Perennial fluctuations of river runoff of the Yesil river basin

The features of perennial fluctuations in the river runoff of the Yesil Basin Rivers are considered. The purpose of study is to analyze the spatial and temporal variability of various characteristics of river runoff in the F river basin. In the presented work, the series of annual and maximum water discharges were analyzed, as well as the series for the minimum runoff, based on analysis of runoff long-term fluctuations of the Yesil river basin and the authors' own researches. Hydrological calculations and statistical analysis were carried out using standard Excel and Statistica packages. Based on the processing of surface-based observations (1933-2016 years), as well as the analysis of literature data, conclusions were conducted about the presence of intra-century cycles in the series of the annual river runoff of the Yesil river basin. It was revealed that the Yesil Basin Rivers are characterized by a cyclic runoff with a period of 15-25 years. As a result of the analysis of the long-term variation of the values of the maximum water discharge, noticeable differences in the long-term variation of both the direct values of the maximum discharge and their absolute variability were revealed; it was revealed that the series of the minimum winter and summer-autumn runoff at a significant part of the gauging stations of the Yesil river basin are heterogeneous. The heterogeneity of the runoff series characteristics in the Yesil basin is due to both factors: climatic changes and anthropogenic pressure.


Introduction
The problem of global climate change and its forecast is now given great attention in the world; this problem is reflected, in particular, in the following scientific works [1; 2; 3-5]. According to scientific studies, given in [4], it follows that, at least since the beginning of the twentieth century, there has been an increase in the global problem -by smoothed values on 0.75°С. After a temporary cold snap from the middle of 1940s to the mid-1960s of the last century, there was already a continuous rise in temperature, but this is very indicative, an extremely powerful warming has been taking place since the middle of 1970s. This phenomenon was noted much earlierso, O.A. Drozdov [4][5][6][7][8][9] pointed out that a new warming in the world began in 1973, and on this basis, doubts were expressed about the possibility of predicting future water resources based on long-term observation series. According to the researches of V.P. Meleshko [2;10] the probability of warming since the middle of the twentieth century is associated with the concentration of greenhouse gases over 90 %, it follows that warming will be continue. Furthermore, the gases emission into the atmosphere constantly is increasing, but even with a reduction in emissions, the greenhouse gases already accumulated there anyway will cause an increase in temperature. Consequently, a return to the situation that was, for example, in the middle of the last century, until the mid-70s, is unlikely, and the temporary hydroclimatic situation should be assessed based on the data of only the last 10 years. In Russia, warming is greater than the global average, for the period 1972-2006 years the surface air temperature increased by 1.35±0.4°С [3; 11-20], while during the cold period it increased on average even by 2.5°С [4]. In Kazakhstan, by the 1990s the temperature rise was 1-1.3°С [21; 22], according to [23] only for the period 1954-2003 years the annual air temperature increased by 1.5°C, as showing by the data of long-term series meteorological stations, and at some meteorological stations (Pavlodar, Semipalatinsk), an increase by 2-2.5°C. A significant change of temperature naturally entails changes in other meteorological characteristics, as well as in river runoff, and these changes have been especially noticeable since the 1970s, in particular, since the middle of 1970s the repeatability of zonal forms of atmospheric macro circulation is systematically increasing. At comparing water resources in a significant part of the European territory of Russia for the period 1978-2005 years with the previous period 1946-1977 years it increases is observing [24]. Scientific studies [2; 3; 25-30] contain the following statement in the water resources distribution in the future: in areas of excessive moisture, water resources will be increasing, and in areas where water availability is now insufficient, its further decrease is forecasting. Apparently, this dynamic feature of water resources is also typical for Kazakhstan. Indeed, in the inland regions of middle latitudes, an increase in temperature is causes an increase in evaporation, a decrease in the period of snow accumulation [9; 31-42], which negatively effects on the river runoff.

Material and methods
The Yesil River is a left-bank tributary of the Ertis, the river length is 2450 km, the catchment area is 177000 km 2 , and including an active is 141000 km 2 . The main climatic factors that determine the value of the spring and, consequently, the annual runoff of the Yesil Basin Rivers are snow reserves in the river basin by the beginning of the spring flooding, the intensity of snow melting, rainfall during floods, the degree of autumn moisture and the depth of freezing of soils in the catchment area. Snow reserves are the main source of power for the river. Precipitation during the spring flooding period is of secondary importance in the formation of runoff in the consideration area it amounts to 5-10 % on average, and only in rare years is consist 20-30 % of the amount of snow reserves [43][44][45][46][47][48]. An important formation feature of river runoff in the Yesil river basin consists in the surface retention of melt water, which is quite degree facilitated by a relatively flat relief and a large number of macro depressions. In dry years, almost all runoff is spending on filling the relief depressions, runoff is negligible, and hydrological conditions approach those to the deserts. In high-water years, the relief depressions overflow and produce runoff into the main channel, the existing drainage area is increasing many times over. The main determinants of the minimum runoff are climatic and hydrogeological conditions. The territory of the Yesil river basin is characterized by a sharply arid climate and deep bedding groundwater. Due to the extremely limited reserves of groundwater in river basins, a significant part of the watercourses dries up in summer and the runoff to it resumes only in the spring season of the next year. The rivers runoff, in the feeding of which the groundwater takes a noticeable part, also periodically ceases, but mainly only in winter due to freezing of ripple areas, and sometimes reaches [49][50][51][52][53][54][55]. Low-water period on the Yesil River along its entire length lasts an average of nine months (from July to March). In the summer period, the minimum water discharges were observed in July-August, in the winter periodin January-March. Least of the minimum water discharges accounted on the winter low-water period. In the presented work, the series of annual and maximum water discharges were analyzed, as well as the series for the minimum runoff (minimum average monthly and minimum daily water discharges). Initial materialsobservational data on the network of the RSE Kazhydromet published in the Hydrological Yearbooks, the State Water Cadastre ( Figure 1, Table 1) (annual data on the regime and surface water resources of land) [10; 11; 12; 13 ;14; 56-60]. Selecting the boundaries of the calculation periods is based on the results of an earlier analysis of runoff longterm fluctuations of the Yesil river basin and the authors' own researches [15; 16; 61; 62-66]. Hydrological calculations and statistical analysis were carried out using standard Excel and Statistica packages; maps are executed by using means of software ArcGIS 10.6 (spatial analysis method). All calculations were performed in accordance with the normative document Code of Rules CR 33-101-2003 "Determination of the main calculated hydrological characteristics" and "Methodological recommendations for determining the calculated hydrological characteristics in the presence, insufficiency, absence of hydrometric observations and for assessing the homogeneity of hydrological characteristics and determining their values based on heterogeneous data" [17; 18; 19; 67-70]. Analysis of long-term fluctuations in Yesil river runoff is includes an assessment of quasiperiodicity, trend and statistical homogeneity of the series. For the studied series were obtained estimates of the mathematical expectation Q 0 , variance S 2 , standard deviation S, variation coefficient Cv, autocorrelation coefficients of the study series between the runoff of adjacent years r 1 and their errors [20; 71]. The evaluations were carried out using the moment's method. For autocorrelation, offset corrections are introduced. Difference integral curves were used to isolate phases of increased and decreased water content. The series homogeneity (stationarity) was checked by using the Student and Fisher criteria. Student's test is used to check the homogeneity of hydrological series in terms of mathematical expectation. Applying by the Fisher criterion is estimated the series homogeneity in terms of variance, the Fisher criterion is designed to analyze independent series obeying a normal distribution, the asymmetry coefficients Cs and autocorrelation r 1 were taken into account in the calculation [21; 72; 73].

Results and discussion
Assessment of changes in air temperature and precipitation. According to WMO data for 1906-2005 years the air temperature in the earth's surface has increased by 0.74°C, the last 50 years, the temperature has increased at a rate of 0.13°C every 10 years. In [22], estimates of changes in the annual and seasonal surface air temperature are presented. The increase in air temperature in the winter season of the Yesil water basin is reaches 0.43°C/10 years, the spring air temperature increased at a rate of 0.44°C/10 years, in the summer air temperature the trend was 0.19°C/10 years, the increase in air temperature in the autumn season was 0.46°C/10 years. Modern climate changes on the territory of the considered region are researched in [23; 24; 22; 74-85]. In general, in the last 20 years in Kazakhstan were prevailed significant positive deviations of the average annual temperature of the surface layer and this is typical for all seasons of the year (0.39°С/10 years). On the territory of the Yesil water basin, the annual precipitation increases everywhere up to 300-380 mm. The maximum precipitation in the annual cycle occurs in the summer period, the minimumin the winter with large amplitude. The precipitation of the warm period is 2. Secondly, the river runoff is influenced by economic activity, but it is not the same in different parts of the territory and changes significantly over time. Further, it is quite obvious that general hydroclimatic laws should be better appeared in large basins. In almost all large and medium-sized rivers of Kazakhstan, the climatic runoff is strongly distorted, in particular, by reservoirs, including in the Yesil river basin. At analyzing, it is necessary take account of the features of the runoff series in a significant part of Kazakhstan: exceptional, due to no analogues, low water period in the 1930s, and a very high runoff in the 1940s. Difference integral curves are widely applied to identify the phases of increased and decreased water contents of rivers, the moments these phases change. However, it should be taking account of that it illustrates the course of the accumulated anomaly only relative to the average sample. Analysis of the river runoff dynamics of the Yesil river shows that runoff fluctuations occur cyclically (Table 2,     For the rivers of the Yesil water basin, a rather pronounced tendency to the formation of groups of high-water and low-water years was discovered, the probability of following a high-water low-water year is 50%, after a low-water low-water year 55%. On the other hand, after a high-water year, a dry year should be expected on the river. Yesil with a probability of only 14%, for a dry year the probability of a high-water year is 30%. Thus, on the lowland Kazakhstan rivers is a clearly expressed tendency to the formation of a group of low-water years. Here, in the 30s, the low water period lasted until 10-11 years. The duration of the highwater cycles of the rivers of Central Kazakhstan is short and is 3-5 years. Analysis of the maximum runoff fluctuations. For most of the considered hydrological gauging stations, the following features prevail: a decrease over a long-term period in the values of both Q max itself and its absolute variability σ Q . A decrease in its absolute variability to a certain extent is associated with a number of features of the long-term runoff in general and maximum runoff norm in particular: the proximity in time of very lowwater years 1930s and several outstanding maximum runoffs in 1940 year (3750 m 3 /s -1948 in the Petropavlovsk gauging station). Naturally, in the series that includes both these decades, the runoff variability is increased. It was the presence of several high-water years in 1940 the general subsequent decrease in Q max with time can also be explained. However, these are only the most general features of the long-term course; in different sections, its own characteristics can be observed. In the Yesil river basin after a decline in Q max values (1.5-2 times) from the 20th anniversary of 1956-1975 yearsa new increase in the maximum discharge values, and along the Kalkutan rivermore than 2 times. On the Zhabay River is observed a slight decline in Q max over a long period, for Imanburlyk River since 1950 is a slight increase in the maximum runoff is observed. Relative to the absolute variability of runoffon the rivers of the Yesil, Zhabay and Kalkutan basins, the decrease in σ Q was more gradual, the indicator of runoff variability decreased by half by the 20th anniversary of 1973-1992 years, then its growth began, but in general the value of σ Q is decreased. Analysis of minimum runoff fluctuations. Regarding the minimum water discharges of the Kazakhstan Rivers, its long-term course and state in the modern period is still complete uncertainty. The low-water phase on rivers is observed during the summer-autumn and winter periods. Its total duration can reach 200-300 days or more. On the territory of the Yesil river basin, not only small streams, but also relatively large rivers with a catchment area of the order of 5000-10000 km 2 are drying up. The frequency of drying up of watercourses significantly depends on the hydrogeological characteristics of river basins and it sizes. A number of small tributaries of the Yesil, Silety and Zhabai rivers belong to the non-drying small streams. The cessation of runoff in the summer low-water period is typical for watercourses in the southern part of the catchment. All watercourses located south of the Yesil River, to one degree or another are drying up. On the territory of the catchment's northern part, where moisture conditions are more favorable, along with temporary watercourses, there are also non-drying rivers. The lack of runoff in summer is mostly observed on rivers with a relatively small catchment e area (F<3000 km 2 ). Drying up of larger rivers is observed only in some years. In particular, during the 80-year observation period, the Yesil River dried up for a short time 8 times, and more than a monthonly one time. The Zhabai River near the Atbasar city for the 1937 to 2016 period dried up only twice. Cessation of runoff due to freezing is typical for all watercourses in the territory that retain it runoff until the beginning of the winter season. Small non-drying streams freeze through annually in the second half of November (Yesil river -Prishimskoye). The lack of runoff is observed for about 120-140 days. On relatively large rivers (the Yesil River -Nur-Sultan and the Zhabai River -Atbasar) the duration of freezing is reduced to 80-100 days. During the period with a natural runoff regime (up to 1973) in some rare years, runoff on these rivers was observed once for 5-10 years and less often throughout the year. After 1980, as a result of releases from reservoirs, runoff in these rivers is observed throughout the year. The minimum daily water discharges of the Yesil River -Nur-Sultan during the winter period did not have zero values (after 1980) and varied from 0.018 to 1.73 m 3 /s, the average runoff was 0.54 m 3 /s. On the Zhabay river -Atbasar the minimum daily water discharge varied (after 1980) within the range from 0.094 to 0.80 m 3 /s and averaged 0.45 m 3 /s. Low-water period on the Yesil river is lasts an average of nine months along its entire length (from July to March). In the summer period, the minimum water discharges were observed in July-August, in the winter periodin January-March. The smallest of the minimum water discharges accounted for on the winter low-water period.

Conclusion
For flat Kazakhstan, the following trends are expected: 1. The comparatively simple structure of the surface favors the consistency of fluctuations in both meteorological characteristics and river runoff. 2. The large size of the territory can contribute to certain meteorological and hydrological differences. 3. The Yesil water basin rivers are characterized by a cyclic runoff with a 15-25 years period. 4. It is expedient to calculate the runoff norm in two versions: for a long-term period and for a period characterizing the current climate phase and the current level of anthropogenic influences on runoff. The modern phase of the climate is mainly characterized by the period from the mid-1970s. 5. As an analysis result of the long-term variation of the values of the maximum water discharge, the following conclusions can be drawn: a) Noticeable differences in the long-term course of both the direct values of maximum water discharges and its absolute variability. b) For most rivers, the long-term variation of Q max partly repeats the features of the course of the annual runoff (a general decrease in water discharge, a noticeable decrease since the first half of 1970, an indefinite course even a slight increase in water discharges in the last 10 years).
As a result of the analysis of the long-term variation of the values of the minimum discharges, the following conclusions can be received: It was revealed that the series of the minimum winter and summer-autumn runoff at a significant part of the gauging station of the Yesil river basin are heterogeneous. The inhomogeneity of the runoff characteristics series of the Yesil basin is due to both factors: climatic changes and anthropogenic pressure. So, the concept of runoff stationarity at assessing water resources cannot be accepted as the only one. In a continental climate and poor groundwater supply, many rivers are temporarily active, it dries up in summer and freeze to the bottom in winter. The total duration of the period without runoff can reach 10-11 months.