Climate Change And Its Impacts On Runoff Process For Typical River Basin In The Southern Slopes Of Tianshan Mountains

Global warming has become an indisputable fact, and water resource is directly and significantly affected by the climate change. As there are mountains and basins in arid region of Xinjiang, and they are particularly sensitive to climate change, the global warming intensifies water cycle, to make the climate change in this particular region more complex, exacerbating the uncertainty on hydrology and water resources of inland river basin. Therefore, the strengthening of assessment of climate change process and its impact on runoff in typical catchment of arid region, and prediction of runoff trend under future climate change scenarios will provide practical significance for rational allocation and sustainable utilization of water resources, as well as promoting social and economic sustainable development in arid region.Xinjiang’s rivers originate in the mountains, especially Tianshan Mountains honored as the "water tower of Central Asia" and "solid reservoir" is the birthplace of the major rivers. Kaidu River and Aksu River originate in the Tianshan Mountains, the runoff supply is mainly from glacial meltwater and precipitation, and the temperature and precipitation changes have a significant impact on the runoff from mountain pass. Located in southern slope of Tianshan Mountains, Kaidu River and Aksu River are also two main origins of Tarim River. Changes in runoff from mountain pass not only affect the water supply of downstream industry and agriculture, but also relate to the regional social & economic sustainable development and ecological safety maintenance. Thus, a typical catchment of Kaidu River and Aksu River in the southern slope of Tianshan Mountains was selected as the study target region in this paper, and the integrated use of multi-method, multi-scale modeling techniques was made for runoff process and mechanism study, with climate- runoff process as the main line, and runoff response to climate change as the core. The main conclusions are as follows:(1) The annual and seasonal temperature and precipitation experienced an overall significant upward trend from 1960 to 2012 in the southern slope of Tianshan Mountains, with the fastest temperature increase in winter and the maximum precipitation increase in summer. Through the analysis of abrupt change points, the annual average temperature and precipitation have undergone a jump respectively in 1990 and 1987 from low to high. While the nonlinear analysis showed, the annual and seasonal temperature and precipitation process was a complex nonlinear system featured with periodicity, multi-scale, fractal and chaos phenomena. Over 50 years, its temperature and precipitation presented a significant nonlinear upward trend, its changes were featured with obvious inter-annual scale and inter-decadal scale, and the inter-annual oscillations held dominant position in the long-term changes of temperature and precipitation. Different regional and seasonal temperature & precipitation change trends showed the linear upward or downward trend, and presented the nonlinear upward-downward and downward-upward trends. The correlation dimensions of annual average temperature and precipitation in southern slope of Tianshan Mountains were 3.2612 and 3.2926 respectively, greater than 3 and less than 4, indicating that at least four independent variables are required to describe its annual average temperature and precipitation process from the perspective of dynamics. Such process has a long memory property, i.e., the warm and humid trend will continue in the future period of time in this region.(2) The annual runoff of Kaidu River and Aksu River presented significant increasing trends from 1960 to 2012, with the maximum runoff increase in summer of the four seasons. The annual runoff of Kaidu River and Aksu River abrupt change point appeared in 1995 and 1993, respectively, which occurred later than the regional temperature and precipitation abrupt changes, indicating that runoff mutation respond to climate change with a lag time in the region. While the nonlinear analysis showed, the annual and seasonal runoff of Kaidu River and Aksu River process were complex nonlinear systems featured with periodicity, multi-scale, fractal and chaos phenomena. Over 50 years, the overall runoffs of Kaidu River and Aksu River both have showed significant nonlinear upward trends, and their changes have obviously exhibited an inter-annual scale (quasi-3 and quasi-5-6 years) and inter-decadal scale (quasi-11～13 and quasi-27～28 years). Variance contribution rates of each component showed that the inter-decadal change held a dominant position in the overall runoff change for both river, and the inter-annual variation also played an important role in the overall runoff change. The correlation dimensions of annual runoffs for Kaidu River and Aksu River are 2.9629 and 3.1006 respectively, suggesting that at least three independent variables for Kaidu River and four independent variables for Aksu River are required to describe their annual runoff process from the perspective of dynamics. Such process has a long memory property, i.e., the increasing trends of runoff variation for both rivers will continue a period of time in the future.(3) The correlation analysis showed, the temperature and precipitation in the region of Kaidu River and Aksu River origin had significant positive correlation with runoff at annual time scale, and the correlation coefficient of precipitation and runoff was greater than that of temperature and runoff in Kaidu River catchment, but vice versa in Aksu River catchment. Path analysis, cross wavelet and wavelet coherence analysis further confirmed the greater influence of precipitation on runoff in Kaidu River catchment and dominant temperature effect on runoff in Aksu River catchment. Furthermore, the significant correlation areas between precipitation and runoff, as well as temperature and runoff for both rivers are distributed mainly in the 1990s. In the region of Kaidu River and Aksu River origin, AMO, the northern hemisphere polar vortex area VPA and XZH(the Xizang High) showed a significant correlation with runoff, precipitation and temperature; through comparison of the atmospheric circulation AMO, VPA, XZH with wavelet coherence spectrum of temperature and precipitation over Kaidu River, the significant correlation intensity of atmospheric circulation with temperature and precipitation as well as range were greater than those of atmospheric circulation with runoff, indicating the atmospheric circulation influenced the runoff of Kaidu River and Aksu River through influencing regional temperature and precipitation.(4) The reconstructed inter-annual variation trends of runoffs for Kaidu River and Aksu River could describe the fluctuation state of original runoff during the study period; the reconstructed inter-decadal variability effectively revealed that the runoff for both rivers changed over the years, namely the state of abundance and low water period appear alternately. In addition, we found the annual runoff for both rivers has a positive correlation with annual precipitation and the annual average temperature at both inter-annual scale and inter-decadal scale, and the correlation is stronger and more significant at inter-decadal scale, indicating the inter-decadal scale is more suitable for investigating the responses of runoff dynamics to climate fluctuation. EEMD artificial neural network model built was used to simulate nonlinear relationship between annual runoff and annual average temperature & precipitation, and the results further confirmed the inter-decadal scale was more applicable to evaluate the nonlinear response process of runoff on climatic fluctuations.(5) By the use of sensitivity analysis method, and through prediction of future runoff change trend at different time scales under assumed future climate change scenarios, the runoff of Kaidu River was more sensitive to precipitation than temperature at annual, inter-annual and inter-decadal scales, while the runoff of Aksu River was more sensitive to temperature than precipitation, and the runoff for both rivers was most sensitive to precipitation and temperature at inter-decadal scale. Runoff sensitivity to climate factors had scale difference, and the sensitivity difference of runoff also existed under the same conditions of climate change before and after climatic shift, especially after 1987, the runoff sensitivity to temperature and precipitation of Kaidu River and Aksu River at inter-decadal scale increased significantly.(6) Through analysis on the response mechanism of runoff to climate change, Kaidu River runoff is more sensitive to precipitation due to its runoff component dominated in rain and snowmelt runoff, while Aksu River runoff is more sensitive to temperature mainly because of its runoff component playing a major role in glacier meltwater runoff, and they are underlying causes for different responses of two rivers runoff to the temperature and precipitation. In the mid-and-late 1980s after the regional climate transition, the increase in snow and glaciers melting caused by the climate warming has directly resulted in significant increase of Kaidu River and Aksu River runoff. In fact, changes in runoff from mountain pass of Kaidu River and Aksu River are not only subject to the dual influences of precipitation and temperature, but also under the combined effects of potential evaporation,0℃ level height and other climatic factors, and the rise and fall of 0℃ level height in summer have become a crucial factor to affect two rivers runoff changes, especially for Aksu River mainly relying on glacial meltwater supply. In addition, the global warming mitigation in recent years and climate change complexity in Tianshan Mountains have increased the uncertainty of future climate-hydrological processes in the region, and the inflow from origin area shows overall increase while the amount of water into the mainstream of Tarim River decreases, bringing worries for the future security of water resources; therefore, the management and control of water resources shall be strengthened in making research on applicable countermeasures of water against climate change.