, the simulation results showed a difference in the range of 0 to 9 mg/lit between the surface and deep water layers. with the start of stratification, oxygen concentration decreases gradually with depth., such that in july the lowest 10m layer of the basin becomes completely anaerobic, providing conditions for production of undesired odors, tastes and color. during the period of 1998 to 2004 (period of data analysis and simulation run), the water quality of the basin was observed to be uniform in autumn and winter months indicating a complete mixing condition in the basin. key words: water quality models, thermal stratification, water salinity, dissolved oxygen, dam’s basin.