The main purpose of the paper was to identify day-and-night variations of the selected physico-chemical water characteristics (discharge, electrical conductivity, temperature, pH and dissolved oxygen) of a small urban river that had been heavily transformed by human activity (Sokołówka river catchment with the area of 7.71 km²). The data came from measurements conducted in the period between 4 October 2011 and 3 October 2012, using the YSI 6920 V2 (multi-parameter probe) and ISCO 2050 (automatic current meter), in 15- and 10-minute intervals, respectively. The research has proven the existence of rhythms of diurnal and seasonal variability to the selected water characteristics. The variability of the hourly values of the tested characteristics in particular days depends to a large extent on the value of the analysed feature itself. The variability coefficients of discharge and conductivity increase with the increase in the values thereof. For dissolved oxygen, pH, and water temperature, the variability coefficients decrease with the increase in the values thereof. In the case of water pH, its conductivity, and temperature, the correlations between the given feature and its variability observed at different times of day are not linear, but they run along loops of varying curvature. The average day-and-night variability of the Sokołówka river’s SEC and pH was significantly changed as a result of human impact. The maximum readings of the conductivity and the pH shifted to the morning hours, with no change to the minimum readings.
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