# Spatio-Temporal Variability of Water Temperature of Arctic Rivers in Russia over the Past 60 Years
## 4 Discussion  
The studies of long-term changes in water temperature in the Baltic countries are most similar to our study in terms of the type of initial data and partially in terms of their methods of processing. Figure 13: R\({}^{2}\) of regressions of air (ERA 5) and water (hydrological stations) temperatures anomalies for 1991–2020. The studies [57; 58] show a very slow increase in the average annual water temperature of \(\pm\)0.01-0.02 \({}^{\circ}\)C/decade (1945-2010) in the rivers of the Baltic countries. The water temperature increases during this period at a rate of \(\pm\)0.02-0.03 \({}^{\circ}\)C/decade in July. In the hydrographic regions of the north of the European part of Russia and the north of Western Siberia, the average July temperatures are 1-2 \({}^{\circ}\)C lower than those in the Baltic states, but the increase in water temperature over the 1961-2021 period averages 0.41-0.49 \({}^{\circ}\)C/decade, which is an order of magnitude greater. At the same time, a significant and almost ubiquitous trend towards an increase in average annual water temperatures (using the same measuring technologies) has been confirmed in this region of the Baltic states [59]. To date, the intra-annual distribution of water temperature and changes within each month of the warm season has not been described, as far as we are aware. A great deal of attention has been paid to the issue of long-term changes in water temperature in China. However, there have been no publications on this topic for the mountainous regions of Northwestern China that could be used as an analog for comparison with the Arctic part of Eastern Siberia. [PERSON] [37] describes changes in air temperature in this area and the intensity of these changes, which are similar to those in the southern parts of the hydrographic regions of Eastern Siberia that we are considering. The analysis of long-term water temperature observations in Europe and China has been carried out for many rivers, primarily based on daily measurements. In most studies, the leading factor in determining water temperature is air temperature, which can be influenced by factors such as the terrain, the amount of precipitation, and the elevation of the catchment area. The intensity of changes in water temperature over the past few decades, as studied in certain areas of China and southern Europe, has shown that the increase in average annual water temperatures is significantly lower than in some Arctic regions (for example, [60; 61; 62]). An analysis of long-term data from Central Europe [63] showed that the average annual increase in water temperature is slower than the increase in air temperature. This finding is confirmed in our study of the Arctic territories at the level of individual months. Meanwhile, air temperature data from meteorological stations or specially installed meteorological sensors are often used in our work. In particular, we use reanalysis data, which, if the weather station is within 50 km, show similar relationships with water temperature. Reanalysis data are more often used to create global models of water temperature formation. The analysis of the results from global models of river water temperatures is challenging, primarily because they mostly provide information about the average annual temperature [64] or use data from hydrological stations on the largest rivers only [65]. However, for the average annual water temperature, high-resolution global models show trends in the Arctic region (1960-2014) that are similar to those observed for the average monthly temperature in our study (tenths of a degree), at least for the major tributaries of major Arctic rivers. According to the IPCC report [47], the water temperature in rivers around the world varied between \(-\)1.21 and +1.076 K per decade in the period from 1901 to 2010. In the northern regions, above 60 degrees north, the rate of change was approximately +0.35 K per decade. For the Russian Arctic, this report used information from the lower reaches of major rivers, primarily from [35].