Abstract
This study analyses potential trends in river water temperature (Tw) over a period of 50 years (1960–2009). The study area is located in the Raba catchment in the Polish Carpathians. The aim of the study was to evaluate long-term trends in daily Tw for each season of the year as well as to compare the direction of these trends for sites located some distance upstream and downstream from the Dobczyce Reservoir (DR), built in 1986 on the Raba River. For autumn and winter, increases in mean, median, and minimum Tw are significant for the downstream site (39 km from the DR), with a rate of change of 0.18–0.26 C per decade. Contrasting results were obtained for the spring and summer seasons; significant increases (0.22–0.47 C per decade) in mean, median, and maximum Tw were found for the upstream site, in concordance with seasonal air temperature trends, but a significant decrease in maximum Tw (–0.42 and –0.35 C per decade, respectively) was identified for the downstream site. The revealed discrepancies in the direction of seasonal trends for the upstream and downstream sites studied suggest an anthropogenic impact on Tw downstream from the DR. Overall, a significant increase in summer Tw suggest the need for appropriate flowing water management that would mitigate adverse effects of climate warming on the fluvial environment.
Highlights
Water temperature (Tw) in streams and rivers is one of the most important determinants of aquatic life conditions
For summer (June–August), increases in mean, median, and maximum Tw are significant for the upstream site (0.22–0.29 °C per decade, Figs. 5a–c); in contrast, a significant decrease in maximum Tw was identified for the downstream site (–0.35°C per decade, Fig. 5d)
In the described analysis pertaining to changing thermal conditions in a mountain river, long-term (50year) trends in river water temperature for each season of the year (1960–2009) were found
Summary
Water temperature (Tw) in streams and rivers is one of the most important determinants of aquatic life conditions. The exchange of heat across the water surface is determined by shortwave and long-wave radiation, evaporation, and conduction between water and ambient air; changing meteorological conditions such as air temperature, solar radiation, cloud cover, relative humidity, and wind speed induce changes in water temperature [2,3] to an extent dependent on insulating and buffering processes, which operate within a stream system [4]. The world-wide warming trend in mean surface air temperature of 0.12 °C per decade (1951–2012) [14] together with likely changes in precipitation [15] may create flow and thermal conditions unfavourable for native aquatic biota. Even temperature variation produced by habitat heterogeneity [17] may be insufficient refuge for them
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