Abstract
Cities alter the thermal regime of urban rivers in very variable ways which are not yet deciphered for the territory of Romania. The urban heat island of Suceava city was measured in 2019 and its impact on Suceava River was assessed using hourly and daily values from a network of 12 water and air monitoring stations. In 2019, Suceava River water temperature was 11.54 °C upstream of Suceava city (Mihoveni) and 11.97 °C downstream (Tişăuţi)—a 3.7% increase in the water temperature downstream. After the stream water passes through the city, the diurnal thermal profile of Suceava River water temperature shows steeper slopes and earlier moments of the maximum and minimum temperatures than upstream because of the urban heat island. In an average day, an increase of water temperature with a maximum of 0.99 °C occurred downstream, partly explained by the 2.46 °C corresponding difference between the urban floodplain and the surrounding area. The stream water diurnal cycle has been shifted towards a variation specific to that of the local air temperature. The heat exchange between Suceava River and Suceava city is bidirectional. The stream water diurnal thermal cycle is statistically more significant downstream due to the heat transfer from the city into the river. This transfer occurs partly through urban tributaries which are 1.94 °C warmer than Suceava River upstream of Suceava city. The wavelet coherence analyses and ANCOVA (analysis of covariance) prove that there are significant (0.95 confidence level) causal relationships between the changes in Suceava River water temperature downstream and the fluctuations of the urban air temperature. The complex bidirectional heat transfer and the changes in the diurnal thermal profiles are important to be analysed in other urban systems in order to decipher in more detail the observed causal relationships.
Highlights
An increase in water temperature of rivers all over the world due to global warming is depicted in numerous studies [1,2]
We aim to describe the thermal impact of Suceava city on its stream waters, especially Suceava River, by using high temporal resolution data and multiple analysis methods
We suggest that 2019 can be taken into account as a case study year, which is valuable especially because studies using hourly data on thermal pollution of urban rivers are missing in Romania
Summary
An increase in water temperature of rivers all over the world due to global warming is depicted in numerous studies [1,2]. The increasing urban population worldwide leads to a greater and more territorially focused impact of cities on the environment. This impact, enhanced by the current climate changes, is exerted on urban rivers through multiple anthropogenic stressors such as the increasing imperviousness of catchments or the diminishing of areas with riparian vegetation [3,4]. The higher temperature of urban stream waters reduce their self-purification capacity by affecting the aquatic biota and the amount of dissolved oxygen in the water [10]. Changes in fish populations’ structure and diversity are identifiable in rivers switched to an urban regime, and species that better tolerate warmer waters replace the previous ones [11,12]
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