Climate transformation to warm-humid and its effect on river runoff in the source region of the Yellow River

YongChao Lan , HuiJun Jin , ChengFang La , Jun Wen , Jie Song , JinPeng Liu

1. Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Lanzhou,Gansu 730000, China

2. Institute of the Yellow River Source, Yellow River Conservancy Committee, Lanzhou, Gansu 730000, China

1 Introduction

The Yellow River drainage basin is China’s second-largest river basin. Although the Yellow River runoff accounts for only 2% of the total river runoff in China, it is the largest water supply source in northwestern and northern China, providing water resources for 12% of the population and irrigation for 15% of the arable land in China (Haoet al., 2006).Although the source region of the Yellow River accounts for only 16.2% of whole area of the Yellow River Basin, it provides more than 40% of the runoff(Liet al., 2010). Therefore, the changes in natural runoff in the source region play a quite important role in the social and economic development of the Yellow River Basin (Wanget al., 2002). From the 1990s onward, the source region has experienced low water levels, which are not sufficient to meet the demand for water resources in the Yellow River Basin.

Most of the source region of the Yellow River has a monsoon climate in which river runoff mainly comes from rainfall in the annual flood period(June-September) (Lanet al., 2007, 2010). Therefore,regional climate change and its influence on river runoff and surface water resources in the region have drawn close attention from many hydrological and meteorological researchers because of its importance in water science and in developing strategies for adapting to climate change, specifically climate warming (Wanget al., 2002; Huang and Zhao, 2004;Zhanget al., 2004; Lanet al., 2010; Liet al., 2010;Sunet al., 2010; Jinet al., 2013). Many scholars are pessimistic about the future of water resources in the source region of the Yellow River, believing that the river runoff will continue to decrease, the regional temperature will continue to rise, and the amount of evaporation will continue to increase (Zhanget al.,2004; Liu and Chang, 2005; Liuet al., 2012; Jinet al.,2013).

However, the study suggests that this is not actually the case, based on our analysis of observational data series of the average annual precipitation and annual runoff at 10 main hydrologic stations and rainfall stations in the source region of the Yellow River during 1960–2012. The analyzing results indicated that the significant signal of climate transformation from warm-dry to warm-humid that appeared in the mid-2000s was corresponding to the transformation of climate change that began at the end of the 1980s in the western part of northwestern China (Shiet al., 2003).

2 General characteristics of the study area

The source region of the Yellow River is located above the Tangnag hydrologic section on the main channel of the Yellow River, located in Qinghai Province and the northeastern part of the Qinghai-Tibet Plateau (95°50′E–103°30′E, 32°30′N–30°00′N). The region is the primary runoff-yield area in the Yellow River Basin, with a catchment area of 12.19×104km2occupying 16.2% of the Yellow River Basin (Niu and Zhang, 2005). The long-term (1956–2012) average annual runoff is 198.8×108m3, which comprises about 42.0% of the runoff of the Yellow River Basin in the corresponding period (Liet al., 2010). Most of source region of the Yellow River is at an elevation of more than 3,000 m a.s.l., where the climate is quite cold and human activities are sparse. Therefore, the river runoff in the region is mainly influenced by climate, and the runoff observational data at the Tangnag hydrological section (the watershed outlet) can objectively reflect the natural conditions of river runoff in the region. Continuous island permafrost and seasonal frozen soil are widely distributed in this zone. The elevation of most of the mountain fields in the region is below the snowline altitude, except Animaqin Mountain (6,282 m a.s.l.), so there are relatively few modern glaciers (only 125.17 km2) in the region (Liuet al., 2002).

There are many lakes, swamps, grasslands, and quite rich groundwater resources in the region. Precipitation is comparatively abundant, the relative humidity is moderate, and the extent of vegetation coverage is adequate. The direct surface runoff mainly originates from rainfall during the annual flood period(June-September, the active summer monsoon period)(Wanget al., 2006). Water vapor usually enters into the source region by two air layers. In the upper layer,the warm-moist current from the western Pacific Ocean moves west along the western Pacific subtropical high and arrives in the western part of Sichuan Province, affected by the southerly current over the Hengduan Mountains. Then the warm-moist current is driven to the source region of the Yellow River by the eastern-northern current over the southeast edge of the Qinghai-Tibet Plateau from the areas of Jiuzhi, Maqu,and Ruo’ergai. Thus, the study area becomes a high-value area of annual precipitation, receiving as much as 800 mm or more in some years.

In the higher layers (500 hPa), water vapor carried by the southwest current from Benga Bay in the Indian Ocean enters into the Plateau from the valley of the Yarlung Zangbo-Brahmaputra River, surmounts the Bayankala Mountains, and enters into the source region of the Yellow River. Stable precipitation weather forms in the region after the tropical cloud cluster joins with the cold front cloud systems of the westerlies (Xuet al., 2004; Tanget al., 2006;Wanget al., 2006; Lanet al., 2007; Zhouet al.,2012). Regional precipitation data (1960–2012) provided by the Upstream Hydrology and Water Resources Bureau of the Yellow River Water Resources Commission indicate that the mean annual precipitation is 504.7 mm and the mean annual runoff is 204×108m3in the corresponding period, which means the runoff coefficient in the region is about 0.32. The rainfall, groundwater (base flow), and snow and frozen melt water recharge account for 63.15%,26.18%, and 9.17 %, respectively, of the annual runoff (Liuet al., 2009). Therefore, the river runoff in the source region of the Yellow River mainly originates from atmospheric precipitation. The White River and the Black River, located in the area between the Jimai and Maqu sections of the main channel of the Yellow River, are the two largest tributaries of the Yellow River in the source region.

3 Data and methods

3.1 Data

Temperature, precipitation, and runoff data used in this paper were provided by the China Meteorolog-ical Data Sharing Service Network and the Upstream Hydrology and Water Resources Bureau of the Yellow River Conservancy Commission. In order to ensure the continuity and reliability of the data, the monthly precipitation and runoff observation data during the period 1960–2012 at 8 hydrological and rain stations (Table 1) in the source area of the Yellow River were adopted, and the short-term data at some stations were interpolated according to long-term serial data at the neighboring stations; all interpolated data passed the significance test ofα= 0.05. The temperature observation data of the corresponding period at 10 weather stations in the region were also used (Lanet al., 2010).

Table 1 Hydrologic stations on the main channel in the source area of the Yellow River and rain stations in the same area

3.2 Analysis methods

We used the Mann-Kendall trend analysis and sudden change test, the Spearman rank correlation test,and cluster analysis and linear trend analysis methods(Yamamatoet al., 1986; Fu and Wang, 1992; Wei and Cao, 1995) to analyze the evolution trend and change characteristics of temperature, precipitation, and runoff in the source area of the Yellow River.

4 Data and trends of climate change in the source region

4.1 Temperature change

Temperature changes can: (1) affect regional total evapotranspiration; (2) affect glacier and snow melting; (3) change precipitation forms in alpine regions;(4) change temperature differences between underlying surfaces and ground air layer, and form the regional microclimate; and (5) change the underlying surfaces and result in changes in soil moisture, regional evaporation, and water infiltration. In a word,temperature change will change runoff generation and the runoff environment in catchments. Previous analyses have shown that, on the whole Qinghai-Tibet Plateau, the interannual variation of mean temperature in the source region of the Yellow River(located in the northeastern part of the Plateau) presents a general rising trend, especially in the past 10 years, and the trend rate of mean temperature increase is greater than 0.33 °C/10a (Lanet al., 2010;Liuet al., 2012; Jinet al., 2013), which is above the rise rates of global average temperature and the average temperature in China (Renet al., 2005; Tang and Ren, 2005). The rising state of temperature can be more clearly observed from the interdecadal variations of mean temperature (Figure 1). Overall, the interdecadal mean temperatures were below the long-term average in every 10-year period before the 1990s; they started to rise quickly in the 1990s; and they have been higher than the long-term average since the 1990s. Corresponding to the change of average temperature in the whole Qinghai-Tibet Plateau in the 1990s, a sudden change in the average temperature in the source region of the Yellow River occurred in 1997, since then the average temperature rose 1.2 °C. The trend rates of the average temperature series before and after 1997 were 0.155 °C/10a and 0.448 °C/10a, respectively.

Figure 1 Interdecadal change of mean temperature in the source region of the Yellow River

4.2 Precipitation change

Trend and mutation analyses of the annual precipitation series at the hydrologic stations on the main Yellow River channel, and at some rainfall stations on the main tributaries of the Yellow River in the source region, indicate that the annual precipitation in the main runoff-yielding areas such as Jiuzhi, Maqu, and Ruo’ergai has been decreasing in different degrees since the 1960s, and the precipitation in other some areas such as Huangheyan, Jimai, and Tangnag has been increasing in different degrees (Table 2). The declines of annual precipitation at Mentang and Tangke, located in the high-precipitation area, were the greatest, being -15.1 mm/10a and -13.2 mm/10a,respectively; conversely, the precipitation increase at Jungong, not far from the basin outlet, was the largest,20.6 mm/10a (Figure 2).

From the interannual variation perspective, the mean annual precipitation series presents a moderate decline on the whole (-1.88 mm/10a, Figure 2a), and a mutation appeared in the mean annual precipitation series after 1989. The mean of the average annual precipitation series during 1990–2012 (after the mutation) reduced to 26.6 mm compared with the mean of the average annual precipitation series during 1960–1989, when the trend rate was 11.0 mm/10a(Figure 2b). The precipitation sharply decreased after 1989 in the region. The average annual precipitation in 1990 was 404.5 mm, which was the minimum value since the beginning of observations, and it decreased by 34.6% compared with the precipitation of 1989 (620.8 mm), and by 19.9% compared to the long-term annual precipitation in the region (504.7 mm). The average annual precipitation started to increase slowly after 1990 and was greater than long-term average beginning in 2005. The average annual precipitation in most of the years since then was greater than long-term average in the region of the Yellow River after that (Figure 2c).

Table 2 Changing features of annual precipitation in the source region of the Yellow River from 1960 to 2012

Figure 2 Interannual change of average precipitation in the source region of the Yellow River

5 Response of runoff to climate transformation and its possible future change in the source region

5.1 Response of runoff to climate transformation

From the interannual variation perspective, runoff through the hydrologic sections above Jimai were basically stable and showed no significant trend, whereas the runoff from the hydrologic sections below Jimai declined over the past 50 years (Figure 3a). The runoff through the Jungong hydrologic section had the greatest decline due to a sharp decrease in precipitation in the area above that hydrologic section; the corresponding trend rate was -6.322×108m3/10a. Our analysis results show that there was an obvious non-linear relation between annual precipitation and annual runoff in the source region due to the influences of the different space-time distributions and intensities of precipitation and the different antecedent soil pondages. In other words, large or small annual precipitation did not always have a one-to-one correspondence with annual runoff. The minimum record of runoff occurred at all hydrologic sections in the source region except Huangheyan in 2002, and the annual runoff through all of the hydrologic sections declined during 1960–2002 (Figure 3b). The runoff through the Jungong section, located in the downstream of main runoff-yielding area, had the largest declines (except Tangnag) due to a sharp decrease in precipitation in the area; the trend rate was-11.276×108m3/10a. The trend rate of annual runoff at Tangnag was -12.349×108m3/10a due to an accumulative total runoff decrease at all the hydrologic section above Tangnag. The annual runoff through all the hydrologic sections began to rise gradually from 1990 and reached a maximum in 2012.The trend rate of annual runoff in the source region has been 27.316×108m3/10a since 1990 and 118.55×108m3/10a since 2002. Observations have demonstrated that all the annual runoffs through every hydrologic section in the source region were basically greater than the long-term mean runoff after 2007,which means the river runoff has now entered into a high-flow period.

5.2 Possible future changes of river runoff

How long the current high-flow status of river runoff in the source region of the Yellow River can continue is not only of academic and public interest but is also important for rural governance. The high or low flow of river runoff in the region depends mainly on the regional climate, especially on change of precipitation in the region. Global mean temperature and the temperature in the vast majority of local areas will continue to rise for a foreseeable quite long period,which is the final conclusion presented by the Intergovernmental Panel on Climate Change in 2007(IPCC, 2007). Changes in precipitation are highly uncertain and, at present, there is no mature method which can accurately predict precipitation change.

Most areas of China are in the monsoon climate zone, although the climate of southern China, such as in the Yangzi River and Huaihe River watersheds, is more directly and significantly affected by the summer monsoon and the distribution of drought and flood in summer in northern China (located at the edge of monsoon activity); the source region of the Yellow River also has a close relationship with the strength and path of summer monsoon (Wanget al.,2000; Hu and Qian, 2007; Lanet al., 2007). Zhangetal. (2002) analyzed the influence of the East Asian monsoon on air temperature and precipitation in northwestern China. Their results show that the impact of the East Asian monsoon on the climate of northwestern China, including that of the source region of the Yellow River, is quite obvious. A strong winter monsoon will bring cold and rainless weather to northwestern China, which will result in decreased precipitation and lower runoff flows in winter, spring,and summer, whereas a strong summer monsoon can bring rainy weather in summer in the southeastern part of northwestern China and rainless weather in many parts of west-central and northwestern China.

The expectations for the future change of Asian summer monsoon activity are that the East Asian summer monsoon will continue to strengthen in the 21st century, especially after the 2040s, which will lead to rain bands moving to the north (Jiang and Tian,2013). Observational data also have shown that the interannual variability of the East Asian winter monsoon has waned in the past 20 years or so (He, 2013).It is estimated that the time scale of climate transformation to warm-humid is likely decadal, considering the above prediction of Asian monsoon decadal changes and forecasts for future runoff changes (Zhaoet al., 2010). This means that a rainy and warm climate will continue for a long time, and that perhaps an even longer rainy and high-flow period, similar to that of 1961–1989, will again occur in this century.

6 Discussion

Changes in river runoff in the coming decades can be simulated or forecasted qualitatively or quantitatively by use of certain statistical inferences based on the evolution laws of runoff. Here, we used the periodic mean superposition method to simulate and assess the change situation of river runoff in the source region of the Yellow River in the coming decades(Figure 4). The fundamental principle of this method is that the hydrological time series is separated into several periodic waves, and the prediction results are derived by extrapolating the periodic waves and linear superposition. We predict that the average runoff during 2013–2062 in the source region of the Yellow River will be 636 m3/s, which is larger than the current long-term runoff average (615 m3/s during 1920–2012).Thus, the average runoffs during 2013–2022,2023–2032, 2033–2042, 2043–2052, and 2053–2062 will be 687 m3/s, 657 m3/s, 581 m3/s, 594 m3/s, and 662 m3/s, respectively. These compare to the current runoff average as +10%, +5%, -7%, -5% and +5%, respectively. On the whole, the runoff in the next 50 years will be larger than the current runoff and there will be many more high-flow periods than low-flow periods in the source region of the Yellow River.

Figure 3 Interannual change of runoff in the source region of the Yellow River

Figure 4 Change processes of observed, simulated, and forecasted annual runoff in the source region of the Yellow River

7 Conclusions

Some preliminary conclusions, based on observational data of regional climate parameters and river runoff, and analysis of changing trends of local runoff and the Asian monsoon decadal variability, can be obtained as follows:

1) Corresponding to global temperature changes and temperature changes on the whole Qinghai-Tibet Plateau, the temperature in the source region of the Yellow River (located in northeastern part of the Plateau) has been rising for the past 50 years, especially in the past 10 years. On the whole, the regional temperature before the 1990s was lower than the long-term average, it rose sharply in the 1990s, and has been higher than the average since the 1990s.Corresponding to a mean temperature jump that occurred in the 1990s in the whole Plateau, there was also a mean temperature jump in the source region of the Yellow River in 1997, and the mean temperature after that jump has risen by 1.2 °C. The trend rates of mean temperature rise before and after that jump are 0.155 °C/10a and 0.448 °C/10a, respectively.

2) Although precipitation has been slightly decreasing over the past 50 years in the source region of the Yellow River, a jump occurred in 1990 when the precipitation sharply decreased, and the precipitation began to gradually increase after 1990. If variation of over the past 50 years in the source region is divided into two periods, that is, 1960–1989 and 1989–2012. All precipitation during 1960–1989 and during 1990–2012 presented an increased trend, and the precipitation in 2005 had exceeded the long-term average to enter another pluvial period in the source region of the Yellow River in second period. The trend rates of mean precipitation in the source region period 1960–1989,1990–2012 and 1960–2012 are 11.4 mm/10a and 43.1 mm/10a and -1.88 mm/10a, respectively.

3) As a whole, the change trends of runoff flowing through the Huangheyan and the Jimai hydrologic cross sections are unremarkable, while the runoffs flowing through the Maqu, the Jungong and the Tangnag hydrologic cross sections have decreased in the past 50 years, The runoffs flowing through all the hydrologic cross sections decreased during 1960–2002, affected by continued precipitation decreases after 1989. The runoffs began to rebound quickly after 2002, affected by gradual precipitation recovery; the downstream increases were greater than upstream, and in 2012 the runoff in the source region of the Yellow River reached its maximum value since 1990.

4) By certain signs we may presage that the climate transformation to warm-humid in the region may not be short-term; rather, it is likely to be decadal or longer in time scale, which means a warming and rainy climate in the source region of the Yellow River will continue in the coming decades.

This research is supported by the Key Deployment Project of the Chinese Academy of Sciences (Grant No. Y322G73001), the Major Research Projects of the National Natural Science Fund Project (Grant No.91225302), and the National Natural Science Foundation of China (NSFC) (Grant Nos. 41240002 and 91225301). Authors are grateful to Prof. Hu Xinglin of the Hydrology and Water Resources Reconnaissance Bureau of Gansu Province and Prof. Ma Quanjie of the Institute of the Yellow River Source, Yellow River Conservancy Committee for their constructive comments.

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