Urban Stream Temperature Surge—Streamwater Temperature Variability after Rainfall in Suceava City Metropolitan Area

Suceava city is a territory under the urban heat island effect that leads to warm runoffs into small urban streamwaters that are highly impacted by the thermal pollution due to a combination of the small stream discharge and important runoff from impervious urban surfaces. This research detects the frequency and specificities of temperature surges in the Suceava city metropolitan area–40 surges were certainly identified in ~2 years-long time series. We analyzed the diurnal cycles of the stream temperature and correlated the atmospheric precipitation with some heated surface runoffs that resulted in aperiodic increases in the stream temperature. The high variability of rainfalls in the urban area meant that the stream temperature surges occurred rarely in all streams during the same rainfall event, despite the small distances between the watersheds (1–5 km between adjacent water monitoring points). The surges lasted up to 10 h and had an amplitude of up to 5.2 °C; they usually lasted 3.9–5.6 h after 5.8–7.7 mm rainfalls, causing temperature peaks of 0.5–1.4 °C. Additionally, they were easier to detect in the Cetăţii Creek due to its colder-than-natural waters during the warm season—a result of important wastewater discharge. Overall, the surges occurred mostly during the warm and wet semester of the year (87.5%) and especially during the summer (50%). Because Suceava is an average Romanian city, our findings could be taken into account as relevant data at least for cities located in the Moldavian Plateau.