Characterization of long-term seasonal climate and its impact on runoff of Kharkhiraa River in Western Mongolia

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Characterization of long-term seasonal climate and its impact on runoff of Kharkhiraa River in Western Mongolia

Otgon Shinebayar, Master student at University of Chinese Academy of Science and Xinjiang Institute Ecology and Geography; Li Lanhai, Professor at Xinjiang Institute of Ecology and Geography, Shareef Muhammad, PhD graduate (researcher) at Xinjiang Institute Ecology and Geography, Chinese Academy of Science Mukanyandwi Valentine Master student at Xinjiang Institute Ecology and Geography, Chinese Academy of Science

Abstract

The present study intended to assess the impact of changing climate on runoff of the Kharkhiraa River in western Mongolia by analyzing long-term climate and runoff data, which mainly included daily mean temperature, total precipitation and daily mean runoff for the period of 1975-2015. Regression and path analysis tools were used to analyze the annual and seasonal changes in regional climate, and its direct and indirect impact on river runoff. Results revealed that annual daily mean temperature significantly increased by 2.85°C from 19752015 with an increase of 4.22°C in winter, 4.79°C in spring, and 1.63°C in the autumn. In contrast, total mean annual precipitation significantly decreased during study period by -20.91 mm, with the maximum of -25.78 mm in summer season.

Consequently, the daily mean runoff decreased by -0.17m³ s^-1, with a maximum of -1.35 m³ s^{- 1} in summer season during 1975-2015, which can be directly linked with long-term variation in precipitation and indirectly with temperature of the Kharkhiraa river basin. The impact of precipitation on winter and spring runoff was more pronounced than other seasons. Likewise, path analysis also showed more distinct direct effect of annual, winter and summer precipitations on daily mean runoff, than daily mean temperatures of neither annual nor seasonal, except in winter. Thus, it is concluded that the impact of total precipitation on runoff generation of Kharkhiraa river has been much larger than that of daily mean temperature.

Keywords: Climate change; Kharkhiraa river basin; Path analysis; River flow.

Introduction

Water resources in arid and semi-arid regions are normally generated from geographically elevated mountainous areas where, the precipitation occurs either as rainfall or snowfall. In the past century, evidences of global climate change have been reflected in increasing temperature, increased variability of precipitation, and reduction in the areas of both glaciers and snow cover [1-3]. In addition, the increased population and associated human activities have aggravated changes in the water resources and their spatial and temporal distribution within the region, and also affected the local ecological conditions [4]. Several studies have explored the effects of changing temperature and precipitation on water resources and the runoff of rivers [5, 6]. In the United States, an increase in the average temperature and precipitation was observed through the 20^th century, where the climatic change affected both the volume and time-based pattern of river runoff [7].

Temperature rise, especially in the cold seasons, is negatively correlated with snowpack [11, 12]. However, the snowpack dynamics are affected by the changes in precipitation forms and snowmelt regimes, which are primarily influenced by the variation in temperature [13]. The water sources of rivers in mountainous basins basically comprise of melt-water from the snow-covered peaks or glaciers in the high-altitude mountains [14], seasonal snowmelt in the mid-range Mountains, and occurrence of rainfall in low elevation mountains [15]. The seasonal snowmelt and precipitations often result in the highest river flow during both spring and summer seasons [16]. The highest flow occurring due to seasonal snowmelt during the months of April and May is referred as spring snowmelt highest flow [17], and the highest flow driven by the precipitation occurring between June and August is referred as summer highest flow [16]. Both the spring snowmelt peak flow (SSPF) and summer peak flow (SPF) can cause severe damage by floods in lowland areas, which could be reduced or avoided through a thoughtful and precise understanding about the characteristics of longterm climatic variation and efficient mitigation strategies in a specified region.

In Mongolia, the mean annual air temperature increased by 1.56°C from 1940s to early 1990s, particularly because the winter and spring/fall temperatures increased by 3.6°C and 1.5°C, respectively, while the temperature in summer slightly decreased by 0.3°C [18-20]. However, temperature rise during 1940-2003 was recorded even higher, indicating clear warming from the beginning of the 1970s and intensifying towards the end of 1980s, which reveals the general warming trend for the entire Mongolia [21]. In addition, it is also pointed out that such changes in temperature varied both in space and time from 1961 to 2001, in which the warming was 4°C in winter and 0.9°C in summer in the Khovd region of the Altai Mountains, and only 0.8°C in winter and 0.5°C in summer in Dalanzadgad of the southern Gobi [22]. A slight increase in precipitation during the second half of the 20^th century also occurred, and these changes were statistically insignificant [23]. However, the scenario generated by existing climate change models shows a temperature rise with 2-8°C from 2000 until 2099 for Mongolia, both in summer and winter [24].

Compared with the baseline time period of 1961-1990, the time slices of 2020s, 2050s and 2080s, Batima [23] proposed a future winter warming of 0.9-8.7°C and summer warming of 1.3-8.6°C, with a precipitation change by -3% to 11% in summer, and by 13-119% in winter, whereas, snow cover would decrease by 27-51%, and the evapotran- spiration increase by 13-91%. The changing pattern of temperature and precipitation and its consequences on melting of glaciers and ice covers are obvious, which have eventually affected the rivers runoff all over the globe. Different opinions have been reported on the impact of increasing temperature on runoff volume. Some studies showed that increasing temperature can reduce the streamflow in snow melting season because of less glacier mass caused by glacier retreat [25-27]. Others concur that increasing temperatures tend to induce the glacier retreat and increase runoff through a rise in melt-water amount [28-30]. The forest-steppe ecotone of semi-arid Mongolia located at the northern fringe of Central Asia between the arid Gobi-desert zone in the south and cold-humid Taiga of Siberia is the western part of Mongolia, which is highly sensitive to environmental changes.

The climatic conditions in this region are usually controlled by global circulation system of different climatic components [31-33]. Topographically, the region is dominated by Kharkhiraa and Turgen mountains, which are covered by a thick layer of snow, occurring from late autumn till early spring [34]. The melt water coming from these mountains is the source of two different rivers, which later join together and are known as Kharkhiraa Turgen river basin. However, the pattern of long-term seasonal climatic variation in this region and its impact on the flow of the Kharkhiraa river has not been reported in literature so far. Therefore, the objectives of the present study were to analyze the variations in seasonal temperature and precipitation patterns, identify the key climatic factors influencing seasonal and annual runoff, and their direct and indirect impact on the Kharkhiraa river runoff through path analysis with long-term climatic and runoff data.

Study area

This study takes the Kharkhiraa River basin as study area, which belongs to central Asian drainage basin in the north-west of Mongolia that includes the Uvs Lake and Tes river basin. With a peak altitude of 4037m above sea level (asl), the river originates from the Kharkhiraa mountain ranges and drains after 126.5 km into Uvs Lake. The Kharkhiraa mountain coordinates are 49°35'8.36 "N, and 91°22'3.49 "E with snow cover above a height of 3078 m asl, from where the river originates: so, named as Kharkhiraa river. The river begins from 2982 m asl of the Kharkhiraa Mountains as an unnamed small river with some frozen white swamps, which at 2922 m asl, combines with small pins of snow, ice, glacial, and rainwater. The rivers flow from the mountainous steppes of the Kharkhiraa Mountains, and along with some other small unknown rivers drains into the Uvs Lake with the coordinate of 50°04'13.5” N, 92°27'20.5” E. The total catchment area of Kharkhiraa river basin is 3332.7 km². However, the total area above Tarialan hydrology station is 738 km² at 2502 m asl, which has been the trust source of river runoff and climatic data. The mean width of river is 10-30 m, the depth is 0.25-0.5 m, and the mean flow rate is 1.0-1.5 m/s.

Methodology

This study employed the long-term weather data of annual and seasonal temperature (°C) and precipitation, obtained from a meteorological station located in Uvs province as it is the only one in the area and the runoff data of Kharkhiraa river (m³/s), obtained from a hydrological station which situated near the Kharkhiraa river. Both stations are located in the areas around the Kharkhiraa river, whose geographical information are listed in (Table 1).

Figure. 1 Geographical location of the study area and stations (Tarialan hydrological station and Ulaangom metrological station) on the map of Mongolia.

Table 1. Geographical information of the hydrological and meteorological stations from where the climate and runoff data were obtained

Regression analysis

Simple regression analysis was used to define the types of correlations and describe the changes in characteristics of temperature, precipitation and runoff time series in this study [35]. While, the linear equation of regression analysis, developed by Xuemei, Lanhai [35] was used to estimate hydrological and climatic parameters.

Where, y is precipitation, temperature (°C) or runoff (m³/s), and Я1 and Я0 refer to regression slope and the intercept, respectively, while t indicates the time.

Path analysis

Correlations among the climatic and hydrological parameters were analyzed in this study, which is the measure of relationships between dependent and independent variables. However, path analysis is found to be the most suitable method for segregating direct and indirect effects of correlating variables [36]. The relations in the path analysis is sketched in a diagram (Figure. 2) in which the double-headed arrow shows correlation between the effect causing variables, while the single-headed arrows point from the causing variable to the affected variable. This analysis gives a path coefficient which reveals direct effect of the variables of cause on the variable of effect. The coefficients determined from correlation analysis are standardized, while a path coefficients of regression analysis are unstandardized [35].

The estimated path coefficients are inscribed with two subscripts. Path coefficient from the variable X1 to variable X2 is denoted as r12, r21, which indicates the indirect effect on Y (runoff). On the other hand, coefficients from X1 to Y and X2 to Y are denoted as P01 and P02 which reflect the direct effects of independent variables (X) on dependent (Y), respectively. The correlation index R² was measured statistically that reflects the reliability by which the cause variables explain the effect variables.

The path coefficient is defined as, if the correlations between the cause (X1, X2) and effect (Y) variables exist, then they can be estimated using following equations [35].

In these formulas, r10 is the coefficient of correlation between the effect X1 and result Y variable, while r20 indicates correlation coefficient between the cause X2 and result Y variables. However, r12,and r21 are the coefficients of correlations between the cause variables Xi to X2 and X2 to Xi, respectively. Whereas, P01 is path coefficient from variable X1 (temperature) to variable ions in the path analysis is sketched in a diagram (Figure. 2) in which the double-headed arrow shows correlation between the effect causing variables, while the single-headed arrows point from the causing variable to the affected variable. This analysis gives a path coefficient which reveals direct effect of the variables of cause on the variable of effect. The coefficients determined from correlation analysis are standardized, while a path coefficients of regression analysis are unstandardized [35].

?10 = P01 + л12 P02

?20= P02 + л 21P01

Y (runoff) and, P02 is the path coefficient from variable X2 (precipitation) to result variable Y(runoff).

Overall, P01 and P02 in equations 2 and 3 indicated direct effects of cause variables Xi and X2 on result variable Y, respectively. On the other hand, r12 P02 in equation 2 and r21P0i in equation 3 represent the indirect effects of variable X1 (temperature) on result variable Y (runoff) through X2 (precipitation) variable, and the variable X2 (precipitation) on the result variable Y (runoff) through variable X1 (temperature), respectively. The ultimate aim of this path analysis was to understand relationships between the hydrological (runoff) and climatic (temperature and precipitation) attributes of the study area, based on available data.

Results

The analysis of long-term annual and seasonal climate of the Kharkhiraa river basin, western Mongolia reveals an overall warming in the study region (Figure. 3). Trend line shows significant rise in mean annual temperature from -4.0°C (1975) to -1.3°C (2015), indicating an annual daily mean temperature (AN-DMT) rise by 2.85°C from 1975 to 2015.

Figure. 3 The trend analysis of the annual mean temperature (AN-MT) and total precipitation (TP) time series during the period of1975-2015 in Kharkhiraa River basin.

On the other hand, long-term seasonal analysis showed significant (P<0.05) variation in seasonal daily mean temperature (DMT) from 1975-2015 (Figure. 4). It is obvious from the trend lines that DMT of winter (WI), spring (SP), summer (SU), and autumn (AU) seasons changed from -33°С, -4°C, 16.50°C, and 0.7°C in 1975 to -25.0C, 0.38C, 18C, and 2.1C in 2015, respectively. In addition, detailed analysis showed that WI-DMT, SP-DMT, SU-DMT, and AU-DMT increased by 4.22C, 4.79C, 0.86C and 1.63C, respectively (Figure. 4), revealing significant change in the temperature in the study site.

Characteristics of long-term precipitation

Long-term climate data (1975-2015) of the Kharkhiraa river basin show that total annual precipitation successively declined from 157 mm in 1975 to 120 mm in 2015 (Figure 3).

Figure. 4 The trend analysis of temperature and precipitation for season 's (WI-DMT, SP-DMT, A U-DMT, SU- DMT, and WI-TP, SP-TP, SU-TP, AU-TP) time series during the period of1957-2015 in Kharkhiraa River.

However, its declining trend on an average was determined as -0.5099 mm yr^-1. From another aspect, the observations show that average annual total P (AN-TP) during the period of 1975-2015 was 144.20 mm with the maximum total P of276.8 mm in 1986 and the minimum of 69.6 mm in 1980. In addition, the trend lines show that total precipitation in winter (WI), spring (SP), summer (SU), and autumn (AU) changed approximately from 9 mm, 12 mm, 105 mm, and 25 mm in 1975 to 10.8 mm, 17.5 mm, 80 mm, and 25 mm in 2015, respectively (Figure. 4). Furthermore, the seasonal assessment of precipitation data revealed that overall P in winter and spring seasons increased while in summer and autumn seasons substantially decreased. In winter and spring, the mean annual increase in P was 0.0379 mm yr^-1 and 0.1491 mm yr^-1, while its decline rate in summer and autumn was determined -0,6288 mm yr^-1 and -0.0682 mm yr^-1, respectively (Fig. 4).

Long-term annual and seasonal runoff of Kharkhiraa river

The mean values of winter daily mean runoff (WI- DMR), spring daily mean runoff (SP-DMR), summer daily mean runoff (SU-DMR), and autumn daily mean runoff (AU-DMR) of the Kharkhiraa river during 19752015 were 2.37 m³ s^-1, 4.08 m³ s^-1, 24.02 m³ s^-1, and 8.00 m³ s^-1, respectively. Along with, the trend line shows that daily mean runoff of winter (WI), spring (SP), summer (SU), and autumn (AU) changed from

The long-term (1975-2015) runoff analysis of the Kharkhiraa river revealed an average of 5.37 m³ s^-1 for last four decades that was recorded at the Tarialan hydrological station, northwest Mongolia. The period of 1975 to 2015 indicated significant changes in daily mean runoff of the Kharkhiraa river, which is obvious in the trend line that shows 6 m³ s^-1 in 1975 to 5 m³ s^-1 in 2015 with an annual average decline of -0.17 m³ s^-1. On the other hand, these long-term data also showed the maximum total annual runoff of 30.80 m³ s^-1 in 1986, and the lowest 2.11 m³ s^-1 in 2009 (Figure. 5).

2.3 m³ s^-1, 4.2 m³ s^-1, 24.5 m³ s^-1, and 6.5 m³ s^-1 in 1975 to 2.6 m³ s^-1, 3.8 m³ s^-1, 23 m³ s^-1, and 8.5 m³ s^-1 in 2015, respectively (Figure 5). However, the mean values of WI-DMR and AU-DMR showed increase by 0.07 m³ s^{- 1} and 0.83 m³ s^-1, while the mean values of SP-DMR and SU-DMR decreased by -0.84 m³ s^-1 and -1.35m³ s^{- 1}, respectively (Figure.6). Comparatively, the change in SU-DMR was more significant than other seasons.

Figure. 5 They trend analysis of annual daily mean runoff (AN-DMR) time series during the period of 1975-2015 in the Kharkhiraa River.

Figure. 6 They trend analysis of daily mean runofffor seasons (WI-DMR, SP-DMR, SU-DMR, A U-DMR) time series during the period of1975-2015 in the Kharkhiraa River.

Impact of temperature and precipitation on runoff

The impact of climatic components on runoff of the Kharkhiraa river was assessed through regression and correlation analysis. The coefficients of correlations between DMT and DMR, and TP and DMR were calculated on annual and seasonal basis, while, their regression values have been presented in (Table. 2). The correlations showed that impact of long-term AN-DMT on AN-DMR was non-significant, while AN-TP affected AN-DMR significantly (P<0.01) with regression coefficient of 0.5778. DMT and TP are believed as the foremost driving factors of DMR of the Kharkhiraa river, but the effect of TP on runoff was more significant than that of DMT during the period of 1975 to 2015 (Figure. 6). The coefficients of seasonal correlations revealed that DMT of winter affected DMR of Kharkhiraa river, whereas, DMT of other seasons did not affect DMR significantly during 1975 to 2015. Contrarily, the regression analysis also showed that WI-TP, and SU-TP had a significant impact on WI- and SU-DMR, whereas, TP of spring and autumn did not affect mean daily runoff substantially (Table. 2).

Note: *- significant, **highly significant, ^ns -no significant at 0.01 level, AN-DMT- annual daily mean temperature, WI-DMT- winter daily mean temperature, SP-DMT- spring daily mean temperature, SU-DMT- summer daily mean temperature, AU-DMT-autumn daily mean temperature, AN-TP- annual total precipitation, WI-TP- winter total precipitation, SP-TP- spring total precipitation, SU-TP- summer total precipitation, AU-TP- autumn total precipitation, AN-DMR- annual daily mean runoff, WI-DMR- winter daily mean runoff.

The path analysis

The assumption for path analysis is that a significant linear correlation relationship between cause variables and effect variables exists. Table. 3 reveals the direct and indirect impact of climatic factors including temperature and precipitation on runoff of the Kharkhiraa river, western Mongolia. The path coefficients show that direct effect of temperature was found significant only in winter with the highest value of 0.4206, while annual and other season's DMTs did not have significant direct impact on DMR. In contrast, as for as precipitation is concerned, the path analysis showed maximum significant (P<0.01) direct impact of AN-TP (0.5795) on runoff that was followed by Su-TP and WI-TP (Table. 3). However, the indirect impact of T on R through P and P on R through T was non-significant except in autumn season.

Table. 3 The path analysis direct effect values and in direct effect values in Khakhiraa River.

Noted: *- significant, **- highly significant, ^ns - none significant at 0.01evel. T- temperature, P- precipitation, RO-runoff, DMT-daily mean temperature, TP- total precipitation, DMR- daily mean runoff.

Discussion

This study is the first independent characteristic analysis of long-term annual and seasonal climate of the Kharkhiraa river basin and its impact on river runoff. The analysis is mainly based on observed data of daily mean temperature and daily precipitation in the region, and daily mean runoff of the Kharkhiraa river from 1975-2015. As a key climatic factor, the annual daily mean temperature increased by 2.85°C from 1975 to 2015 (Figure. 3). These results accord with 2.26C rise in temperature in the Uvs province of Mongolia during 1940 to 2011 [37] . Moreover, according to long-term seasonal analysis, WI-DMT and SU-DMT increased by 4.22C and 0.86C, respectively (Figure. 4), indicating significantly more warming in cold seasons than in hot seasons. Consistent with these results, in [21] Batima reported warming in winter by 4°C and in summer by 0.9°C in the Khovd region of the Altai Mountains during 1955- 2005 [21].

This temperature rise can be attributed to global warming due to increased anthropogenic activities within the region and, also in those neighboring regions which are supposed to have direct influence on the climate of the study region. In the line of these interpretations, numerous earlier studies have reported significant anthropogenic impact on global warming, that is the leading cause of rising global temperature [38, 39]. The seasonal analysis of this study indicated that, among four seasons, an increase in daily mean temperatures of winter and spring seasons was more pronounced than those in the summer and autumn seasons in the past (Fig. 4). Consistent with these results, Apsite, Bakute [40] also reported that the increased mean air temperature of winter and spring seasons had more significant impact on annual mean air temperature of the region than that of summer and autumn. In the second assessment report on long-term climate change in Mongolia, it has also been reported that, over the period of 1901-2009, the warming trend in Mongolia was particularly strong in the cold season (between

November and March), with an increase of 2.4°C in the mid-latitude semi-arid area of Asia and large warming trends (>2°C per 50 years) in the second half of the 20th century was observed in the northern Asian sector [21].

On the other hand, this study shows that total amount of annual precipitation decreased in the Kharkhiraa river basin during 1975-2015, irrespective of rise in AN-DMT. These results are similar with Chu, Xia [41], whom also reported decreased precipitation with increasing DMT in Haihe river basin of China. In addition, the occurrence of uneven precipitation among the seasons was also observed, showing substantial increase in winter and spring seasons, and decrease in summer and autumn (Figrues. 3 and 4). This increase in daily P of winter and spring might be the consequence of increased evapotranspiration due to increased warming during these seasons in the period of 1975-2015. In accordance with these illustrations, Lee, Nadolnyak [42] also reported that, a drastic temperature rise in a particular place may lead to remarkable change in precipitation amount and pattern as well. Similarly, Houghton, Albritton [43] have also noted the same interaction between temperature rise and total P, while even, there have been an extensive debate on this concern, and a broad consensus among climate scientists is that there would be a notable change in precipitation patterns due to substantial rise in regional temperature. However, despite of an overall increase in regional temperature (2.85C) of the Kharkhiraa river basin during 1975-2015, the decline in annual, summer and autumn total P would have been due to significant reduction in total ET of the region because of declining ET sources, such as snow cover area, surface water, and the vegetation. Accordingly, Nagler, Glenn [44] found that vegetation index of a region is strongly correlated with ET which is directly responsible for developing regional precipitation patterns. While, on the other hand, Khrutsky and Golubeva [45] and Lehmkuhl [46] also agreed that the glacier or snow cover area of Turgen- Kharkhiraa mountains is facing continuous decline, which is the principal source of surface water.

It is obvious from long-term data that the runoff of Kharkhiraa river significantly fluctuated during the period of 1975 to 2015, and among the climate factors, the precipitation affected it directly more than the air temperature. These results also agree with Uniyal, Jha [47], Wang, Zhang [33] and Chang, Zhang [48] who reported that precipitation has a direct significant influence on the stream flow runoff of rivers. Likewise, earlier than that, de la Paix Mupenzi and Li [49] also prospected that changing precipitation pattern may have a strong impact on the runoff of Kaidu Rive in northwest China. However, in the present study, as long-term data showed that total annual precipitation of the Kharkhiraa river basin substantially declined over time, so, the runoff of the Kharkhiraa river also reduced by -0.17 m³ s^-1 (Figure.5). In accordance with these interpretations, Zongxing, Yuanqing [30] explained that snow covering on high mountains is the key water reservoir for building the downstream flow of rivers, while Li, Xu [50] anticipated that increased global warming can strongly affect the hydrological process and precipitation in the glacial parts, resulting in a positive change in rivers runoff.

In conclusion, the present study intended to assess the impact of changing climate on the runoff of Kharkhiraa River in western Mongolia by using longterm climate and runoff data. The annual daily mean temperature significantly increased by 2.83°C during 1975-2015. Precipitation, specifically in summer season, significantly decreased by -25.78 mm, which showed a significant direct effect on the summer runoff with a decrease of -1.34 m³ s^-1 during the period of 1975-2015. On the other hand, observations revealed that runoff of the Kharkhiraa river in western Mongolia was directly affected by changing precipitation and indirectly by changing daily mean temperature. The impact of temperature on runoff was significant only in winter, while other than total annual, the winter and summer precipitations also affected runoff significantly. Together, this study shows that the impact of changing climate in the Kharkhiraa river basin on the river runoff was obvious during the period of 19752015. However, following this study, future forecasting of the impact of this climate change scenario on water resources and runoff of the Kharkhiraa and other related rivers with the use of modelling approaches is inevitable.

Acknowledgements

This work was sponsored by the projects jointly- funded by Xinjiang and National Natural Science Foundation of China (Grant No: U1703241). Besides, the authors feel delighted to acknowledge UCAS international student's scholarship program for sponsoring his study, and the support from the Tianshan Station for Snow cover and Avalanche Research, Chinese Academy of Sciences, for data collection and analysis.

Conflict of interest

It is hereby confirmed that, no conflict of interest exists in the submission of this manuscript, and the manuscript is approved by all authors for publication.

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