Upstream discharge planning of regulated rivers and its relationship with downstream water quality parameters can be a good indicator for quantitative and qualitative surface water resources management, especially in arid and semi-arid regions. The present study aims to evaluate the effects of discharge released from Sefid Rud Dam on downstream water quality parameters using the Qual2kw 6 model and the YASAIw plugin. In this regard, samples were collected from the Sefid Rud River through 12 sampling stations at a distance of about 110 km and measured for 12 months over the period 2007-2020. Of these, eight samples (December 2007, October 2008, November 2008, February 2011, July 2008, April 2011, June 2011, and August 2011) were used for calibration, and four samples (Nov19, Feb20, Aug19, and May20) were used to validate the Qual2kw 6 model. Subsequently, the criteria of scatter index (SI), coefficient of determination (R2), and mean bias error (MBE) were applied to compare the modeling results and the measured data. The modeling results indicated the high accuracy of the Qual2kw model in the calibration and validation steps given scatter index (SI) <0.1 for dissolved oxygen (DO), pH, and temperature parameters. The highest values of R2 in the two stages of calibration and validation were 0.63 and 0.70 for the parameters of discharge and electrical conductivity, respectively. The sensitivity analysis results showed that the parameters of biochemical oxygen demand (CBODu), electrical conductivity (EC), and pH were more sensitive to upstream changes compared to dissolved oxygen (DO), total phosphor (TP), and total nitrogen (TN) parameters in both flow modes (low and high water). Totally, the results showed that it is possible to manage the changes in downstream water quality parameters of the river by controlling the amount of flow released from the Sefid Rud Dam separately during high and low water months.
