Quantitative Identification Of River Runoff Change And Its Attribution In The Arid Region Of Northwest China

Water resource is an important ecological and environmental control factor to be significantly affected by global changes. Currently, the potential impact of climate change and human activities on water resource is an important part of global change research. Water circulation system is an important component of the climate system, and climate change would inevitably lead to temporal and spatial variation of water resources; for many basins worldwide, especially for those in arid and semi-arid regions, the runoff is very sensitive to small changes and fluctuations in the climate. Altering the basin underlying surface, industrial and agricultural water intake & use and other activities by humans, can destroy the natural runoff generation and confluence mechanism within the basin, and directly affect the spatial and temporal distribution of water resources as well. Climate change and human activities, as the two main factors driving the evolution of water resources, have interaction and feedback mechanism, and its impact on water resources is more complex, intensifying the imbalance between global water supply and demand.The ecosystem in arid region of northwest China is very fragile and susceptible to the influence of various natural factors and human factors with not high stability, so it is extremely sensitive to global changes. Under the background of warm and humid climate in this region, the water resources system based on mountainous precipitation and snow meltwater supply is more fragile, and climate warming accelerates the melting and retreat of glaciers, changes the composition of water, and exacerbates water volatility and uncertainty:on the other hand, due to the continuous expanding of artificial oases, the runoff generated in the upstream mountain has been utilized in great amount through the piedmont plain oasis, human activities has diverted ecological water rights, and the contradiction between social economy and ecological water utilization has been increasingly prominent. In the dual driving action of climate change and human activities, significant changes to unique hydrological processes have occurred. In view of this, this paper, with the arid region of northwest China as the research target, applied Mann-Kendall nonparametric trend and mutation test, ensemble empirical mode decomposition method (EEMD), correlation analysis and a variety of statistical methods for analysis on typical basin runoff evolution characteristics and its response mechanism to climate change in the regional changes environment, identified the effect degree of climate change and human activities on runoff change by constructing an objective quantitative model, and conducted runoff variation attribution study. The study results are expected to provide scientific understanding of the water circulation evolution rule, implement rational and effective development and utilization of water resources, and offer technical support and decision-making reference for scientific development of applicable countermeasures against water resources under climate change. The main conclusions are summarized as follows:(1) The runoff showed an overall significant change trend and mutation characteristicsMann-Kendall nonparametric trend test results showed among 36 runoff control stations at mountain pass, the actually measured runoff at 13 hydrological stations exhibited extremely significant increasing trend (P<0.01), more significant at 5 stations (P<0.05), non-significant at 12 stations (P>0.05), and a decreasing but mostly non-obvious trend at the remaining 6 stations. Overall, the runoff from mountainous watershed in arid region of northwest China mainly showed a growing trend. However, in the four hydrological stations not set at mountain pass, only the actually measured runoff at Baolangsumu station in the Bosten Lake basin showed increasing trend, and the runoff at Aral control station in the mainstream of Tarim River, Zhengyixia Gorge in the mainstream of Heihe and Caiqi hydrological station in the mainstream of Shiyang River showed different degrees of decreasing trend, while the inflow from the above river headwaters except Shiyang River showed a significant increase trend, reflecting a major impact of human activities in the plain oasis region on runoff process.Mutation analysis showed that except individual hydrological station located in Xinjiang, the actually measured runoff at most stations has been subject to mutations in 1990s, which is corresponding to the climate transition time in northwest China from "warm and dry" to "warm and wet", but there is obvious lag effect; the runoff at Changmabao hydrological station of Shule River in Hexi inland river basins experienced the mutation in 1998, synchronous with mutations in most rivers runoff; however, Heihe River basin is an exception, rivers in larger runoff generally have two mutation years, earlier in around 1980 and later in the mid-2000s, while Hongshuihe and Mayinghe in smaller runoff experienced the mutations in the late 1970s, more consistent with early mutations time for rivers in larger runoff.(2) Significant changes to regional climate backgroundDuring 1960 and 2012, the average surface temperature and precipitation have shown an increasing trend, wherein the Z statistical value of the average annual temperature variation under Mann-Kendall trend testing is 5.38, showing a very significant warming trend, and the rate is approximately 0.33℃/10a, significantly higher than the national and global levels; the Z statistical value under the annual precipitation variation trend testing is 3.90, showing extremely significant increasing trend, and the rate is approximately 6.66mm/10a. Mutation analysis showed that the average annual temperature experienced two mutations respectively in 1988 and 1997, of which 1988 is consistent with the climate transition time in northwest China, and a response to global warming mitigation occurred in 1997, while the annual precipitation experienced mutations in 1986. In addition, the surface temperature and precipitation changes have obvious characteristics of cyclical fluctuations, and the temperature is mainly dominated by interdecadal fluctuations, while precipitation changes are subject to greater impact of interannual fluctuations. Over the same period, the temperature of the upper atmosphere has also undergone a significant change, and the warming of middle and lower troposphere as well as temperature drop of upper troposphere, middle and lower stratosphere are important features of environmental change in the upper atmosphere. Corresponding to temperature changes of upper atmosphere,0℃ level height in summer has also been subject to significant fluctuations.(3) Significant impact of climate change on the mountainous runoff The response of mountainous runoff in arid region of northwest China to surface temperature and precipitation is mainly shown as follows:with respect to the period, runoff changes have a similar period to temperature and precipitation over the same period (2-3a and 5-8a) on the interannual variability of nature, and there are some differences in the interdecadal cyclical variability of nature; from the relevant statistical point of view, the impact of precipitation on runoff was significantly greater than the temperature on the interannual scale, and vice versa on the interdecadal scale; for rivers in smaller proportion of the glacial meltwater runoff supply, its runoff response to precipitation is more intense in view of the runoff component, the runoff response to temperature will be more intense for those in larger proportion, and its response to precipitation and temperature are intense for those in comparative proportion.0℃ level height variation in summer has significant impact on the mountainous runoff. During 1960 and 2009, the excellent positive correlation was exhibited between the mountainous runoff and 0℃ level height in summer in four typical areas of arid region of northwest China, i.e., the northern slope of Kunlun Mountains, the southern and northern slopes of Tianshan Mountains and the northern slope of Qilian Mountain; the sensitivity analysis also showed that the summer runoff was very sensitive to 0℃ level height variation, and the sensitivity coefficients of the mountainous runoff in summer to 0℃ level height were 7.19,6.63,3.80 and 2.79 respectively in the above four areas, indicating the rise and fall of 0℃ level height in summer have become a critical factor to affect the mountainous runoff changes.(4) Contributions of climate change and human activities to the runoff Based on Budyko water-heat balance coupling framework, the impact estimation model of climate change and human activities (mainly the underlying surface impact in mountainous areas) on runoff was established in this paper, and contributions of climate change and human activities to runoff were quantitatively separated by means of this model.For 39 typical river headwaters, the relative contribution of climate change to runoff variation for 21 rivers was more than 60% with a positive effect, collectively showing an increasing trend of mountainous runoff; the relative contribution of underlying surface change to runoff variation for 9 rivers was more than 60% with a negative effect, collectively showing an decreasing trend of mountainous runoff. In terms of relative contribution rate of 50%, the relative contribution of climate change to runoff variation for 26 rivers was more than 50%, accounting for 66.7% of the total. Rivers significantly affected by climate change, usually originate in the medium-to-high mountains, and they are mainly large and medium rivers; while those largely affected by the underlying surface changes, are primarily medium and small rivers originating in the medium-low mountains.Over the period 1970-2009, the relative influence of human activities on the inflow reduction in the mainstream of Tarim River remained above 60%, and water consumption by human activities, partially offset by increasing the runoff due to warm and humid climate, was the dominant factor in the reduction of runoff into the mainstream; the rapid development of oasis agriculture at three river headwaters in the upper reaches of Tarim River has accelerated the excessive development and utilization of water resources, ecological water rights of the mainstream have been thus occupied in a large amount, and the influence of human activities on inflow of the mainstream was far-reaching; Located in the mainstream of Shiyang River, the inflow of Hongyashan reservoir has been almost entirely controlled by human activities since 1980s; for the Bosten Lake, the relative influence of human activities on the runoff into the lake has remained above 96% from 1970 to 1989, which was the main factor in the reduction of runoff into the lake, while the warm and humid climate is a major factor in the increase of runoff into the lake from 1990 to 2009, and the relative impact of human activities on the runoff into the lake showed an overall decreasing trend in the whole period of 1970 to 2009, which was related to the gradual improvement in utilization efficiency of agricultural irrigation water.