The effect of environmental drivers on summer spatial variability of water temperature in Polish lowland watercourses

Abstract

Thermal properties of streams and rivers, due to significant impact on biota and other physico-chemical water properties, were broadly recognized in hydrological literature last years. Nevertheless, water temperature of temperate lowland watercourses has received relatively small attention, despite the development of new measurement devices and techniques. Thus, the current study attempt to document spatial water temperature variability of lowland watercourses and examined the effects of environmental drivers on their thermal heterogeneity. For this purpose, water temperature was monitored from May to October 2017 with the use of digital data loggers in 20 sites located in central Poland, representing two spatial scales—main rivers (MR) and tributaries (TR). On the basis of the measurement data, statistical distribution of magnitude and variability water temperature parameters was presented, whereas cluster analysis (Ward method) was used to group sites similar in their thermal properties. Furthermore, selected catchment and channel metrics were computed using GIS software for each site, which in combine with the principal component analysis allowed to assess the effect of such metrics on thermal parameters. Then, to support the findings of PCA and assess meteorological dependence of the water temperature, linear regression between daily mean water and air temperatures was performed. The results indicate that in terms of magnitude and variability parameters TR scale sites demonstrated clear thermal heterogeneity, particularly in comparison to MR sites, characterized by similar thermal properties even between separate, independent catchments; in such sites the highest thermal contrast were related to anthropogenic impacts, such as reservoir releases and sewage inflows. Clear longitudinal zonation of water temperature parameters was found as presented by first two principal components, which was related to transition from small headwater sites to the largest, downstream catchments, driven mainly by changes of catchment area, mean slope, and width:depth ratio. The increase of the catchment area also resulted in a shift in linear regression parameters, which suggested higher meteorological control in the downstream direction and simultaneously, lower impacts of groundwater inflows. The obtained results provide new insight into lowland watercourses temperature behavior, being of primary significance in the context of fisheries and environmental management, particularly in the face of climate warming and increasing anthropopressure.