One way to monitor watercourses regarding water quality is by using Water Quality Indexes (WQIs). Currently, there is a lack of information about their behavior in the diurnal cycle as sampling is often carried out in the morning. Also, few papers focus on assessing the urban impact on the spatial variability of WQIs in tropical first-order streams. Such streams receive many pollutants varying in intensity according to population habits, justifying the possible diurnal variation in water quality, in addition to climatic attributes. This paper aimed to evaluate the fluctuations in the Brazilian WQI and its parameters [temperature, turbidity, total solids (TS), hydrogen potential, dissolved oxygen (DO), biochemical oxygen demand (BOD), total phosphorus (TP), total nitrogen (TN), and E. coli] between the morning (8 am), afternoon (2 pm), and night (7 pm) periods for an urban first-order tropical stream. Overall, the lowest DO concentrations and highest values of TS, turbidity, BOD, TP, TN, and E. coli were obtained in the morning, possibly representing population habits: the greatest generation of pollutants occurs overnight and early morning since there are clandestine domestic wastewater inputs into the stream, whose hourly periodicity generates a similar periodicity in the WQI of the evaluated stream. Although there was a significant variation in WQI average values between morning (15.50 ± 1.97) and afternoon (20.83 ± 5.42) only during the dry season (p < 0.05), different results ​​were common throughout the day in all months and the water quality was often classified in distinct categories: ‘very bad’, ‘bad’, and ‘regular’. Our findings present another dimension to be considered when assessing urban water quality, leading to direct benefits to the management and use of urban waters.

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