Characteristics and changes of streamflow on the Tibetan Plateau: A review Academic research paper on "Earth and related environmental sciences"

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Journal of Hydrology: Regional Studies

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Review

Characteristics and changes of streamflow on the Tibetan Plateau: A review

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Lan Cuoa *, Yongxin Zhangb, Fuxin Zhua, Liqiao Lianga

a Key Laboratory of Tibetan Environmental Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China

b Research Applications Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

ARTICLE INFO ABSTRACT

Article history:

Received 5 June 2014

Received in revised form 7 August 2014

Accepted 13 August 2014

Keywords: Streamflow River basins Tibetan Plateau Climate change Human activity

Study region: The Tibetan Plateau (TP).

Study focus: The TP exerts great influence on regional and global climate through thermal and mechanical forcings. The TP is also the headwater of large Asian rivers that provide water for billions of people and numerous ecosystems. Understanding the characteristics and changes of streamflow on the TP will help manage water resources under changing environment. Three categories of rivers (the Pacific Ocean, the Indian Ocean, and the interior) on the TP were examined for their seasonal and long term change patterns. Outstanding research issues were also identified. New hydrological insights for the region: Streamflow follows the monthly patterns of precipitation and temperature in that all peak in May-September. Streamflow changes are affected by climate change and human activities depending on the basins. Streamflow is precipitation dominated in the northern, eastern and southeastern basins. In the central and western basin either melt water or groundwater, or both contributes significantly to streamflow. Human activities have altered streamflow in the lower reaches of the eastern, northern and western basins. Long-term trends in streamflow vary with basins. Outstanding research issues include:

(1) What are the linkages between streamflow and climate systems? (2) What are the basin-wide hydrological processes? And (3) What are the cryospheric change impacts on hydrological processes and water balance?

© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

* Corresponding author at: Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Building 3, Courtyard 16, Lincui Road, Chaoyang District, Beijing 100101, China. Tel.: +86 010 84097091. E-mail address: lancuo@itpcas.ac.cn (L. Cuo).

http://dx.doi.org/10.1016lj.ejrh.2014.08.004

2214-5818/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Contents

1. Introduction..............................................................................................................................................................................................50

2. Characteristics and changes of streamflow............................................................................................................................51

2.1. Basins and their classifications........................................................................................................................................51

2.2. The Pacific Ocean rivers......................................................................................................................................................52

2.3. The Indian Ocean rivers......................................................................................................................................................54

2.4. The interior rivers..................................................................................................................................................................58

2.5. General characteristics of the streamflow on the TP..........................................................................................59

3. Relationships between streamflow and precipitation/temperature..........................................................................60

4. Outstanding issues................................................................................................................................................................................61

4.1. Relationships between streamflow and climate systems................................................................................61

4.2. Water cycle and hydrological processes....................................................................................................................61

4.3. Impacts of cryospheric component changes............................................................................................................62

5. Conclusions ..............................................................................................................................................................................................63

Conflict of interest................................................................................................................................................................................64

Acknowledgments................................................................................................................................................................................64

References ................................................................................................................................................................................................64

1. Introduction

Often referred to as the "Roof of the World" or the "Third Pole" or the "Water Tower of Asia", the Tibetan Plateau (TP) is the source region of major rivers in Southeast and East Asia that flow down to almost half of humanity. With an area of 2.5 x 106 km2, the TP is the largest and the highest plateau on Earth, and exerts great influence on regional and global climate through thermal and mechanical forcing (Manabe and Broccoli, 1990; Yanai et al., 1992; Liu et al., 2007; Nan et al., 2009; Lin and Wu, 2011). The TP also has the largest cryosphere outside the Arctic and the Antarctic (Zhou and Guo, 1982; Zhou et al., 2000; Cheng and Jin, 2013). Vast areas of snow, glaciers, permafrost and seasonally frozen ground distribute over the TP throughout the year.

Different from the Arctic and the Antarctic, climate change and the induced hydrological and cryospheric changes on the TP directly affect the lives of people and animals that depend on the rivers originating from the TP. It is important to examine the changes in hydrology in the context of climate change over the TP for understanding the links between the changes and for developing a sustainable water resource strategy for the region.

Streamflow of major rivers is an important component of fresh water resource that is crucial for both human societies and natural ecosystems. Streamflow is the product of the integrated processes of atmosphere, hydrosphere, pedosphere and cryosphere in a basin, and is directly affected by climate change and human activities (Wigley and Jones, 1985; Milly et al., 2005; Barnett et al., 2005). Understanding the characteristics and long-term changes of streamflow on the TP is therefore essential for water resource management and ecosystems in the whole region. This work, with a focus on the hydrological changes, will rely on the published literature and draw conclusions on the hydrological changes and the links to climate change. Based on a number of the published literatures, we synthesize the long-term streamflow records for the rivers that originate on the TP and summarize the major characteristics and changes of streamflow, and the relationship between precipitation/temperature and streamflow. We also strive to point out the outstanding issues and possible directions for future research in hydrology on the TP.

Being the first of its kind, by reviewing the vast Chinese and English literatures on the hydrological changes over the TP, most of which are not easily accessible to international readers, this work provides up-to-date knowledge of the findings and the understanding in the characteristics and changes of streamflow over the TP for the international researchers and readers, and can serve as a foundation for focused surface hydrological studies on the TP. It is our hope that this work could further encourage interests and promote collaboration in the TP surface hydrology research. Also, we hope that through the review we could raise the awareness of the importance of hydrological data sharing among scientific communities in China.

2. Characteristics and changes of streamflow

2.1. Basins and their classifications

Rivers on the TP (Fig. 1 ) can be grouped into three categories depending on where they ultimately flow to: (1) the Pacific Ocean, (2) the Indian Ocean, and (3) within the plateau or the surrounding

10' -------------

70" 80° 90° 100° 110° 120° 130° 140'

70° 75° 80° 85° 90° 95° 100° 105°

^^^^^ | | | | | I Elevation (m)

0 1000 2000 3000 4000 5000 6000 7000

Fig. 1. Geographic location of the Tibetan Plateau (a). Major river basins on the Tibetan Plateau and stream gauges on tributaries examined in the paper (b). YLR, the Yellow River; YTR, the Yangtze River; MKR, the Mekong River. BPR, the Brahmaputra River; SWR, the Salween River; IDR, the Indus River. CTB, the Changtang basins; CQB, the Chaidamu and Qinghai Lake basins; TRB, the Tarim basins; and QMB, the northern Qilian Mountain basins. Stream gauges in each river basin are coded and are listed as follows. In YLR, (1) Huangheyan, (2) Jimai, (3) Tangke, (4) Maqu, (5) Tangnaihai, (6) Xunhua, (7) Haiyan (Huangshui River), (8) Xining (Huangshui River), (9) Minghe (Huangshui River), (10) Gadatan (Daitong River), (11) Lanzhou. In YTR, (1) Tuotuohe,

(2) Zhimenda, (3) Xiaodeshi (Yalong River), (4) Shigu. In MKR, (1) Xiangda, (2) Changdu. In BPR, (1) Nugesha, (2) Yangcun, (3) Nuxia. In SWR, (1) Jiayuqiao, (2) Daojieba. In TRB, (1) Aksu, (2) Shaliguilanke, (3) Kaxigar, (4) Keleke, (5) Shaman, (6) Yarkant, (7) Yuzimenleke, (8) Wuluwati, (9) Hotan, (10) Qarqan. In QMB, (1) Yingluoxia (Hei River), (2) Changmabao (Shule River),

(3) Zamusi (Shiyang River). In CQB, (1) Buhahe Kou (Buha River), (2) Gangcha (Shaliu River), (3) Golmud (Golmud River), (4) Xiangride (Xiangride River), (5) Chahanwusu (Chahanwusu River), (6) Delingha (Bayin River).

lowland (Shen and Chen, 1996). This classification is based on the fact that river basins are physical entities that can be delineated based on topography. The Pacific Ocean oriented rivers consist of the Yellow River (YLR), the Yangtze River (YTR), and the Mekong River (MKR). The Indian Ocean directed rivers include the Salween River (SWR), the Irrawady River (IWR), the Brahmaputra River (BPR), and the Indus River (IDR). The Tarim basins (TRB), the Chaidamu and Qinghai Lake basins (CQB), the northern Qilian Mountain basins (QMB), and the Changtang basins (CTB) are interior basins and do not have confluent rivers.

On the other hand, classification of the river basins on the TP based on climate zones is less straightforward. Climatologically, the TP is affected by the mid-latitude westerlies, the Indian (South Asia) monsoon, the East Asia monsoon, El Nino - Southern Oscillation (ENSO), the North Atlantic Oscillation (NAO), the Arctic Oscillation (AO) and local weather systems (Yanai et al., 1992; Tian et al., 2007; Wang and Li, 2011; Cuo et al., 2013b; Yao et al., 2013; Hudson and Quade, 2013; Gao et al., 2014; Molg et al., 2013). These weather systems impact the TP either collectively or independently at various time scales and spatial scales (e.g., Cuo et al., 2013b), rendering it difficult to identify the exact influence domain or boundaries of each of the systems. Attempts made by Yao et al. (2013), Wang and Li (2011), and Tian et al. (2007) in locating the boundaries of the westerlies, the Indian monsoon and the East Asia monsoon provide some guidance for dividing the watersheds on the TP into the Indian monsoon, East Asia monsoon and westerly dominated watersheds, although such division does not take into account the influence by ENSO, NAO, AO and the local circulations. The Pacific Ocean oriented rivers (YLR, YTR, and MKR) are located in the eastern TP and are primarily affected by the East Asia monsoon in summer and westerlies in winter; the Indian Ocean directed rivers (BPR, IWR, and SWR) located in the southern plateau are predominantly influenced by the Indian monsoon in summer and westerlies in winter; whereas IDR and the interior river in the northern (QMB), western (TRB) and central TP (CQB and CTB) are to some extent westerly dominated all year round.

In the monsoon dominated basins over the TP, precipitation mainly occurs in May-October, whereas in the westerly dominated areas, the seasonal pattern of precipitation varies among basins, for example, TRB has two peaks in spring and summer, respectively, while QMB and CQB have one peak in June-August (Ding et al., 1999; Sorget al., 2012; Yao et al., 2013; Palazzi et al., 2013). The seasonal patterns of precipitation and temperature determine the streamflow regime to a great extent. In basins where precipitation and temperature peak during May-October, a great portion of annual streamflow will come directly from rainfall. In basins with one peak in temperature but double peaks in precipitation, streamflow is not only contributed directly by rainfall but also by melt water that is dictated by temperature. Therefore, for different river basins, the impacts of precipitation changes and temperature changes could be different.

Due to the inherent uncertainties associated with the climate zone classification, the review summaries provided in the following sections are organized based on the rivers' destinations. Because of the lack of the published literature, IWR is not included in this review. Also, we focus primarily on the basins located within China. Hydrometric stations located inside and along the boundaries of the plateau are chosen for the review (Fig. 1). Here the boundaries are set at approximately 1000 m above the sea level, hence, all the river basins included in the review correspond to their upper parts, for example, YLR refers to the upper YLR basin. Among the river basins considered, CTB is the largest and IDR is the smallest (Fig. 1 and Table 1). BPR and IDR have the largest and the smallest annual discharge, respectively (Table 1). Contributions to the annual total streamflow in river basins above the hydrometric stations are presented in Table 2. Streamflow trends during the study periods for the river basins over the TP are represented in Table 3.

2.2. The Pacific Ocean rivers

In YLR, the June-October discharge accounts for 55-72% ofthe annual total streamflow at all stations (Fig. 1; Yan, 2000; Dong et al., 2007), with rainfall being the dominant contributor to the annual total (Table 2). YLR's annual streamflow decreased at the stations during a study period of 1956-2009, and the decrease was statistically significant at Tangke, Maqu and Lanzhou (Table 3; Yan, 2000; Chen et al., 2006; Xie et al., 2006a; Chang et al., 2007; Cuo et al., 2013a). The reduction in streamflow is due to the combined effects of increasing evaportranspiration, decreasing precipitation in major

Table 1

Total area (km2), annual total discharge (x108 m3), glacial area (km2), and percentage of glacial area (%) for major river basins on the Tibetan Plateau.

Categories Rivers Sub-basins/stations Area Annual discharge Glacial area Percentage Sources of of glacial area and area discharge

The Pacific YLR Huangheyan 26,541 7.14 171.41 0.08 1

Ocean Rivers Jimai 57,000 40.1 2,3

Maqu 109,000 145.1 2,3

Tangnaihai 122,000 203.9 2,3

Lanzhou 220,000 306.9 2,4

YTR Zhimenda 137,704 126.8 1895.00 0.89 5, 6, 7

Shigu 213,304 423.8 8,9

Xiaodeshi 128,440 580.0 10

MKR Changdu 53,800 149.2 316.32 0.35 5

Point leaving plateau 90,633 280.3 11

The Indian BPR Lhasa 32,588 92.7 18,161.44 7.59 12

Ocean Rivers Nuxia 201,200 605.7 5

Point leaving China 239,288 1359.0 13,14

SWR Jiayuqiao 67,740 213.8 1730.20 1.56 5

Point leaving plateau 110,860 463.9 11

IDR Senge zangbu 27,450 6.9 1451.26 2.89 11

Langqin Zangbu 22,760 9.1 11

The interior CTB Xizang 586,000 194.4 7836.00 1.09 11

basins Qinghai 42,500 26.2

Xinjiang 87,500 15.5

Total 721,182 236.2

CQB 319,260 74.5 1865.05 0.58 11

TRB 159,992 101.7 19,878.00 12.42 11

QMB 60,836 41.7 1334.75 2.19 11

(1) Chang et al. (2007); (2) Cuo et al. (2013a); (3) Niu and Zhang (2005); (4) Huang and Zhou (2012); (5) Zhang et al. (2013b); (6) Zhang et al. (2011a,b,c); (7) Chen (2013); (8) Zhao and Gao (2011); (9) Xu et al. (2010); (10) Chen et al. (2012); (11) Shen and Chen (1996); (12) Lin et al. (2007); (13) Liu (1999); (14) Huang et al. (2007). Glacial areas are from Xie et al. (2006b). YLR, the Yellow River; YTR, the Yangtze River; MKR, the Mekong River. BPR, the Brahmaputra River; SWR, the Salween River; IDR, the Indus River. CTB, the Changtang basins; CQB, the Chaidamu and Qinghai Lake basins; TRB, the Tarim basins; and QMB, the northern Qilian Mountain basins.

runoff production sections of Maqu - Jimai in July-September, and intensified human activities below Tangnaihai, the lower parts of the basin (Yao et al., 2007; Cuo et al., 2013a,b). Statistically insignificant increasing trends are found only atXunhua and Haiyan and for a shorter period of 1956-2000 (Table 3). Whether or not these increasing trends at Xunhua and Haiyan persist beyond 2000 is unknown.

In YTR, the June-September discharge is 72% of the annual total at Zhimenda (Zhu et al., 2011; Jin et al., 2005). At Xiaodeshi in the Yalong River, a tributary of YTR, the June-October discharge is 77% of the annual total (Chen et al., 2012). Rainfall contributes the most to the annual total stream-flow at Zhimenda, Shigu and Xiaodeshi (Table 2). Annual flow showed slightly increasing trends at Zhimenda during 1961-2011 (Li et al., 2012a,b), at Xiaodeshi during 1956-2004 (Cao et al., 2005; Chen et al., 2012), and at Shigu in the lower reach during 1953-2005 (Xu et al., 2010; Zhao and Gao, 2011) (Table 3). The negative trends in annual total are noted at Yushu during 1956-2000 (Table 3) and the reason is unknown (Zhou et al., 2005). The Tuotuo River, the headwater of YTR and located above Yushu, exhibited an increasing trend in streamflow during the late 1950s-2000 (Table 3; Yang et al., 2003; Jin et al., 2005; Zhang et al., 2008; Liu et al., 2009; Bing et al., 2011), indicating that the main contributor to the Tuotuo River is melt water that is enhanced by increasing temperature. The difference in streamflow change between Tuotuo River and Yushu implies that as the basin expands to the lower elevation, melt water contribution diminishes and the other influence becomes more important.

In MKR, the June-September discharge accounts for 70% of the annual total at Changdu, with combined melt water and groundwater contributing much more than rainfall (Table 2; Wang, 2007;

Table 2

Contributions (%) to the annual total streamflow in river basins on the TP.

Rivers Gauges/subbasins Precipitation Melt water Groundwater Sources

YLR Huangheyan a Chang etal. (2007)

Jimai a Cuoet al. (2013a)

Maqu a Cuoet al. (2013a)

Tangnaihai a Cuoet al. (2013a)

Lanzhou a Cuoet al. (2013a)

YTR Zhimenda a Yan and Jia (2003)

Shigu a Yan and Jia (2003)

Xiaodeshi a Yan and Jia (2003)

MKR Changdu 32 68 Guanand Chen (1980)

BPR Nugesha 42 18 40 Liu (1999)

Yangcun 44 20 36 Liu (1999)

Nuxia 30 38 32 Liu (1999)

Nianchu River 31 21 48 Liu (1999)

Lhasa River 46 26 28 Liu (1999)

Niyang River 27 50 23 Liu (1999)

Yigong Zangbu 25 53 22 Liu (1999)

SWR Jiayuqiao a Zhang etal. (2013a,b)

Daojieba a Fan and He (2012)

IDR Senge Zangbu 16 84 Guanand Chen(1980)

Langqing Zangbu a a Guanand Chen(1980)

CTB a Guanand Chen(1980)

CQB Qinghai Lake Basin a Ding and Liu (1995)

Southwest and a Zhou and Dong (2002b)

north of Chaidamu

Southern a Zhou and Dong (2002b)

Chaidamu Basin

TRB 28 48 24 Fu et al. (2008)

QMB a Zhao etal. (2011) and Ma

et al. (2008)

Blanks under precipitation, melt water and groundwater indicates that values are not available. YLR, the Yellow River; YTR, the Yangtze River; MKR, the Mekong River. BPR, the Brahmaputra River; SWR, the Salween River; IDR, the Indus River. CTB, the Changtang basins; CQB, the Chaidamu and Qinghai Lake basins; TRB, the Tarim basins; and QMB, the northern Qilian Mountain basins.

a The dominance but do not have reported numbers available.

Lu et al., 2009). Streamflow change at Xiangda during 1956-2000 showed decreasing trends before 1980 but increasing trends after 1980, though the trends were not statistically significant (Table 3; Zhou et al., 2005). Also, the date of the mid-point of yearly flow shifted earlier at Xiangda during recent decades (Xu et al., 2004; Lu et al., 2009). At Changdu that is located below Xiangda, Cao et al. (2005) found statistically insignificant increasing trends in streamflow during 1968-2000 (Table 3); on the other hand, Zhang et al. (2012a,b) showed that during 1958-2005 streamflow at Changdu exhibited statistically insignificant decreasing trends in annual, flood and non-flood seasonal flows. The differences between Cao et al. (2005) and Zhang et al. (2012a,b) are due to the different datasets, methods and study periods used. It is possible that Cao et al. (2005) only showed a partial change of streamflow over a longer period 1958-2005.

2.3. The Indian Ocean rivers

For IDR, Senge Zangbu and Langqin Zangbu are the headwaters that are fed primarily by groundwater and melt water (Table 2). In Senge Zangbu groundwater and melt water together account for

Table 3

Streamflow changes in basins on the TP. Tr: annual trends derived from the methods listed in the methods column, + denotes increase, - denotes decrease; reported statistically significant trends (p< 0.1) are indicated by superscript letter a (not all studies reported the significance). Blanks under station latitude/longitude, station elevation, control area above station columns refer to missing data. YLR, the Yellow River; YTR, the Yangtze River; MKR, the Mekong River. BPR, the Brahmaputra River; SWR, the Salween River. CTB, the Changtang basins; CQB, the Chaidamu and Qinghai Lake basins; TRB, the Tarim basins; and QMB, the northern Qilian Mountain basins.

Rivers

Sub-basins Stations

Station latitude Station Station Control area

longitude elevation (m) above station

Start year End year Tr Methods

Selected sources

Huangheyan

Tangke

Tangnaihai

Xunhua

Huangshui Haiyan Xining Minhe Daitong Gadatan Lanzhou

Tuotuohe Yushu Zhimenda Yalong Xiaodeshi Jinsha Shigu

34-42' 33-46' 33-25'

33-58'

35-30'

35-50'

36-38'

36-21'

37-26'

36-04'

34-12' 3333-01'

26-45'32.53'' 26-52'26.70''

98-13'

99-39' 102-28' 102-5'

100-09' 102-30'

100-47'

101-46''

102-52' 101-42'

103-49'

92-24'

9797-14'

101-50'18.84'' 99-57'35.70''

4272 26,541 1955 2005 - Linear regression Chang et al. (2007)

3955 57,000 1959 2009 - Mann-Kendall Cuo et al. (2013a)

3435 7800 1981 2009 -a Mann-Kendall Cuo et al. (2013a)

3435 109,000 1960 2009 -a Mann-Kendall Cuo et al. (2013a)

2700 122,000 1956 2009 - Mann-Kendall Cuo et al. (2013a)

177,275 1956 2000 + Linear regression/Mann-Kendall Zhou et al. (2005)

1956 2000 + Linear regression/Mann-Kendall Zhou et al. (2005)

1956 2000 - Linear regression/Mann-Kendall Zhou et al. (2005)

1956 2000 - Linear regression/Mann-Kendall Zhou et al. (2005)

1957 2000 - Decadal difference Yan and Jia (2003) 1600 220,000 1956 2009 -a Mann-Kendall Cuo et al. (2013a)

4533 1959 2000 + Decadal difference Yan and Jia (2003)

3681 1956 2000 - Linear regression/Mann-Kendall Zhou et al. (2005)

137,704 1961 2011 + Linear regression Li et al. (2012a,b)

1063 1960 2004 + Mann-Kendall/Spearman Chen et al. (2012)

1823 1953 2005 + Mann-Kendall, Xu et al. (2010)

Xiangda Changdu

32-14' 31-08'

97-10'

50,608

1956 1968

2000 2000

+ Linear regression/Mann-Kendall + Mann-Kendall

Zhou etal. (2005) Cao etal. (2005)

Table 3 (Continued)

Rivers Sub-basins Stations Station latitude Station Station

longitude elevation (m)

BPR Lazi 29°14'20.57" 87°48'25.59" 4557

Nugesha 29° 16' 91°52' 3720

Yangcun 29°16'48" 91°52'48" 3500

Nuxia 29°16'12" 94°20'24" 2780

Lhasa Lhasa 29°38'31.09" 91°08'48.65" 3659

SWR Jiayuqiao 30°52'41.43" 96°11'53.13" 3182

Daojieba 25°50'57.55" 98°51'12.57" 818

CQB Bayin Delingha 37°22'35.30" 97°22'11.72" 3004

Chahanwusu Chahanwusu 36° 1715.26" 98°5'30.07" 3203

Naijin Nachitai 35°52'19.12" 94°34'09.08" 3603

Germud Germud 36°24'12.75" 94°54'12.23" 2800

Xiangride Xiangride 35°58'42.24" 97°52'19.8" 3096

Buha Buhahekou 36°59'35.70" 99°47'32.12" 3203

Shaliu Gangcha 37°19'44.51" 100°07'29.35" 3299

TRB Aksu Xiehela 41°34'12" 79°37'12" 1427

Shaliguilanke 40-57' 78-36' 1909

Yarkant Kaqun 37°48' 76°54' 1960

Yuzimenleke 37°37'48" 77-12' 1620

Control area Start year End year Tr

above station

Methods

Selected sources

1956 2000 + Decadal difference

1956 2000 - Mann-Kendall

1956 2000 - Mann-Kendall

1956 2000 - Mann-Kendall

1956 2003 + Mann-Kendall

1980 2000 + Mann-Kendall

1958 2000 +•> Mann-Kendall

1957 2000 - Decadal difference 1957 2000 + Decadal difference 1957 2000 + Decadal difference

1956 2000 + Linear regression/Mann-Kendall

1956 2000 +a Linear regression/Mann-Kendall

1956 2007 - Mann-Kendall/Spearman correlation

1956 2007 - Mann-Kendall/Spearman

correlation

1957 2003 +•> Mann-Kendall 1957 2003 +•> Mann-Kendall 1957 2003 + Mann-Kendall 1957 2003 + Mann-Kendall

Liu etal. (2007) Huang etal. (2007) Huang etal. (2007) Huang etal. (2007) Lin etal. (2007)

Yao etal. (2012b) Yao etal. (2012b)

Yan and Jia (2003) Yan and Jia (2003) Yan and Jia (2003) Zhou etal. (2005) Zhou etal. (2005) Li etal.(2010) Li etal.(2010)

Xu et al. (2005) Xu et al. (2005) Xu et al. (2005) Xu et al. (2005)

Kaxigar

Qarqan

Huangyang

Gulang

Tongguziluoke 36°49/12//

Wuluwati

Kalabeili

Keleke

Shaman

36°52'12" 39-33' 38°48' 38°48/ 38° 8'

Yingluoxia 38°48/

Changmabao 39° 49'12"

Zamusi 37°42/

Nanyin

reservoir

Sigouzui

Huangyang

reservoir

Gulang

Dajinxia reservoir

79°55'12"

79°25'48"

75-12'

75°22'48"

75°39'

85°31/

100°10'48"

96°51'

102°34'12"

1650 1800 1900 1960 2330

1674 2112 1495

10,009 10,961 847

840 340 631 660 212

1958 1957

1959 1959 1959 1956

1944 1953 1956 1956 1956 1956 1956 1956

2003 -

2003 -

2005 +•>

2005 -

2005 +

2006 +

2005 +

2005 +

2009 -

2009 -2009

2009 -

2009 -

2009 -

Mann-Kendall Mann-Kendall Linear regression Linear regression Linear regression Linear regression

Linear regression Linear regression Linear regression Linear regression Linear regression Linear regression Linear regression Linear regression

Xu et al. (2005) Xu etal. (2005) Mao et al. (2006) Mao et al. (2006) Mao et al. (2006) Mamat et al. (2010)

Niu etal. (2010) Niu etal. (2010) Zhou etal. (2012) Zhou etal. (2012) Zhou etal. (2012) Zhou etal.(2012) Zhouet al. (2012) Zhou etal. (2012)

about 84% of the annual streamflow, with 55% of the annual flow occurring in July-September (Guan and Chen, 1980). Due to lack of reports on IDR within China, streamflow change is virtually unknown.

In BPR, the June-September flow accounts for 65-75% of the annual total at stations located along the main branch (Liu, 1999). Among the gauged tributaries, the Nianchu River is mostly fed by groundwater, and the Niyang and the Yigong Zangbu Rivers are mostly fed by melt water. BPR at Nuxia essentially equally contributed by precipitation, melt water and groundwater, while the other tributaries are fed mainly by rain (Table 2; Guan and Chen, 1980; Liu, 1999). On average, surface runoff increases toward the lower reaches of BPR (Guan and Chen, 1980). During 1956-2000, the Nugesha, Yangcun and Nuxia stations located in the main tributary showed slightly decreasing annual flow while the Lazi station located in the source region exhibited slightly increasing annual flow (Table 3; Huang et al., 2007; Li et al., 2010). The Lhasa River, a tributary of BPR, presented slightly increasing trends in annual flow during 1956-2003 (Table 3; Lin et al., 2007).

In SWR, rainfall is the major contributor to the annual flow (Table 2; Fan and He, 2012; Zhang et al., 2013b) although in the upper reach above station Jiayuqiao, melt water is also important and accounts for 25% of the annual flow (Zhang et al., 2013b). At Jiayuqiao, both the annual and the monthly streamflow showed increasing trends during 1980-2000 except for June and July and the increasing trends were statistically significant for January-April (Table 3; Yao et al., 2012b). In the lower reach between Jiayuqiao and Daojieba, the annual streamflow also increased during 1958-2000 (Table 3), and the increases in the low flow season (November-February) were statistically significant (Yao et al., 2012b).

In general, streamflow of the Pacific Ocean and the Indian Ocean oriented rivers is rainfall dominated but for the headwaters of these rivers melt water is more important, for example, the Tuotuo River of the YTR (Table 2). It appears that the melt water contribution diminishes as the basins expand from the source region to the lower reaches for both types of rivers. The streamflow changes at various locations along the rivers are different due to the differences in the major contributions to the streamflow and the dominant acting factors such as temperature and precipitation.

2.4. The interior rivers

Historically, all tributaries in TRB flowed to the Tarim River, the main branch. The major tributaries of the Tarim River included the Yarkant, Hotan and Aksu Rivers, which contribute about 3.6%, 23.2% and 73.2%, respectively, to the Tarim River (Chen and Xu, 2004). The Yarkant River used to be the headwater of the Tarim River but it has now lost the connection to the Tarim River except in the extreme flooding season. In TRB, the June-September flow accounts for 72-80% of the annual total (Chen et al., 2003). The major contribution to streamflow in TRB is from melt water, which accounts for approximately half of the annual total (Table 2; Fu et al., 2008), although this number varies among the studies. The lower TRB is desert where precipitation is very limited.

For TRB, during 1957-2003, streamflow increased in the Aksu (statistically significant) and Yarkant Rivers but decreased slightly in the Hotan River (Table 3; Sun et al., 2006; Zhang et al., 2007a,b; Fu et al., 2010). The Kaxigar and Qarqan Rivers are smaller tributaries with generally increased streamflow during 1951-2005 (Mao et al., 2006; Mamat et al., 2010). Streamflow has been heavily but inefficiently exploited in the upper-middle reaches of all sub-basins of TRB resulting in the disconnection between most tributaries and the main branch (Li and Yang, 2002). The influence of human activities in the upper-middle reaches overwhelms the climate change impact (Xu et al., 2005; Chen et al., 2003; Ye et al., 2006) in that streamflow in the Tarim River decreased despite the fact that the upper parts of most sub-basins had increased flow and the regional climate became warmer and wetter (Li and Yang, 2002).

In QMB, the Hei, Shiyang and Shule Rivers are located on the northern slopes of the Qilian Mountains and all flow to the desert. The Yingluoxia station catches the upper Hei River flow and about 80% of its annual flow occurs during May-October (Yang et al., 2009). Annual streamflow at Yingluoxia showed increasing trends during 1944-2005 (Table 3; Wang and Meng, 2008). The Changma River is a major tributary of the Shule River and its monthly streamflow at Changmabao increased during 1953-2005 (Table 3; Niu et al., 2010). Annual streamflow at Shiyang decreased during 1956-2009 at all 6 tributaries (Zhou et al., 2012). The major contribution to the annual streamflow in QMB is

precipitation (Table 2). Although the upper reaches of the Hei River were characterized by increased annual flow, the middle reaches showed decreasing trends due to enhanced agriculture and a chain of dams built in between (Wang et al., 2002; Zhou and Dong, 2002a; Li et al., 2006; Yuan et al., 2006; Yang et al., 2007, 2009; Wang and Meng, 2008). Besides TRB, QMB is another example in the region where human impact overwhelmed climate change impact, and essentially altered the hydrological processes.

CQB, located to the south of the Qilian Mountains, consists of the Chaidamu basin in the west and the Qinghai Lake basin in the east. The Buha and Shaliu Rivers are the two largest rivers that flow to the Qinghai Lake, and together account for 64% of the total lake inflow (Yan and Jia, 2003). The primary contributor to streamflow in the Qinghai Lake basin is rainfall (Table 2; Ding and Liu, 1995). Melt water is the dominant contributor to annual streamflow in the southwest and north of the Chaidamu basin, whereas groundwater is the major contributor to the annual flow in southern Chaidamu basin (Table 2; Zhou and Dong, 2002b; Yan and Jia, 2003). This difference in the contribution between rainfall, melt water and groundwater within CQB may be related to the local geology and the abundance of precipitation. During 1956-2007, the Buha and Shaliu Rivers exhibited insignificant decreasing trends (Table 3; Li et al., 2010). The Golmud and Xiangride Rivers, located in the south of CQB, all showed increasing trends in streamflow during 1956-2000 (statistically significant for the Xiangride River) (Table 3; Zhou et al., 2005). The Chahanwusu and Naijin Rivers located in the southeast also showed large increasing trends during 1957-2000; however, the Bayin River situated in the north CQB exhibited a slightly decreasing trend during 1957-2000 (Table 3; Yan and Jia, 2003).

CTB is located to the south of the Kunlun Mountains and the Tanggula Mountains, and to the north of the Gandise Mountains and the Nianqing Tanggula Mountains. CTB consists of numerous isolated sub-basins and does not have confluence. In CTB, most sub-basins are sized only in hundreds of square kilometers except for the Zhagen Zangbu, Zhajia Zangbu, Cuoqin Zangbu and Bocang Zangbu basins for which the sizes are over 10,000 km2 and are located in the south; most rivers are ephemeral; about 90% of the annual total discharge concentrates in June-September (Chen and Guan, 1989). Annual total precipitation in CTB is only about 150 mm and mostly occurs as snow, which is the reason that the major part of the annual streamflow comes from melt water and groundwater (Table 2; Chen and Guan, 1989). Streamflow characteristics and long-term changes are essentially unknown in CTB due to lack of long-term observations.

2.5. General characteristics of the streamflow on the TP

In summary, streamflow on the TP is concentrated during the flood season of May-October and peaks in July-August (Guan and Chen, 1980), due to the coexistence of the wet and warm seasons, and the dry and cold seasons. In general, the major contributor to the annual total streamflow is rainfall in the north (QMB), the east (YLR and YTR), and the southeast (SWR) of the TP; while melt water or groundwater or their combination dominates in the central (CTB) and west (TRB and IDR) of the TP. BPR and CQB show more complex patterns (Table 2). These regional variations in streamflow contribution are to a large extent related to the climate systems that prevail over the TP. In the eastern and southeastern TP where the East and South Asia monsoons exert strong influence and where precipitation occurs mainly in the warm season of May-October, precipitation is the major contributor to streamflow, and streamflow peaks with precipitation and temperature. In the westerly controlled western TP (e.g., TRB) where precipitation exhibits double peaks in early spring and summer, respectively, melt water is the major contributor to streamflow and melt water peaks when temperature evolves to the seasonal high. On the other hand, in the central TP (e.g., CTB), a westerly dominated area where precipitation is not only low but also solid for the most part of the year, both melt water and groundwater, which peak in the warm season, become important for streamflow.

Based on previous studies, for example Yan and Jia (2003), Zhou et al. (2005), Cao et al. (2005) and Ding et al. (2007) who examined multiple rivers up to 2000 and showed varied streamflow temporal changes, it is reasonable to infer that the up-to-date long-term streamflow in basins on the TP does not have consistent spatial and temporal patterns. Even for the same river system, the streamflow trends could change from sub-basins to sub-basins, and headwater region to downstream reaches. The varied streamflow trends are caused by variations in streamflow components and contributions,

prevailing climate systems, watershed environmental settings, and the influence of human activities. For example, precipitation, an important contributor to many rivers on the TP, shows spatially varying trends on the TP that arise due to the impact of the complex terrain and large- to small-scale circulations affecting the region differentially (e.g., Zhao et al., 2004; Xu et al., 2008; Cuo et al., 2013b). Nevertheless, the quantification of up-to-date long-term streamflow changes for all the basins on the TP and the understanding of the spatial patterns of changes are needed.

3. Relationships between streamflow and precipitation/temperature

Correlation between streamflow and precipitation/air temperature reveals how climate affects hydrological processes and streamflow. For example, positive correlation between streamflow and temperature may indicate the dominance of melt water contribution over evapotransporation, whereas negative correlation would suggest otherwise. Similarly, positive correlation between streamflow and precipitation would indicate that streamflow changes in accordance with precipitation. Likewise, a positive correlation between streamflow and precipitation/temperature indicates that streamflow is dominated by both precipitation and melt water, which most likely happens in basins with precipitation mainly occurring in winter as snow.

Based on linear regression, many studies have analyzed the relationships between annual stream-flow and precipitation/temperature on the TP using available observations (Yan and Jia, 2003; Chen and Xu, 2004; Mao et al., 2006; Huang et al., 2007; Wang and Meng, 2008; Sun et al., 2009; Mamat et al., 2010; Xu et al., 2010; Liu et al., 2012; Li et al., 2012a,b; Yao et al., 2012b). The correlation coefficients between annual streamflow and precipitation are positive and larger than those between annual streamflow and temperature for YLR, YTR, MKR, BPR, SWR, QMB, and CQB (Yan and Jia, 2003; Huang et al., 2007; Xu et al., 2010; Zhang et al., 2011a,b,c; Niu et al., 2010; Liu et al., 2012; Chen et al., 2012; Li et al., 2012a,b; Yao et al., 2012b). A majority of these basins are located in the monsoon controlled eastern and southern TP where rainfall is the major contributor to streamflow. Thus, changes in annual streamflow are strongly affected by changes in annual precipitation in the above basins in that streamflow temporal pattern follows that of precipitation closely (Yan and Jia, 2003; Ding et al., 2007; Niu et al., 2010; Zhang et al., 2011a,b,c). Two basins that reveal slightly negative correlation between annual streamflow and temperature are YLR and QMB (Sun et al., 2009; Niu et al., 2010; Zhou et al., 2012). These two basins are located in the eastern and northern TP where the annual temperature is relatively higher compared to the other basins (Cuo et al., 2013b), indicating the importance of evap-otransporation to some extent. Positive correlation between annual streamflow and temperature is reported for YTR above Zhimenda, BPR, SWR above Jiayuqiao and upper reach ofTRB (Mao et al., 2006; Huang et al., 2007; Li et al., 2012a,b; Yao et al., 2012b), among which TRB, especially its Yarkant and Hotan tributaries (Xu et al., 2009), exhibits the strongest correlation confirming that melt water is a very important source for TRB as noted before.

Notable correlation between streamflow and precipitation/temperature in most basins on the TP demonstrates that streamflow in those basins have been primarily affected by precipitation and temperature changes because of similar annual temporal patterns among streamflow, precipitation and temperature. The exceptions are the lower reaches of YLR, the upper-middle reaches ofTRB and QMB where intensified human activities exert greater influence than climate change and have overwhelmed the climate change impacts (Cuo et al., 2013a; Liu et al., 2013; Li et al., 2008; Huo et al., 2008).

The relationship between streamflow and temperature can be explained by glacier coverage to some extent. In basins that have high glacier coverage, streamflow is positively affected by temperature increases, for example, the upper reaches ofTRB and BPR (Table 1). Streamflow response to temperature changes also depends on the forms and spatial distributions of precipitation. In TRB, annual precipitation increases from the lowland to the mountains in the range of about 20 to 700 mm (Guan and Zhang, 2004; Sabit and Tohti, 2005; Mao et al., 2006; Gao et al., 2010a). Due to low precipitation, the valleys do not generate sufficient water for stream, whereas high precipitation in the mountains is reserved as snow and ice initially and is slowly released as melt water when temperature increases. In the Yarkant sub-basin and the entire TRB, contribution of melt water from the mountains accounts for a major proportion (63% and 48% by some studies, respectively) of the annual total

streamflow, and the contribution is expected to increase as temperature continues to rise (Sabit and Tohti, 2005; Xu et al., 2005; Gao et al., 2010a,b).

Besides precipitation and temperature, actual evapotranspiration is another important factor that affects streamflow. On the TP, studies about actual evapotranspiration were based primarily on water balance equation and potential evapotranspiration adjusted by available moisture content in both soil and vegetation layers (Zhang et al., 2007a,b; Cuo et al., 2013a). Long-term actual evapotranspiration observations virtually do not exist on the TP. The few studies that examined actual evapotranspiration reported that actual evapotranspiration would increase over the TP generally but with spatial variations (Yang et al., 2011; Zhang et al., 2007a,b), and the result would be less available water for streamflow. Cuo et al. (2013a) looked at the impacts of actual evapotranspiration change on stream-flow and found that increases in actual evapotranspiration were larger during May-October when compared to the other months. The same authors noted that actual evapotranspiration change was the second most important factor besides precipitation change in causing the annual and seasonal streamflow decreases in YLR.

4. Outstanding issues

The difficulty in obtaining existing hydrological observations collected and maintained by the Chinese Ministry of Water Resources and the local bureaus of water resources due to their data policies and the harsh environment unfavorable for setting up and maintaining hydrological observational sites on the TP pose great challenges for hydrological research in the region. Overcoming these challenges requires sustained and coordinated efforts from all levels of agencies and researchers alike. In addition, there are other hydrological research topics on the TP that need to be addressed. Among them, three most important scientific issues are discussed below.

4.1. Relationships between streamflow and climate systems

Climate systems dictate precipitation and temperature on the TP, which in turn regulate stream-flow. Large-scale atmospheric systems such as the mid-latitude westerlies, East Asia and Indian monsoons, North Atlantic Oscillation, Arctic Oscillation, ENSO and local circulations all play roles in affecting the weather and climate of the TP (Tian et al., 2007; Cuo et al., 2013b; Yao et al., 2013; Gao et al., 2014). As an example, Wang et al. (2006) showed that above-average annual precipitation in YLR and YTR is caused by enhanced moisture transport by the Indian monsoon when Mongolian low pressure and the westerlies are weak. Li et al. (2007) reported that above normal precipitable water vapor is transported to TRB by the intensifying westerlies as the northerlies become weakened. Any changes in precipitation would have strong implications for streamflow in the basins. Relating streamflow to climate system indices could potentially reveal the impacts of the climate systems on streamflow and help understand the spatial and temporal changes of streamflow over the TP. Ding et al. (2007) compared the annual streamflow changes among YLR, YTR and BPR and found that the changes were out of phase between YLR and BPR, and they attributed that to the differences in the prevailing systems.

4.2. Water cycle and hydrological processes

As the basins on the TP have their unique environmental settings such as high elevation in general, varying coverages of frozen soil, glacier, snow and vegetation, and heterogeneous soil textures, hydro-logical processes and water balances would be different among the basins which could be another reason, besides weather systems, that regional streamflow change pattern varies. For example, in the central TP, the dominant sandy soil would allow infiltrated water drain quickly down to the deep soil; whereas the dominant loam soil in the eastern periphery of the TP could hold more water for soil and vegetation evapotranspiration while desert basins in the northwest would have lower evapotranspiration than forest covered southeastern basins (FAO, 2008). There have been limited studies on the TP about hydrological processes and water balance as most studies have focused on the streamflow climatology and its relation to precipitation and temperature changes. The mechanisms

for streamflow changes could be studied through complementary approaches such as modeling and analyses of field observations including hydrometeorological observations and environmental tracer collections. Hydrometeorological observations can reveal the state and fluxes of hydrometeorological elements such as precipitation and temperature. Environmental tracers such as isotopes and chemicals can be used to separate streamflow into surface, subsurface and baseflow components, and describe the sources of each component (Asano et al., 2002; Michel, 2004; Vache and McDonnell, 2006; Zhang et al., 2009). Unlike the observations that are often collected over points or small-scale basins, physically based hydrological models that are vigorously evaluated can be set up for small or large domains and can be used to study hydrological processes and water balance at various spatial and temporal scales. In other words, hydrological modeling can reveal historical trends and can project future trends of hydrological variables for larger river basins given reasonable forcings (e.g., Cuo et al., 2013a).

Although there are quite a few studies that used isotopes to examine streamflow components on the TP (Nie et al., 2005; Liu et al., 2008; Pu et al., 2013; Meng and Liu, 2013), very few, for example, Nie et al. (2005), used multiple environmental tracers including stable isotopes and chemical tracers combined with hydrometeorological observations such as precipitation, streamflow and groundwa-ter measurements to investigate the sources, components and traveling paths of the components of streamflow on the TP. Sources, components and their paths for most rivers of large or small scales are still unknown on the TP.

Modeling the cryospheric processes from coherent mass and energy perspectives is another important aspect of TP hydrological research. A majority of cryospheric modeling studies on the TP focused on the specific aspects of water and energy balances for frozen soil and glacier (Fujita and Ageta, 2000; Zhang et al., 2004a,b, 2005; Chen et al., 2010; Guo et al., 2012; Zhang et al., 2013a; Molg et al., 2013); while a few other studies looked at the integrated hydrological processes and water and energy balances for the entire basins (Yang et al., 2011; Zhang et al., 2012b, 2013b; Zhao et al., 2013; Cuo et al., 2013a). Valuable as they are, these studies also show that there is still a lot to improve in the simulation of the cryospheric processes such as the thaw and freeze cycles of snow, frozen soil and glacier, glacier volume and movement, extent and depth of snow, frozen soil and glacier, and in the incorporation of the cryospheric processes into physically based hydrological or land surface models that account for both energy and water balances on the TP.

4.3. Impacts of cryospheric component changes

The TP has an abundance of permafrost, glacier, ice and snow. Permafrost occupies about 75% of the entire area (Cheng and Jin, 2013) while glacial coverage equals to 49,873.44 km2 in area (Yao and Yao, 2010). Snow covers the majority of the land during winter (Immerzeel et al., 2009). All cryospheric components contribute to streamflow in one way or another and understanding their roles and impacts of their changes is important for understanding the hydrological processes and hydrological changes as a whole. Yang et al. (1993), Zhang et al. (2003), Tian et al. (2009) and Niu et al. (2010) studied the relationship between frozen soil and streamflow in small-scale basins on the TP. Their findings include (a) frozen soil resulted in a reduction in the lag time between precipitation and peak flow, (b) frozen soil depth and streamflow exhibited positive correlation, and (c) permafrost degradation resulted in a slowdown of peak flow recession.

Glacier and snow are important water resources whose contributions to streamflow differ at tem-porospatial scales. Glacier acts on longer time scales such as years or decades while snow contribution tends to be seasonal and shorter in duration. Glacier contribution to streamflow over decades has been examined for various river basins on the TP using mostly degree-day modeling approaches (Liu, 1999; Kang et al., 2000; Liu et al., 2009; Gao et al., 2010b; Immerzeel et al., 2010; Zhang et al., 2012a, 2013b) and other empirical relationships (e.g. Xie et al., 2006b), but large gaps exist among these studies concerning the quantitative contribution of glaciers and a consensus has not been reached. It is generally accepted that glacier contribution is important mainly for headwaters or basins for which glacier coverage is relatively large. Ye et al. (1999) stated that when glacier coverage is greater than 5%, glacier contribution to streamflow starts to show up in a small basin in Xinjiang, China. However, it is unclear whether or not the criterion of 5% glacial coverage is also applicable for large river basins on the TP. Snow contribution to streamflow is also a topic of debate for this region. Cuo et al.

(2013a) showed that snow contribution is seasonal and is important in mid-spring when up to 40% of the seasonal streamflow comes from snow melt in YLR. Immerzeel et al. (2009, 2010) reported that glacier and snow melt water had a limited role in YLR and YTR but were important for BPR and were extremely important for IDR. In TRB, except for the northeastern area, all the other areas saw increased snow depth and runoff but not in the same phase, indicating that snow is not the major contributor to streamflow in TRB (Xu et al., 2009).

As climate changes on the TP (Wang et al., 2008; You et al., 2008; Cuo et al., 2013b), cryospheric components also change. In YLR, continuous permafrost is becoming discontinuous and isolated, and some of the isolated permafrost is converted to seasonal frozen soil (Jin et al., 2010). Wu and Zhang (2008) found that during 1996-2006, permafrost temperature at 6 m below surface increased by 0.43 °C on average along the Qinghai-Tibet railway. Snowfall is reported to have reduced along the large mountain ranges in the northern TP (Cuo et al., 2013b). Glacial changes are found to be regionally specific and are related to not only climate change but also the local topography (Xu et al., 2009), with the largest and the smallest glacial retreat located in the southeast and interior TP, respectively, and advancement in some glaciers in the western TP and the Pamir Plateau (Yao et al., 2012a).

Cryospheric changes will affect hydrological processes and streamflow, for example, frozen soil degradation causes higher infiltration capacity and soil heat capacity but lower soil thermal conductivity as more ice becomes liquid (Cherkauer and Lettenmaier, 1999). The significance of the cryospheric change impacts on streamflow depends on the quantitative coverages of the components in a basin and the relative contribution of each component to streamflow. Efforts are needed to quantify the coverages of the components and the contribution of each component to streamflow as these are not available for many basins on the TP (see Table 2). Also, more studies on cryospheric component changes and their impacts on hydrological processes for all basins on the TP are needed to help water resources management sector mitigate and adapt to climate change impacts in the region. Questions like how much water can be released by frozen soil degradation and how changes in frozen soil affect soil moisture, evapotranspiration, streamflow and other hydrological processes and water balance in all basins on the TP remain to be answered.

For the aforementioned outstanding issues, one of the limiting factors is the availability of observations. Due to the harsh natural environmental conditions, many areas on the TP are not accessible and in situ field observations are difficult and essentially impossible in some places. While sustained efforts should be devoted to obtain existing observations from various sources, other methods such as remote sensing should be explored and fully utilized for obtaining hydrological measurements. Remote sensing appears to be an ideal tool for hydrological studies on the TP. Missions such as the Surface Water and Ocean Topography (SWOT) and MODIS could have great potential for hydrological application on the TP and should be explored with great effort.

5. Conclusions

The major rivers in Southeast and East Asia that originate from the TP can be categorized into three groups depending on their final destinations: the Pacific Ocean directed rivers, the Indian Ocean directed rivers, and the interior rivers. From the point of view of climate zones, the Pacific Ocean oriented rivers are mainly influenced by the East Asia monsoon in summer and the mid-latitude westerlies in winter; the Indian Ocean oriented rivers are primarily affected by the Indian monsoon in summer and westerlies in winter; while the interior rivers are to a certain degree westerly dominated all year round. The classification of the river basins based on climate zones contains uncertainties since the TP is affected by numerous weather systems and it is difficult to delineate the exact domain of influence for each of the climate systems.

Streamflow change on the TP exhibits similar monthly patterns to those of precipitation and temperature, high in the wet and warm season of May-October and peaking in July-August. The long term streamflow trends vary among the basins on the TP. Even for the same river basin, the streamflow trends could be different from sub-basins to sub-basins, and headwater region to downstream reaches due to the differences in basin environmental settings, prevailing climate systems, components of and contributions to streamflow. Spatially, streamflow is precipitation dominated in basins in the northern (QMB), the eastern (YLR and YTR) and the southeastern (SWR and BPR) TP where the westerlies, the

East Asia and South Asia monsoons exert dominant influence in weather and climate, respectively. In the center (CTB) and west (TRB and IDR) of the TP where the westerlies prevail, either melt water or groundwater, or the combination of both is the major contributor to streamflow. Human activities have overwhelmed climate change impacts in the lower reaches of YLR, upper-middle reaches ofTRM and QMB.

Outstanding research topics that need to be addressed include the linkage between climate systems and streamflow, hydrological processes and water balances in river basins, and the impacts of cyrospheric changes on hydrological processes. These research topics could facilitate the explanation of the regional variations of streamflow and its change patterns as well as the understanding of the water cycle and hydrological processes.

As a natural laboratory with many rivers and various land covers and complex terrain, the TP provides challenges and opportunities as well as tremendous societal significance especially in surface hydrology. The hydrological studies on the TP would be both exciting and rewarding for hydrologists, and concentrated and sustained efforts are needed to overcome the challenges.

Conflict of interest

None declared.

Acknowledgments

This study was supported by the National Basic Research Program (Grant No. 2013CB956004), by the Hundred Talent Program granted to Lan Cuo by the Chinese Academy of Sciences, and by the National Natural Science Foundation of China (Grant No. 41190083).

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