Statistical and graphical analyses of water quality and quantity for the rivers in North Iran (Case study: Polroud and Sefidroud rivers)

Assessing surface water quality characteristics is essential in planning related to water resources management as assessing basin health and managing its changes. Therefore, in this study, a comparative measurement of water quality changes in Polroud and Sefidroud rivers in Gilan province, Iran has been made using statistical and graphical methods. This study used the recorded data of water quality variables in two hydrometric stations of Polroud and Sefidroud rivers from 1999 to 2018. Schuler diagrams were used to evaluate drinking quality, Wilcox for agricultural quality, Piper to use water type, Mann-Kendal method to examine time series, and Pearson correlation test to examine the relationships between parameters. According to the results obtained from the Schuler diagram, the water quality of the Polroud River is in good condition in terms of drinking water over 20 years, and the Sefidroud River is in good condition. The Wilcox diagram of Polroud River showed that the condition of agricultural water consumption of Polroud River is in C2S1 condition, which shows that the water quality condition is suitable for agriculture. Still,  the results of this diagram for Sefidroud River showed that in spring water in The C3S1 area is located, which indicates the average water quality of the river, while in summer, autumn, and winter, the water is located in the C2S1, and C3S1 area, which indicates the fluctuation of water in this area in the good and average range. Based on the Piper diagram, the water of both rivers is corrosive and hard. The results of qualitative statistical analysis of the parameters studied in this study for the two rivers showed that in the period from 1999 to 2018 in the summer, the measured parameters have high values ​​compared to other seasons so that the highest arithmetic mean for the index EC in this season is equal to 290.85 mg L^-1 for Polroud river and 314.66 mg L^-1 for Sefidroud river, Total Dissolved Solids (TDS) index for Polroud river is equal to 246.3 mg L^-1 and for Sefidroud river is 277.5 mg L^-1 and PH index for Polroud river is equal to 7.65 and for Sefidroud River was 7.79. Also, the study of the significance level of the Pearson correlation test for the studied parameters in Polroud showed that the Ca, Mg, Na, and K indices with a significance level of 0.038, 0.042, 0.015 and 0.024, respectively, were directly correlated with the EC index. This is consistent with the results obtained for Sefidroud River that the significance level of Ca, Mg, and Na indices with EC index were 0.024, 0.034, and 0.022, respectively, indicating the direct relationship between them. In general, the results showed that there is not much difference in the qualitative and quantitative changes of the two rivers in the period under study and the role of human influence as a common factor in increasing the pollution in these two rivers is evident.