The Dez River Water Quality Evaluation in the Most Critical Section and Drought Conditions by HEC-RAS

It is necessary to use mathematical models to determine the river's response to pollutants. These models are able to simulate the effect of existing and future pollutants and facilitate related decisions. In this study, the water quality of Dez River and its main feeding drains were extracted and analyzed through sampling and laboratory analysis from Dez diversion dam downstream of Loreh drainage during seven months from March to September 2018. The results of the measured samples were compared with those of the simulated values through the HEC-RAS model. The river path was investigated in the study period and seven sampling stations were determined along the river and six stations at the point of drainage into the river to determine the sampling stations, as well as the sources of pollution of Dez River. The results indicated the lack of self-purification of DO, BOD, NO3, NO2 parameters in Dez River during seven months of 2018 due to the inflow of drains and lower river discharge in this period. Also, the base on NSFWQI index, in drought seasons the first half of the river quality index is less than 68, that indicates average quality, and in the second half of the river, the quality index is less than 50 (poor quality). This decreasing trend of NSFWQI quality index is consistent with the trend of qualitative variables of HEC-RAS simulation and indicates the lack of relative self-purification of the river during the study period.