Changes in the Water Temperature of Rivers Impacted by the Urban Heat Island: Case Study of Suceava City

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.

Water Quality Index of Suceava River in Suceava City Metropolitan Area

“…The hourly distribution of the minimum and maximum values is similar to those of SC and ORP at the upstream station and to those of DO and pH at the downstream station (according to the diurnal cycles of the water quality parameters described in our previous study [35]). The DO oscillation downstream is bound to the water temperature; water temperature downstream is influenced by the Suceava city urban heat island and wastewaters [36]-the higher thermal amplitude might explain the higher influence of the DO diurnal profile on the shape of the WQI diurnal profile at the downstream station. The atmospheric environment does not influence only the water temperature, but also its evaporation.…”

“…At both monitoring stations, the evaporation and evapotranspiration in the riparian area and sub-catchments play an important role in the oscillation of the water level, which, in turn, influences the water quality parameters. The catchments of the urban tributaries of Suceava River in the study area have surfaces belonging to the built-up area of Suceava city that increase the temperature of urban tributaries up to 2 • C [36] above the water temperature of Suceava River at the upstream station, leading to changes in the values of the water quality parameters due to the modified chemical reactivity and water viscosity. Built-up areas have a negative impact on water quality of rivers [49,50] and groundwaters [51].…”

“…We applied the wavelet coherence method of Grinsted [42] for creating WTC scalograms of the selected parameters, which indicate the time when periodicities in two time series are coherent. The wavelet analysis used Morlet as mother wavelet because this nonorthogonal wavelet function has proven to provide good results in wavelet coherence analyses of geophysical time series [35,36]. The cone of influence on scalograms is created by the edge effects specific to finite time series.…”

“…It has an average flow rate of 16.87 m 3 /s at Iţcani station [34]. It collects urban tributaries that transfer the impact of the urban areas into their collector (chemical and thermal pollution [35,36]). Downstream of Suceava city there is the wastewater treatment plant (WWTP) of Suceava city, which collects most wastewaters of the metropolitan area.…”

The influence of treated wastewater from the Lodz city agglomeration on the ice regime and water temperature of the Ner river

“…It also resulted in a visible increase in the amount of relatively warm wastewater being discharged into the river. Untreated industrial and municipal wastewater discharged today to the CWTP LA treatment plant and previously discharged directly to rivers had and has a relatively stable temperature, although, during negative air temperatures (in winter) and after thaws, it is much higher (by about 5 °C) than the temperature of river waters (Brzezińska 2011;Briciu et al 2020). Such a consumption increase was possible due to the commissioning of a second pipeline supplying water to Lodz from the Pilica river (water pumping began at the end of 1963).…”