Trend analysis of River water quality: A case study on Gadar-Chai River

Water quality in rivers is a critical concern in hydrology. This study examines the Gadar-Chai River, situated within Iran's Urmia Lake basin, utilizing data from four selected hydrometric stations: Oshnavieh, Polebahramlu, Bighaleh, and Naghadeh. An improved Water Quality Index (ImpWQI) was calculated for each station using data from ۲۰۰۳ to ۲۰۲۱. Trends in ImpWQI values and the used water quality parameters, including the annual time series of TDS, EC, pH, HCO₃⁻, Cl⁻, SO₄²⁻, Ca²⁺, Mg²⁺, Na⁺, K⁺, and TH, were analyzed in the period ۲۰۰۳–۲۰۲۱. Three significance levels, at ۱%, ۵%, and ۱۰%, were used. The Mann-Kendall (MK), Mod-ified Mann-Kendall (MMK), and innovative trend analysis (ITA) methods, as well as Sen's slope estimator, were used for trend detection and calculating the slope of trend lines. The results showed both upward and downward trends in the time se-ries. MK, MMK, and ITA showed similar results in detecting the trend direction, but differed in estimating trend magnitude. MMK tended to show stronger trends than MK .However, unlike the MK and MMK methods, which only identify monotonic trends, the ITA method is capable of detecting sub-trends. Among all the water quality variables of the Gadar-Chai Basin, the pH parameter had the highest num-ber of positive trends. Trends in the Cl- parameter were negative in almost all the stations. Trends in the ImpWQI index were negative in the three stations, namely Oshnavieh, Polebahramlu, and Naghadeh; however, station Bighale witnessed an upward trend in ImpWQI. A negative trend line slope was observed for the EC pa-rameter in the station Polebahramlou, whereas a positive slope was obtained for the same parameter in the station Bighaleh.

کلیدواژه ها:

ImpWQI ، ITA ، Mann-Kendall method ، Sen' s slope estimator ، Surface water quality