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.

中文翻译：

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环境驱动因素对波兰低地河道夏季水温空间变异的影响

由于对生物区系和其他物理化学水性质的重大影响，溪流和河流的热性质在去年的水文学中得到了广泛认可。尽管如此，尽管开发了新的测量设备和技术，温带低地水道的水温却受到的关注相对较小。因此，当前的研究试图记录低地河道的空间水温变化，并研究环境驱动因素对其热非均质性的影响。为此，从2017年5月至2017年10月，在波兰中部的20个站点使用数字数据记录仪对水温进行了监测，代表了两个空间尺度-主要河流（MR）和支流（TR）。根据测量数据，给出了水温参数的大小和变化的统计分布，而聚类分析（Ward方法）用于对热性质相似的部位进行分组。此外，使用GIS软件为每个站点计算选定的集水量和渠道指标，结合主成分分析可以评估这些指标对热参数的影响。然后，为了支持PCA的发现并评估气象对水温的依赖性，对每日平均水温和空气温度之间进行了线性回归。结果表明，就幅度和变异性参数而言，TR标度位点显示出明显的热异质性，尤其是与MR位点相比，其特征在于即使在单独的，独立的流域；在这些地点，最高的热反差与人为影响有关，例如水库释放和污水流入。如前两个主要组成部分所示，发现水温参数具有清晰的纵向分区，这与从小水源站点到最大下游集水区的过渡有关，这主要是由集水区面积，平均坡度和宽深比的变化驱动的。集水面积的增加还导致线性回归参数发生变化，这表明在下游方向上的气象控制水平更高，同时对地下水流入的影响也较小。获得的结果提供了对低地水道温度行为的新见解，这对于渔业和环境管理具有重要意义，