INTRODUCTION

It is well-known that the river water conditions reflect specific spatiotemporal features of water inflow from the surface of a river basin, which control the stable alternation of higher and lower runoff phases. In turn, the type of water conditions is based on seasonal differences in the river supply sources due to long-term and seasonal changes in the weather and climate conditions. In particular, precipitation accumulation as snow and temperature peculiarities of the cold period affect the basic features of the river water conditions in the Ural River basin: spring floods and winter low water [1].

The main incoming component of the river runoff is atmospheric precipitation; in this regard, long-term fluctuations are considered as the major factor in the development of high- or low-water cycles. In contrast to atmospheric precipitation, the surface layer temperature is an indirect river runoff factor. Alongside with that, temperature variations lead to transformation of the moisture and material exchange processes between altitudinal–spatial differentiation elements of river basins (atmosphere, vegetation, aeration, and saturation zones) [2]. As a result, the climate conditions affect the seasonal precipitation and evaporation variability, as well as basic features of intra-annual variability, accumulation, and consumption of moisture reserves in the river basin [3].

The long-term climate transformations (first of all, an increase in average annual air temperatures) are an indisputable and confirmed fact. The most intensive global warming has been observed over the past 30–40 years starting from the 1980s; each subsequent decade was warmer than the average for the previous period (since 1850) [4]. Within Russia, this increase has been more than 0.45°C/10 years [5], and by the middle of the twenty-first century, the annual increase may reach from 0.7 to 2.6°C depending on the season and region [6]. In addition, Russia is dominated by the trend toward an increase in annual precipitation: it is 2.2%/10 years, on average, for the period of 1976–2019. The greatest increase in precipitation is observed in spring (5.7%/10 years), and the lowest one is in summer (0.7%/10 years) [4]. In general, according to the World Meteorological Organization, 2011–2020 was the warmest decade on record. It should be noted that 2020 was the warmest year in Russia for 130 years of regular meteorological observations. The absolute temperature maximum was reached almost in all federal districts (except for the North Caucasian region) [4].

The global climate changes cause variations in hydrological cycle components and, first of all, an increase in atmosphere moisture capacity and preci-pitation rate [7]. The precipitation rate, as well as the atmosphere moisture, increases under climate warming [8]. Nevertheless, the current warming is characterized by a lower temperature gradient between the pole and the equator, regional circulation lowering, and a higher frequency of atmospheric blocking events [9].

The global climate transformations are characterized by notable regional features [10]. It should be noted that the regional response to a long-term increase in the average temperature of the climate system is nonuniform due to the interaction of local weather–climatic and physical–geographical conditions (topography, landscape structure, forest cover, etc.). The higher average temperature and the lower air humidity play an important role in the warm period in the steppe drainage areas of the Ural River. First of all, these conditions result in a reduction of river runoff and an increase in compensatory groundwater losses. In addition, the recorded steady increase in winter season temperatures is a major factor in the increasing low-water runoff in the rivers of the basin under study.

In connection with the foregoing, the purpose of this study is to identify regional responses to temperature variations of the surface air layer and atmospheric humidification conditions within the Ural River catchment area.

RESEARCH OBJECTS AND METHODS

The initial data included meteorological observation ranges for a 70-year period (1950–2020) at 14 meteorological stations (MSs) located in the Ural River basin and adjacent areas situated within large geographic regions such as Obshchii Syrt, the Cis-Urals, Southern Urals, and the Trans-Urals. The parameters analyzed included the temperature of the surface air layer and the amount of precipitation in daily, monthly, and annual terms. The conclusions on the regional features and the climate trends in the basin studied were made on the basis of standard statistical analysis methods. We calculated the linear trend coefficients and evaluated their statistical signi-ficance through the determination ratio (R2), taking into account two reliability levels, such as p < 0.01 (1%) and p < 0.05 (5%).

The deviations of considered meteorological parameters were calculated at some MSs. According to the World Meteorological Organization’s (WMO) recommendation, the 30-year period of 1961–1990 was taken as the baseline. Anomaly values such as N.A. Bagrov’s coefficient and V.G. Tokarev’s index were calculated [11].

RESULTS AND DISCUSSION

In terms of the average annual temperature rate, the Ural River basin belongs to the same zone as the European part of Russia characterized by an increase of 0.4–0.5°С/10 years [5]. According to the multimodel estimates, it is expected that, compared with the baseline period (1980–2000), the annual temperature in the Ural River basin will increase by 1.6°С by 2030 [6]. The steady increase in surface temperature within the study area is confirmed by the scenario forecasts obtained from the CMIP 5 ensemble of global climate models (Table 1).

Table 1. Scenario forecasts of surface air temperature growth within the Ural River basin (Russian part) based on CMIP 5 models [12]
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The expected climate warming in the study basin will be due to higher temperatures in all seasons, but the maximum increase in temperature is predicted for winter and spring. These predictions are generally consistent with the current regional trends of European Russia. By the period of 2080–2099, within the Ural River basin, the annual air temperatures are expected to increase from +3.6°С (RCP 4.5 scenario) to +6.3°С (RCP 8.5 scenario) with maxima in winter +4.3°С and +7.6°С, respectively.

The annual average temperature trends of surface air are statistically significant: the trend component (R2) varies from 12% (Kuvandyk) to 25–26% (Aidyrlya and Belyaevka) and 36–42% (Bredy, Uralsk, Orenburg, and Atyrau). The long-term annual and seasonal precipitation dynamics is devoid of unidirectional and statistically significant trends.

The climate warming is confirmed by statistically significant coefficients of the linear trend of the average monthly temperatures (Fig. 1). The coefficients are in the range of 0.30–0.38°С/10 years for most MSs. The exception is the Kuvandyk MS (0.15) situated in a specific basin area and three MSs located in different parts of the basin (Sharlyk, Belyaevka, and Aktobe): 0.27–0.29°C/10 years. The highest increase in temperature was recorded in the lower sector of the basin (Ilek, Uralsk, and Atyrau MSs), as well as in the Trans-Urals (Bredy MS): 0.33–0.34°С/10 years. The maximum contribution to the growing average annual temperatures is made by the first three months of the calendar year (January–March) and the autumn months (October–November). The most significant and ubiquitous increase in temperatures is observed in March (0.70°С/10 years, on average).

Fig. 1.
figure 1

Monthly coefficients of the linear trend of season temperatures in 1950–2020 (in terms of ten years). Black columns, 1% of confidence level; gray columns, 5%; white columns, no statistical significance.

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In general, the recorded temperature variations of the surface air layer in the Ural River basin correspond to the macroregional trends revealed in European Russia [13].

It should be noted that winter is an important period to accumulate snow and water resources in the preflood period and to replenish groundwater supplies that feed the river in the summer low-water period.

According to the study results, the increasing temperatures during the cold season lead to a widespread increase in the thaw occurrence frequency, duration, and rate. The long-term trend recording the onset of thaws is specific at different MSs within the studied basin and is related primarily to the features of their geographical location. Meanwhile, despite the spatial heterogeneity, the temporal distribution of thaw dates is general and synchronous. Below there are diagrams of the total positive air temperatures for decades of winter months in terms of ten years distributed over 30-year periods (Fig. 2).

Fig. 2.
figure 2

Distribution of total positive air temperatures for decades of winter months (XII–II) over 30-year periods.

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A steady upward trend in the thaw rate was recorded in the recent period (1991–2019). First of all, it is necessary to note an increase in the total positive temperature on the first ten days of December, reaching the maximum (–49.2) at the Orenburg MS (1961–1990, –36.5). An almost identical situation was observed in February, mainly in the third decade: the maximum (–30.0) was obtained at the Zilair MS (1961–1990, –8.7).

One of the consequences of temperature variations in winter, in particular, in the Ural River basin, is rising river water during the winter low-water period [14]. As a result, the basic characteristics of water conditions have been transformed in Kazakhstan-type rivers: the reduction in spring floods and the increase in low-water periods caused, in turn, a certain alignment of the annual runoff hydrograph.

In addition to a steady and statistically significant increase in the average annual air temperature, the positive normalized anomalies also confirm long-term changes in the regional climate in the Ural River basin (Table 2).

Table 2. Changes in the temperature anomaly indices of the surface air layer in the Ural River basin
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According to Table 3, normalized anomalies of the average annual air temperature (1991–2020) at MSs located within different parts of the Ural River basin have a positive sign from 0.8 (Aktobe) to 1.1 in the lower reaches of the Ural River (Atyrau and Uralsk). The anomaly indices also confirm the identified trend of long-term temperature variations within the study basin. First of all, it is necessary to note a steady growth of Tokarev’s index: the maximum increase in days with abnormally high temperatures is observed in the period from 1991 to 2020. In this period, Tokarev’s index has a positive sign at all MSs, changing from 1.28 and 1.30 at Orenburg and Belyaevka MSs to 1.56, 1.89, and 1.90 at Uralsk, Atyrau, and Zilair MSs, respectively.

As noted above, the spatiotemporal specificity of the annual and seasonal amounts of precipitation plays a leading role in the river runoff. In contrast to the steady increase in surface air temperatures, no similar long-term trends of atmospheric precipitation were revealed in the Ural River basin. A statistically significant increase in annual precipitation was noted only in the western part of the Ural River basin (Ilek MS, 10 mm/10 years; Sharlyk, 13 mm/10 years) and adjacent areas (Buzuluk MS, 18 mm/10 years). No statistically significant changes in the atmospheric humidification conditions were revealed in the rest of the study area. The absence of statistically significant trends in the annual and seasonal humidification in the steppe regions of European Russia is explained by the mutual compensation of multidirectional moisture variations in different seasons [15].

The intra-annual precipitation distribution in the Ural River basin is characterized by an inconsistent course of changes in normalized atmospheric precipitation anomalies (Fig. 3).

Fig. 3.
figure 3

Seasonal deviations of the amount of atmospheric precipitation (1961–1990), Orenburg MS.

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Negative normalized precipitation anomalies were recorded in summer and autumn. For instance, the average values were –14.3 mm and –4.6 mm in 1990–2020, respectively, at Orenburg MS. On the contrary, normalized anomalies of spring and winter precipitation were dominated by positive values (5.1 and 0.3 mm, respectively) in this period.

The predominant positive anomalies of atmospheric precipitation in spring and winter in the Ural River basin are consistent with the macroregional trend of higher precipitation in the cold period, which is common for most of European Russia. Within the study basin, this trend was revealed in all sectors of the catchment area, except for the lower reaches (Atyrau MS). In 1990–2020, a maximum increase in precipitation in the cold period was recorded in the middle reaches of the Ural River (Orenburg MS, +10%; Akbulak, +13%) and in the mountain forest catchment areas (Kuvandyk MS +7%; Zilair +10%).

Alongside with that, it should be noted that humid conditions of the prewinter period are of great importance for the water content of the Kazakhstan-type rivers due to the relatively rapid setting of stable negative temperatures. In particular, melting of the temporary snow cover at low positive air temperatures and slight evaporation contribute to an increase in moisture reserves in the soil cover [16]. In the basin under study, the last 30 years were characterized by degra-ding natural moisture conditions in the prewinter period. This fact is confirmed by the widespread prevalence of negative normalized anomalies of November precipitation. The maximum deviations were determined in the mountain forest areas (Kuvandyk MS, 24.6 mm; Zilair, 27.5 mm), while the minimum occurred in the lower reaches (Atyrau MS, 8 mm). It should be noted that the precipitation amount decreases in the prewinter period against the background of a steady and statistically significant increase in average temperatures of the surface air layer.

CONCLUSIONS

The analyzed regional effects of global climate changes in the Ural River basin confirm the stable transformation trends of temperature conditions clearly determined in the river runoff parameters. In particular, the warming during the cold season results in a widespread increase in the thaw frequency and duration followed by a water level rise in the rivers during the winter low-water period. Alongside with that, diverse physical and geographical conditions of the regional climate result in a spatial nonuniformity of the response to global and macroregional changes. The climate is the most stable in the low mountains of the Southern Urals including a large river runoff area in the basin under study. In general, the analysis of regional climate changes is relevant for promptly sol-ving the problems of guaranteed supply of water resources to the steppe zone and developing measures to adapt water consumption to changing conditions.