View source for Ejin River

{{Use dmy dates|date=August 2019}}
{{Infobox river
| name               = Ejin River
| native_name        = {{native name|zh|额济纳河}}
| name_other         = Heishui, Etsin-gol, Ruo He, Ruo Shui ({{native name|zh|弱水}})
| name_etymology     = Named for the seasonal flows in its lower course
<!---------------------- IMAGE & MAP -->
| image              = Taklimakan & Badain Jaran.jpg
| image_size         = 300px
| image_caption      = Satellite view of the deserts of northern China. The Ruo River is visible as the faint green trace to the right of the image.
| map                = HeiHe basin.png
| map_size           = 
| map_caption        = 
| pushpin_map        = 
| pushpin_map_size   = 
| pushpin_map_caption= 
<!---------------------- LOCATION -->
| subdivision_type1  = Country
| subdivision_name1  = China
| subdivision_type2  = State
| subdivision_name2  = [[Gansu]], Inner Mongolia
| subdivision_type3  = 
| subdivision_name3  = 
| subdivision_type4  = 
| subdivision_name4  = 
| subdivision_type5  = City
| subdivision_name5  = [[Zhangye]]
<!---------------------- PHYSICAL CHARACTERISTICS -->
| length             = {{convert|630|km|mi|abbr=on}} 
| width_min          = 
| width_avg          = 
| width_max          = 
| depth_min          = 
| depth_avg          = 
| depth_max          = 
| discharge1_location= 
| discharge1_min     = 
| discharge1_avg     = 
| discharge1_max     = 
<!---------------------- BASIN FEATURES -->
| source1            = Kan Chou and Hsü Chao springs
| source1_location   = [[Qilian Shan]], Gansu
| source1_coordinates= {{coord|38|00|04|N|100|54|45|E|display=inline}}
| source1_elevation  = {{convert|3650|m|abbr=on}}
| mouth              = Badain Jaran Playa
| mouth_location     = [[Badain Jaran Desert]], Inner Mongolia
| mouth_coordinates  = {{coord|42|18|50|N|101|04|20|E|display=inline,title}}
| mouth_elevation    = {{convert|900|m|abbr=on}}
| progression        = 
| river_system       = 
| basin_size         = {{convert|78600|km2|abbr=on}}approx.
| tributaries_left   = Dang He, Beida He
| tributaries_right  = 
| custom_label       = 
| custom_data        = 
| extra              = 
}}

'''Ejin River''' ({{zh|c=额济纳河|l=}}), also '''Etsin Gol''', '''Ruo Shui''' ({{zh|c=弱水|l=weak water, weak river}}) or '''Ruo He''' in ancient times, is a major river system of northern China. It flows approximately {{convert|630|km|mi}} from its headwaters on the northern [[Gansu]] side of the [[Qilian Mountains]] north-northeast into the [[endorheic basin|endorheic]] [[Ejin Basin]] in the [[Gobi Desert]]. The river forms one of the largest inland [[River delta|delta]]s or [[alluvial fan]]s in the world.<ref name="deserts"/> Its [[drainage basin]] covers about {{convert|78600|km2|mi2}} in parts of the Chinese provinces of [[Gansu]] and [[Inner Mongolia]], which flows within the [[Zhangye]] area of Gansu; when it flows across [[Jiuquan]] area, it was renamed as Ruo Shui; when it flows across [[Alxa League]], it is called Ejin River.

==History==
About 2,000 years ago, the river was said to have a much more abundant flow than it does today and thus its perennial reaches stretched much farther out into the desert than it does today.<ref name="imhist">{{cite web|url=http://depts.washington.edu/silkroad/texts/weilue/notes1_3.html|title=Draft Translation of the ''Weilüe'' (2004) by John E. Hill. Section 1 – The Di 氐 Tribes|publisher=University of Washington|access-date=29 October 2010}}</ref> Parts of the river flow through the [[Hexi Corridor]], a valley which once formed a significant portion of the [[Silk Road]]. The upper section of the river, also known as the '''Heihe''' (or 'Black River'), was first settled by the Chinese in about 100 B.C. Many outposts were created to protect Silk Road traders from the frequent attacks of [[barbarian]]s, as the water-abundant valley of the river in relation with the aridity of the surrounding terrain provided an easy route for [[Hun]]s and [[Mongol]]s to launch raids. Since then, the river valley has been intensely cultivated and logged. However, clearing of the landscape has caused increased erosion, leading to [[desertification]] of the region and a gradual reduction of the river flow.<ref>{{cite news|last=Hou|first=Ren-zhi|title=Ancient city ruins in the deserts of the Inner Mongolia Autonomous Region of China|journal=Journal of Historical Geography|volume=11|issue=3|date=July 1985}}</ref>

The lower part of the river was once confused to be the middle reach of the [[Yellow River]], which lies farther east but flows for a significant distance in the same direction paralleling the Ruo River.

The former [[Tangut people|Tangut]] city of [[Khara-Khoto]], now deserted, lies near the lower end of the river.<ref name="Kychanov">{{cite journal |last= Kychanov |first= E. 
|author-link=Evgenij Ivanovich Kychanov
|year= 1995 |title=Wen-Hai Bao-Yun: The book and its fate |journal= Manuscripta Orientalia|volume= 1|issue= 1|pages= 39–44|issn= 1238-5018|url= http://www.orientalstudies.ru/eng/images/pdf/a_kychanov_1995.pdf|access-date=3 July 2009 }}</ref> According to legend, the city was abandoned after [[Ming dynasty|Ming]] forces diverted the river away from the city in 1372.

European explorers that visited the area include [[Pyotr Kuzmich Kozlov]] (1907–1909) and [[John DeFrancis]] (1935).

In the 20th century, China's main spaceport, the [[Jiuquan Satellite Launch Center]] was constructed in the lower course of the Ruo Shui, with launch pads on both sides of the river.

==Basin characterization==
The basin subdivision reflects the different hydrological dynamics typical of the numerous natural contexts to be found within the basin. Most of the population is found in the midstream area, together with the oases and the agricultural consortia.
{| class="wikitable"
! !! Upstream area !! Midstream area !! Downstream area
|-
| Area || {{convert|27376|km2|abbr=on}} || {{convert|25391|km2|abbr=on}} || {{convert|99839|km2|abbr=on}}
|-
| Population (2015)<ref>{{cite web|url=https://sedac.ciesin.columbia.edu/mapping/popest/gpw-v4/|title=Population data from SEDAC for 2015}}</ref> || 3,169,747 || 23,869,741 || 4,053,878
|}

===Upstream area===
The main course of the river starts in the Qilian mountains at an elevation of 3650 [[Sea level|m asl]]. The [[loam]]y sediment load gives the waters a dark colour which, during time, has contributed to the river being described as ''Hei'' (黑), that is ''Black''. The [[orography]] in this part is obviously quite steep, with elevation peaks up to 5544 m asl and vegetation mainly made up of wild shrubs. Most of the [[precipitation]] fallen annually over the basin is concentrated in this area, creating two main river courses that join just north of the [[Stream gauge|hydrometric station]] of Yingluo.

===Midstream area===
The midstream area is characterized by the presence of wide ranges of cultivated fields, mainly in the oases that follow the principal route of the river. The city of [[Zhangye]], main centre of the region, is located next to a wide north-west bend of the river. The high population (940 in/km<sup>2</sup> in 2015) and croplands density implies a high [[Water footprint|water demand]], estimated at {{convert|2400000|m3|abbr=on}}/year in the 2001–2012 time frame. This demand is mainly satisfied using the river (71%) and groundwater (29%).
Notwithstanding the water influx from the rivers descending from the north-west side of the Qilian mountains, the total balance of the river in this part is negative (that is more water is lost than acquired).

===Downstream area===
After the stream gauge at Zhengyi, the river describes a wide north-east [[Meander|turn]], developing its last 400&nbsp;km in the western fringe of the [[Badain Jaran Desert]].<ref name="deserts">{{cite web|editor=Alles, David L.|url=http://fire.biol.wwu.edu/trent/alles/ChinaDeserts.pdf|title= China's Deserts|publisher=Western Washington University
|date=7 September 2010|access-date=29 October 2010}}</ref><ref name="imhist"/> Apart from the physiological riparian vegetation, this last part of the hydrological basin is mainly desertic, and the water losses caused by both evaporation and deep percolation cause the water flux to weaken. This pattern has caused the river to be named ''Ruo'' (弱, ''Weak'').<ref name="RS"/> After the Langxinshan stream gauge, the river splits into two entities: the ''Dong He'' (東河, ''Eastern river'') and the ''Xi He'' (西河, ''Western river'') that follow on in the desert for another 220&nbsp;km before emptying into two terminal lakes, the ''Sogo Nur'' and the ''Gaxun Nur'', respectively, collectively forming the [[Juyan Lake Basin]].<ref name="imhist"/><ref name="RS">{{cite news|author1=Walker, A.S. |author2=Olsen, J.W. |author3=Bagen |title=The Badain Jaran Desert: Remote Sensing Investigations|journal=The Geographical Journal|date=July 1987|volume=153|pages=205–210}}</ref> The delta is made of the [[alluvial fan]] of the river itself, crossed by hundreds of dry channels entrenched by the river during its frequent course changes. Because the climate of the area has followed a drying trend over the past 10,000 years or so, the delta is no longer considered active (i.e. deposits are not being built up significantly) and is slowly being eroded by wind and water action.<ref>{{cite web|url=http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_4/GEO_PLATE_F-18.shtml|archive-url=https://web.archive.org/web/20090205040441/http://disc.sci.gsfc.nasa.gov/geomorphology/GEO_4/GEO_PLATE_F-18.shtml|url-status=dead|archive-date=5 February 2009|title=Alluvial Fans in Asia|work=Geomorphology From Space|publisher=NASA|date=9 September 2009|access-date=29 October 2010}}</ref>

===Hydrological balance components===
The analysis of the different components of the hydrological balance in the three main areas of the basin allows to distinguish the different geoclimatic mechanisms at work.

{| class="wikitable" style="margin-left:auto; margin-right:auto;"
! Data about the 2001–2012 period [km<sup>3</sup>/year]<ref name="li18">{{cite journal |last1=Li |first1=Xin |title=Hydrological Cycle in the Heihe River Basin and Its Implication for Water Resource Management in Endorheic Basins |journal=Journal of Geophysical Research: Atmospheres |doi=10.1002/2017JD027889}}</ref> !! Upstream area !! Midstream area !! Downstream area 
|-
| [[Precipitation]] (rain and snow) '''P'''
| 8.66 || 2.48 || 4.53
|-
| [[Surface runoff|Superficial inflow]] '''R<sub>in</sub>'''
| --- || 2.59 || 1.10
|-
| Subsurface inflow '''G<sub>in</sub>'''
| --- || 0.24 || 0.09
|-
| [[Evapotranspiration]] '''ET'''
| 5.54 || 4.28 || 5.69
|-
| Superficial outflow '''R<sub>out</sub>'''
| 3.04 || 1.10 || ---
|-
| Subsurface outflow '''G<sub>out</sub>'''-
| 0.04 || 0.04 || ---
|-
| Variation of the water content of the sub-basin '''&Delta;W'''
| +0.08 || −0.11 || +0.03
|}

The hydrological balance, in its most complete version and formulate for a generic sub-basin, is structured as follows: <math>\Delta W = P + R_{in} + G_{in} - ET - R_{out} - G_{out}</math>.

Most of the precipitation is found in the upstream area, also showing a positive [[correlation]] with elevation.<ref name="li18"/> The evapotranspiration volumes are almost the same notwithstanding the high area difference among the three areas because of the consistent vegetation presence in the upstream (wild shrubs) and midstream (croplands) areas. In the <math>ET</math> term, when shifting from the vegetated (upstream and midstream) to the arid (downstream) areas, the [[transpiration]] element decreases in importance in favour of the simple [[evaporation]]. The volumetric balances, instead, show different signs: in the mountain area the input-over-output excess can be explained with the important precipitation volumes; in the midstream area the cultivations require a great amount of water that is extracted from the aquifers, increasing the water deficit; finally, in the downstream area the positive effects of the water replenishment programme of the Chinese government are visible in the increase of the stored water volume in the 2001–2012 period.

===Hydrometry===
Along the river course a series of stream gauges are located. For each, the following table states position and average of the (yearly cumulated) discharge.<ref name="zhang15">{{cite journal |last1=Zhang |first1=A. |title=Analysis of streamflow variations in the Heihe River Basin, northwest China: Trends, abrupt changes, driving factors and ecological influences |journal=Journal of Hydrology: Regional Studies |doi=10.1016/j.ejrh.2014.10.005|doi-access=free }}</ref>
[[File:Stream gauges HeiHe.png|thumb|Map of the stream gauges along the HeiHe]]
{| class="wikitable"
! Station !! Code !! Lon [°E] !! Lat [°N] !! Volume [hm³/year] !! Temporal series !! Area
|-
| Qilian<ref name="zhang15"/> || QL || 100.23 || 38.20 || 457 || 1968–2010 ||rowspan="2"| Upstream
|-
| Zhamashenke<ref name="zhang15"/> || ZM || 99.98 || 38.23 || 716 || 1957–2010
|-
| Yingluoxia<ref name="zhang15"/> || YL || 100.18 || 38.82 || 1584 || 1945–2012 || ''limit''
|-
| HeiHe Bridge<ref name="li18"/> || HB || 100.38 || 39.02 || ''n/a'' || ''n/a'' ||rowspan="3"| Midstream
|-
| Gao'ai<ref name="zhang15"/> || GA || 100.40 || 39.13 || 1034 || 1977–2010
|-
| Pingchuan<ref name="li18"/> || PC || 100.10 || 39.99 || ''n/a'' || ''n/a''
|-
| Zhengyixia<ref name="zhang15"/> || ZY || 99.42 || 39.79 || 1017 || 1957–2012 || ''limit''
|-
| Shaomaying<ref name="zhang15"/> || SM || 99.96 || 40.75 || ''n/a'' || ''n/a'' ||rowspan="3"| Downstream
|-
| Langxinshan<ref name="zhang15"/> || LX || 100.36 || 41.08 || ''n/a'' || ''n/a''
|-
| Juyan Lake<ref name="zhang15"/> || JY || 101.11 || 42.21 || ''n/a'' || ''n/a''
|}

Following the available [[Discharge (hydrology)|discharge]] data, a flow increase is present in the upstream area (up until Yingluoxia), followed by gradually decreasing values in the midstream area. This flow weakening is the result of the net balance between the tributaries descending from the north-east side of the Qilian Shan and the considerable derivations related to the cropfield area.<ref name="zhang15"/>

==See also==
*[[List of rivers of China]]
*[[Shule River]]
*[[Tarim River]]
*[[Weak River (mythology)]]

==References==
{{Reflist}}

{{China Rivers}}
{{Authority control}}

[[Category:Rivers of Gansu]]
[[Category:Rivers of Inner Mongolia]]