Studieson eco-environm ental change in source regionsof the Yangtze and Yellow Riversof China:presentand future

JianPing Yang

State Key Laboratory ofCryospheric Sciences,Northwest Institute ofEco-Environment and Resources,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

ABSTRACT The source regions of the Yangtze and Yellow Rivers are important in the field of eco-environmental change research in China because of its distinct alpine ecosystem and cryosphere environment.At present,there are three different concepts on the extentof source areas of the Yangtze and Yellow Rivers:hydrological,geographical,and eco-environmental.Over the past decades,annual average air tem perature has w armed significantly;moreover,the temperature rise rate increases notably with increase of time of the data series.Annual precipitation hasno obvious increase or decrease trend,and the climate has become warm and dry in the source regions.As a result,the cryosphere in the regions has shrunk significantly since 1960s.A warm and dry climate and changing cryosphere together induced a substantial declination of alpine wetlands,marked decrease in river runoff,significantdegradation of alpine grassland,and a reduction of engineering stability.The ecologicalenvironment,however,hasa tendency for restoration in the regionsbecause the climate hasbecome gradually warm and wet since 2000.Thus,studies on eco-environmental change is transform ing from a single element tomultidisciplinary integration.Climate change-cryopshere change-physicaland socioeconom ic impacts/risk-adaptation constitute a chain ofmultidisciplinary integration research.

Keywords:cryosphere change;hydrological system;alpine grassland;the source regions of the Yangtze and Yellow Rivers;presentand future

1 Introduction

The headwaters of the Yangtze River are between 32°30′N and 35°40′N and between 90°30′E and 96°00′E,with a watershed area of 12.24×104km2(Ding et al.,2003).The headwater region of the Yellow River is between 33°00′N and 35°35′N and between 96°00′E and 99°40′E,covering an area of 4.49×104km2(Ding etal.,2003).Both areas are located in the hinterland of the Tibetan Plateau with a mean height above sea level of>4,000 m.The lofty topography results in a very cold climate.Mean annual air temperature ranges from-1.0°C to-6.0°C and annual precipitation decreases from about 550 mm in the southeast to about200mm in the northwest(Yang et al.,2007a).As a result,there are extensive permanently frozen ground(permafrost),modern glaciers in themountains,numerous rivers and lakes,distinctive alpinemeadow and steppe in the source regions.There are 753 glaciers with a total area of 1,276.02 km2in the source region of the Yangtze River and 58 glaciers with an area of 125.0 km2in the source region of the Yellow River(Yang et al.,2003a,2003b).Predom inantly continuous permafrost ismainly distributed in the source region of the Yangtze River,while themajority of the source region of the Yellow River is dominated by patchy permafrost(Yang etal.,2004b).Rivers,lakes and marshland(including high-cold wet meadows),supplied by precipitation and meltwater from glaciers and snow cover,are also found mainly in the source region of the Yangtze River,where they account for 69.6%,71.8%,and 74.5%of total rivers,lakes,and marshlands,respectively,in the combined source areas.In the source region of the Yellow River there are relatively few of these components,and they are found largely in the western part of the region(Yang et al.,2007a).A lpine steppe are dom inated by

Stipa purpurea,Carex moorcroftii,and Littledalea racemosa species,and alpine meadow dom inated by Kobresia pygmaea,K.humilis,and K.tibetica in the region.The specific geographical location in the hinterland of the Tibetan Plateau and distinctive constituents of the ecological environment for source regions of the Yangtze and Yellow Rivers have become important in terms of Chinese research since the 1980s.The general public and scientists are highly concerned about this area when dealing with glaciers,frozen ground,snow cover,hydrological elements,and alpine vegetation(Wang et al.,1998;Wu,2000;Zheng,2000;Sha et al.,2001;Wang et al.,2001a;Wang and Cheng,2001;Wang et al.,2001b;Liu et al.,2002;Lu etal.,2002;Xie etal.,2003;Yang etal.,2003b;Liu and Zhang,2004;Wang et al.,2004a;Yang et al.,2004a,2004b;Yang,2006;Yang et al.,2006a,2006c).

The cryosphere includes portions of the Earth's surface where water is in a solid form and includes rivers,lakes,sea ice,snow,glaciers and ice caps,ice shelves,ice sheets,and frozen ground(Lemke et al.,2007).The main components of the cryosphere are glaciers,frozen ground(including permafrost),and snow cover in China.The important components of the source regions for the Yangtze and Yellow rivers are situated in the core area of the Chinese cryosphere.In recent years,under global warming,increased shrinkage of the Chinese cryosphere has induced prominent water and ecological problems in these two source regions,with importantconsequences for the future.The impacts not only affect eco-environmental and socio-econom ic development in the source regions,but on water and ecological safety in m iddle and downstream areas of the watersheds(Qin et al.,2006).Therefore,studies on individual components of the cryosphere have notbeen able to explain and solve many practical problems.The integration ofmultidisciplinary and transdisciplinary research has emerged in the source regions of the Yangtze and Yellow rivers since the 2010s(Ding and Xiao,2013).A lthough in an initialphase,atpresent the two aforementioned disciplinary studies coexist in the source regions.

This study focuses on three aspects:(1)evolution of the extentof source regions of the Yangtze and Yellow rivers;(2)present conditions of eco-environmental change research;(3)new and future study trends in the source regions.

2 Evolution of the extent of source regions of the Yangtze and Yellow Rivers

2.1 Geographical and hydrological

The source regions of the Yangtze and Yellow rivers are a watershed or geographical concept,not an adm inistrative area(Fang et al.,2009).From the 1950s to the early 1980s,geographical and hydrological extent of these source regions are defined after several large scale comprehensive investigations(Institute of Geography,CAS,1990).In the geographic concept,the source of the Yellow River is the valley above Duoshixia(Figure 1;right black triangle)with an area of 2.2×104km2,and the source of the Yangtze River is the valley above convergence of Chumaer and Tongtian rivers(Figure 1;leftblack triangle)with an area of 10.2×104km2(Jing and You,1982).The hydrological range of the Yangtze and Yellow rivers are the watersheds above Zhimenda and Tangnaihai hydrologicalstations,respectively(Figure 1;black dots).

2.2 Eco-environmental

The eco-environment and protection have received w idespread attention with increasing ecological research in the source regions of the Yangtze and Yellow Rivers since the 1990s.At that time,the ecoenvironmental boundaries set by previous research in this region was inconsistent due to individual bias.(Sun et al.,1990;Liu,1995;Sun and Tang,1995;Wang et al.,2001c).As a result,there is a great deal of confusion in terms of overestimating or underestimating the extentof these boundaries in the source regions of the Yangtze and Yellow rivers.In view of the aforementioned confusion,Ding etal.,(2003)defined the extent of the eco-environmental concept based on a comprehensive analysis of topographical characteristics,climate conditions,vegetation distribution and hydrological features in the upper Yangtze and Yellow rivers,respectively.The area above Dari hydrological station is the source region in the watershed of the Yellow River,including Maduo,Dari,and Maqin counties and the partof Gande County in Guoluo Prefecture of Qinghai Province,with a watershed area of 4.49×104km2(Ding et al.,2003).The area above the convergent site of Nieqiaqu and Tongtian rivers is the source region in the watershed of the Yangtze River,including Qumailai and Zhiduo counties as well as Tanggula village belonging to Glomud city,with a watershed area of 12.24×104km2(Ding et al.,2003;Figure 1).

Figure 1 Geographical,hydrological,and eco-environmentalextentof source regionsof the Yangtze and Yellow rivers,China.Black dots representhydrologicalstations,black triangles representplace names

3 Present conditions in eco-environm ental change research

3.1 Overall research trend

The use of three keyword search phrases of"The source regions of the Yangtze and Yellow rivers","the source area of the Yangtze River",and"the source area of the Yellow River"in the China National Know ledge Infrastructure(CNKI)resulted in 218 articles in various Chinese periodicals for 1998-2018.The number of articles studying eco-environmental change in the source regions of the Yangtze and Yellow rivers from 1998 to 2018 is represented graphical as a bell curve(Figure 2).There are less than five articles in 1998,after which the number of articles peaked at31 in 2004,then obviously decreases to less than 10 per year for 2010-2018(Figure 2).2001-2009 is the"golden"period for eco-environmental change research and attention paid to environmental problems in the source regions of the Yangtze and Yellow rivers.

Figure 2 The numberof ecologicalenvironmentchange articles in the source regionsof the Yangtze and Yellow rivers for1998-2018

3.2 Climate change research

Climate change is an important factor for eco-environmental change in the source regions of the Yangtze and Yellow rivers,and is reflected in the importance of eco-environmental research in this region.Observed data at various meteorological stations show s that mean annual air temperature has increased significantly over the past decades in this region(Table 1).The tem perature rise rate,however,relies on time series of data.This data shows a steady increase in the change rate ofmean annual air temperature from 1960 to 2013 in source regions of the Yangtze and Yellow rivers(Table 1).During 1961-2000 the ratewas 0.18°C/10a to 0.22°C/10a,increasing to 0.32°C/10a to 0.36°C/10a during 1961-2007,then increasing further to 0.57°C/10a to 0.62°C/10a during 1982-2013(Table 1).Furthermore,prior to 2007/2008,the rate was 0.18°C/10a during 1961-2000,0.24°C/10a during 1962-2004,and 0.32°C/10a during 1961-2007 in the source area of the Yangtze River,respectively,all lower than those in the source area of the Yellow River during the same time periods(Table 1).In contrast,after 2007/2008,the rate was 0.41°C/10a for 1969-2013,and 0.62°C/10a for 1982-2013 in the source area of the Yangtze River,respectively,all higher than those in the source area of the Yellow River during the same time periods(Table 1).This indicates that the response of both source areas to climate change are different,relatively slower for the Yangtze River than for the Yellow River before 2007/2008,after which is relatively faster for the Yangtze River than for the Yellow River.Thus,2007/2008 may reflect a shift from the Yellow to the Yangtze River in terms of the change rate ofmean annualair temperature.

Tab le1 Change inmean annualair temperature in source regionsof the Yangtzeand Yellow Riversbased on different time series

In terms of annual precipitation,studies show that,whether increase or decrease,there is no obvious change trend in the source regions of the Yangtze and Yellow rivers over the past decades(Yang et al.,2004a;Li et al.,2006;Wang et al.,2009;Wang,2016)(Table 2).Since 2000,however,annual precipitation has increased significantly at a rate of 104.4 mm/10a and 75.48 mm/10a in the source regions of the Yellow and Yangtze rivers,respectively(Wang,2016).

Overall,in recent decades significant increase in air temperature and slight increase in precipitation has produced a climate trend of warm and dry in the source regions of the Yangtze and Yellow rivers(Hu et al.,2007;Du et al.,2015a,2015b;Wang,2016),but shifting to warm and wet since 2000(Jiang et al.,2012;Wang,2016).This result is based on respective analysis of air temperature and precipitation data.However,the study result based on standardized precipitation evapotranspiration index(SSPEI)shows that the source area of the Yangtze River has become hum id since 1969,obviousmore wet since 2008,while the source area of the Yellow River has seen a dry trend during 1961-2014(Wang etal.,2017).It can be seen thatexisting study results describes general variation characteristics and pattern of climate change in the source regions of the Yangtze and Yellow rivers from different perspectives,butwith a certain difference in study results.Thus,it is crucial to carry out a comprehensive study of climate change in the source region of the Yangtze River based onmultiple climatic elements.

3.3 Cryosphere change research

3.3.1 Glacier change

The source regions of the Yangtze and Yellow rivers are located in the hinterland of the Tibetan Plateau,China.Glaciers,frozen ground(including permafrost)and snow cover are well developed in these regions.There are 753 glaciers,which aremainly distributed in the Kunlun Mountain's southern slope,Tanggula Mountain's northern slope,and Sederi Peak in the source region of the Yangtze River;and 58 glaciers upstream of the Yellow River,distributed in the A'nyêmaqên Mountains(Yang et al.,2003a,2003b).Themajority of glaciers has retreated in the source region of the Yangtze Riverwith an averagely retreatof 108.3 m and area shrinkage of 5.3%during 1969-2002(Xu et al.,2013).The change in glaciers,however,are various in differentparts and in the different periods.The Geladandong area is the headwater of the Yangtze River,where large glaciers are concentrated,such as the south branch of Jianggudiru Glacier with an area of 34.77 km2.In this region glacial area has decreased by 1.7%during 1969-2000(Lu et al.,2002;Yang etal.,2003a,2003b),4.78%in 1969-2002(Ye et al.,2006),6.8%in 1964-2010(Wang et al.,2013),9.3%in 1973-2009(Zhang et al.,2015),and 8.6%in 1992-2015(Meng etal.,2018).In general,the amplitude in glacier shrinkage increases with an increase of time.In Sederi Peak,located in the southeastern part of the source area of the Yangtze River,change rate in glacier area is the largest in the source area,reaching 12.9%during 1969-2002(Xu et al.,2013).Compared to glacier retreat in the source region of the Yangtze River,the shrinkage in glaciers ismore significantupstream of the Yellow River.Glaciers shrunk by 17%in A'nyêmaqên Mountains for 1966-2000(Liu et al.,2002;Yang et al.,2003a,2003b).

Table 2 Change in annualprecipitation in source regionsof the Yangtze and Yellow Riversbased on different time series

3.3.2 Frozen ground change

Permafrost in source regions of the Yangtze and Yellow rivers has been degrading in recent decades.This degradation is embodied in shrinkage of permafrost range,active layer thickness,permafrost thinning,increase in annual mean ground temperature,and rising of the lower lim itof permafrostdistribution(Yang etal.,2004b;Wu etal.,2005a;Jin etal.,2010;Zhang and Wu,2012a).As a result,predom inantly continuous permafrost is gradually changed into islands,patchy permafrost,and seasonal frozen ground due to continuous and significant climatewarm ing.In the future,under different climate scenarios,permafrost w ill further degrade(Nan et al.,2005;Zhang and Wu,2012a;Wang et al.,2014;Ma et al.,2017).Recent studies have notonly focused on practical,but also theoretical processes and modes of permafrost degradation(Wu et al.,2009),with in-depth analysis in the relationship between permafrost degradation and alpine eco-system change(Wu et al.,2003;Wang et al.,2004b;Yang et al.,2004b;Wang et al.,2006;Wang et al.,2011;Feng etal.,2016).Of high priority is engineering stability induced by permafrost degradation(Wang et al.,2004b;Wu et al.,2005b;Niu et al.,2007;Wu and Niu,2013).

3.3.2 Snow cover change

Snow-cover area and duration,and snow depth are three elements characterizing snow cover.In studies on snow cover change in source regions of the Yangtze and Yellow rivers,two parameters of snow depth and snow-cover duration are primarily used(Yang etal.,2006a;Yang etal.,2007b;Wang et al.,2016).Snow cover certainly increased in the source regions over the past decades(Yang et al.,2006a;Yang etal.,2007b;Wang etal.,2016).The annualaccumulated snow depth,i.e.,theaccumulation of daily snow depth throughout the year,significantly increased by 2.4%/a between 1960 and 1999(Yang etal.,2007b).The annual snow-cover duration,which exceeded 270 days in the source regions,has an increase of 0.06%/a overa 40 year period(Yang etal.,2007b).

3.4 Hydrological system change research

In this study region,a hydrological system primarily includes rivers,lakes,and alpinemarsh meadows.Study results over the past decades shows that the hydrological system is in a degradation status in source regions of the Yangtze and Yellow rivers(Table 3).In particular,annual average river runoff decreased by 15.2%during 1961-2000(Wang et al.,2007)and by 12.4%for 1986-2000(Yang et al.,2006b)in the source area of the Yangtze River,and by 19.0%during 1986-2000 in the source area of the Yellow River(Yang et al.,2006b).The area in alpinemarsh meadows decreased by 12.5%between 1969 and 2000(Wang et al.,2007),by 27.6%from 1986 to 2000(Yang etal.,2006b),and by 29.3%during 1969-2013(Du et al.,2015b)in the source area of the Yangtze River.The area in alpine marsh meadows in the source area of the Yellow River decreased by 7.5%for 1986-2000(Yang et al.,2006b)and 29.7%for 1969-2013(Du et al.,2015b).The area in rivers and lakes decreased at varying degrees in source regions of the Yangtze and Yellow rivers for 1986-2000 and 1969-2013(Yang etal.,2006b;Du etal.,2015b)(Table 3).It is predicted that for periods of 2030-2049 and 2080-2099,average annual runoff w ill increase in source regions of the Yangtze and Yellow rivers(Cheng etal.,2010)(Table 3).

An increase in air temperature,caused by climate change,is themajor factor of alpinewetland degradation in source regions of the Yangtze and Yellow rivers(Du et al.,2015a,2015b).During 1980-2000 river runoff reduction was directly attributed to a decrease in precipitation,while an increase in melting ice and snow induced by temperature rise was not enough to offset the impact of precipitation decrease on runoff(Cao et al.,2007;Xie and Qi,2007).It is worth noting that the degradation rate of alpine wetland has decreased gradually since 2000,especially after 2007 because lake area increased due to an increase in precipitation(Du etal.,2015b;Wang 2016).

Table 3 Change in alpinewetlands in source regionsof the Yangtze and Yellow riversover the pastdecades

3.5 Alpine vegetation change research

Alpine vegetation has two primary categories:alpine steppe and alpinemeadow,also named as alpine grassland,accounting for 70%of the total area in source regions of the Yangtze and Yellow rivers.The alpine grasslands are in a degradation trend with reduction in coverage,increase in fragmentation,and desertification over the past decades(Du et al.,2015b).The total area ofm id-and high-cover alpine grasslands and alpine meadow has decreased by 16.33%,and 3.75%from 1969 to 2013,respectively(Du et al.,2015b).However,the change in alpine grassland is various in different periods.Prior to 2000,the alpine grassland was significantly degraded.The distributing areas of alpine steppe and alpine meadow with higher vegetation cover decreased by 15.82% and 5.15% in the source regions during 1986-2000(Wang etal.,2004a),decreased by 13.5%and 3.6%in the source area of the Yangtze River and by 23.2%and 7.0%in the source area of the Yellow River from 1967 to 2000,respectively(Wang et al.,2009).The rate of degradation has been consistently decreasing since 2000(Du et al.,2015a,2015b).Some studies indicate that alpine grassland had a tendency of restoration in the source regions after 2000(Wang,2016;Cao etal.,2017).

4 Study trends in source regions of the Yangtze and Yellow rivers

In recent years natural science research has been transform ing to interdisciplinary integrated research such as nature,humanities,and socioeconom ic in terms of global environmental change in order to serve sustainable development(Reid et al.,2009.2010).This integrated tendency includes eco-environmental change studies in source regions of the Yangtze and Yellow rivers.In 2007,the National Basic Research Program of China(973 Program)launched the"Cryospheric Processes in China and Their Climatic,Hydrologic and Ecologic Effects and Adaptation(2007-2011)"project in the field of cryospheric research.Since then,source regions of the Yangtze and Yellow rivers have seen a substantial increase in comprehensive,multidisciplinary studies.

4.1 Transformation from single element to multidisciplinary integration studies

The aforementioned eco-environment elements,such as climate,alpine vegetation,glaciers,frozen ground,and snow cover,are studied separately.These studiesmainly concentrated on analyzing the processes of change,exploring themechanism of change,and attributing,that is,on levels of physical science.In fact,the eco-environment in source regions of the Yangtze and Yellow rivers is a kind of alpine environment based on the cryosphere.Climate change firstly results in change in the cryosphere,resulting in variations of other environmentalelements.Thus,the cryosphere,in relation to a chain of climatic change factors,is the key for carrying out integration research in this region.Climate change is the starting pointof this chain,cryosphere change the connecting point,and adaptation the end of the chain.

The effects of cryosphere change are various,including hydrological,ecological,climatic,engineering,and disasters.Historically,studies on engineering,disasters and physical aspects have been themain focus.Over the past ten years,socio-econom ic impacts of cryosphere change have gained prom inence.Change in the permafrost influences the structure and function of alpine grassland through changing water and heat in the active layer,which influences animal husbandry and herdsmen's livelihood in source regions of the Yangtze and Yellow rivers.Fang et al.(2011),in studying the impacts of frozen ground change on rangeland productivity and sustainable livelihood due to physical changes in permafrost,proposed a number of adaptive initiatives for the source region.Fang et al.(2016)explored the impacts of snow disaster,one of themajor natural disasters in the source regions,on meat production of animal husbandry and identified effective adaptation measures.In addition,change of grassland ecosystem and affecting factors are analyzed using sociological methods under the conditions of cryosphere change(Fang et al.,2012).

4.2 Transformation from describing disaster to quantitatively evaluating disaster risk of cryopshere change

The source regionsof the Yangtze and Yellow rivers experience a large number of cryosphere disasters,such as snow and freezing-thaw ing of the permafrost zone.In recent years disaster research has developed towards quantification.According to historic snow disaster data,current risk of snow disaster is integratedly assessed using a combination of index system and logistic regression methods in source regions of the Yangtze and Yellow rivers(Wang et al.,2014).In terms of permafrost disaster,Zhang et al.(2012)analyzed the formationmechanism of various disasters and theirmain factors,evaluated freezingthaw ing disaster risk using the Analytical Hierarchy Process(AHP)method,and predicted thermal hazards in permafrostzones(Zhang andWu,2012b).A lthough a quantitative trend in the study of cryosphere disaster risk has emerged,current evaluation methods are mainly index systems and statistical methods.Furthermore,because of different comprehension of risk,existing risk assessments,in fact,are only present conditions of risk,not potential or future risks.As a result,these assessment results have lim ited guidance on adaptation.

5 Conclusion and summ ary

The source regions of the Yangtze and Yellow rivers have gained prom inence in the field of eco-environmental change research in China.Based on existing study results in this region in recentdecades,present conditionsof eco-environmental change are generalized and future research trends are proposed in this study.The source regions of the Yangtze and Yellow rivers are a watershed or geographical concept.At present,there are three different concepts on the extent of source areas of the Yangtze and Yellow rivers:geographical,hydrological,and eco-environmental.2001-2009 saw a spike in eco-environmental change research in this region.

Annual average air temperature has increased significantly,with a temperature rise rate of 0.18°C/10a-0.22°C/10a in 1961-2000,0.32°C/10a-0.36°C/10a for 1961-2007,and 0.57°C/10a-0.62°C/10a for 1982-2013.The temperature rise rate increases notably with a gradual increase in time of the data series.Annual precipitation has no obvious change trend in the region over the past decades.Thus,climate in the region takes on awarm and dry trend.As a result,glaciers are retreating in the region with increased amplitude.The glaciers shrunk by 1.7%for 1969-2000,4.78%in 1969-2002,6.8%in 1964-2010,9.3%in 1973-2009,and 8.6%in 1992-2015,in the source region of the Yangtze River,while falling by 17%in the source region of the Yellow River for 1966-2000,larger than those in the source region of the Yangtze River.Continuous permafrost has degraded into islands,patches,and to seasonal frozen ground,and w ill further degrade under different climate scenarios in the future.Snow cover significantly increased with a rise of 2.4%/a for annual accumulated snow depth and of 0.06%/a for annual snow-cover duration between 1960 and 1999.In recent years,warm and dry climate and changing cryosphere influenced degradation of the hydrological system and alpine grasslands in this region.Annual average river runoff decreased by 12.4%-19.0%during 1986-2000.The area of alpine marsh meadows decreased by about 29.0%and the area of rivers and lakes decreased at varying degrees during 1969-2013.The area ofm id-and highcover alpine grasslands and alpinemeadow decreased by 16.33%and 3.75%from 1969 to 2013,respectively.The degradation rates of hydrological systems and alpine grassland have been consistently decreasing and alpine grassland had a tendency of restoration in the source regions since 2000.

The eco-environmental change study of the source regions has transformed from single element tomultidisciplinary integration studies.The cryosphere,in relation to a chain of climatic change factors,is the key for carrying out integration research in this region.Climate change is the starting pointof the chain,cryosphere change the connecting point,and adaptation is the end of the chain.

Acknow ledgments:

This work is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences,Grant No.XDA23060704.The authors thank reviewersand editors for their comments.