# Spatio-Temporal Variability of Water Temperature of Arctic Rivers in Russia over the Past 60 Years
## 1 Introduction  
River water temperature is an important hydroecological factor. Water temperature primarily determines the magnitude of river heat flux and, in part, its spatial distribution, and, through heat flux, the processes of ice formation and breakup in rivers [1] and in coastal zones of seas [2]. To some extent, it also determines the extent of ice cover in Arctic seas [3, 4]. Water temperature also determines the oxygen saturation of river waters and the intensity of various biochemical processes [5], the biological productivity of fish, especially salmonids, and the spatial distribution of their populations [6]. In the Arctic, the role of water temperature in increasing bank erosion and sediment flow has also been noted [7]. Studies on the water temperature in the rivers of the Russian Arctic were initiated in the early 20 th century [8, 9]. During the implementation of the GOELRO plan (construction of numerous power plants) and the beginning of the development of the Northern Sea Route, several expeditions of the All-Union Arctic Institute (now AARI) were carried out. Based on the results of these studies, estimates of water temperatures in major Arctic rivers were made within the framework of assessments of the thermal flow of rivers and its role in their ice regime. In the same years, the network of the hydrological gauging station began to expand, and their program included water temperature measurements. In 1951, the seminal work on river water temperature in Russia of [PERSON] was published [10]. This included spatially differentiated statistics; river water temperature characteristics (monthly mean values, time of water temperature transition by 0.2 \({}^{\circ}\)C); a description of the main determinants of water temperature, intra-annual, and diurnal temperature fluctuations; and classifying thermal river regimes based on the ratio between river water and ambient air temperature. Owing to the rather short observation series at that time and the complete absence of observations in the north of Siberia and the northeast of the USSR, data on Arctic rivers were practically not taken into account. Regional generalizations of the temperature regime of rivers, its governing factors, and characteristics were published in the 1960s and 70s [11; 12; 13; 14; 15; 16]. Multi-year monthly characteristics of water temperature in rivers were often given for observation periods of up to 20 years. Cartographic material of individual regions of the USSR was generally not coordinated with that of neighboring regions. From 1960 to the late 1980s, a number of studies explored actual and potential anthropogenic impacts on the thermal regime of rivers, primarily related to the construction of hydroelectric power stations (HPS) and potential flow diversions [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Since the 1990s, most of the research focused on the effects of climate change on water temperature and the modeling of river water temperature, primarily in connection with the heat flux of Arctic rivers and the assessment of the thermal regime of poorly studied and ungauged river basins [4; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. Most models of water temperature changes on regional scale are created for the lower reaches of the Lena River [7; 33; 37; 42; 43; 44]. Recent water temperature models often do not use hydrological network data, because the majority of models require water temperature data that are at least of daily resolution, while the State Water Register (SWR, in Soviet times--GWR) only publishes 10-day average values (based on twice-daily measurements) and aggregated monthly averages of river water temperature. These data are used only for validation of modeling results, but are often treated as single measurements taken once every 10 days. Observational network data are also used to estimate thermal discharge, most often using the dataset that is limited to 2005 [45].