Ruo Shui


Ruo Shui is a major river system of northern China. It flows approximately from its headwaters on the northern Gansu side of the Qilian Mountains north-northeast into the endorheic Ejin Basin in the Gobi Desert. The river forms one of the largest inland deltas or alluvial fans in the world. Its drainage basin covers about in parts of the Chinese provinces of Gansu and Inner Mongolia.

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. 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'

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.
Upstream areaMidstream areaDownstream area
Area
Population 3,169,74723,869,7414,053,878

Upstream area

The main course of the river starts in the Qilian mountains at an elevation of 3650 m asl. The loamy sediment load gives the waters a dark colour which, during time, has contributed to the river being described as Hei, 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 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 and croplands density implies a high water demand, estimated at /year in the 2001–2012 time frame. This demand is mainly satisfied using the river and groundwater.
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.

Downstream area

After the stream gauge at Zhengyi, the river describes a wide north-east turn, developing its last 400 km in the western fringe of the Badain Jaran Desert. 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. After the Langxinshan stream gauge, the river splits into two entities: the Dong He and the Xi He that follow on in the desert for another 220 km before emptying into two terminal lakes, the Sogo Nur and the Gaxun Nur, respectively, collectively forming the Juyan Lake Basin. 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 and is slowly being eroded by wind and water action.

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.
Data about the 2001–2012 period Upstream areaMidstream areaDownstream area
Precipitation P8.662.484.53
Superficial inflow Rin---2.591.10
Subsurface inflow Gin---0.240.09
Evapotranspiration ET5.544.285.69
Superficial outflow Rout3.041.10---
Subsurface outflow Gout-0.040.04---
Variation of the water content of the sub-basin ΔW+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:.
Most of the precipitation is found in the upstream area, also showing a positive correlation with elevation. 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 and midstream areas. In the term, when shifting from the vegetated to the arid 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 discharge.
StationCodeLon Lat Volume Temporal seriesArea
QilianQL100.2338.204571968–2010Upstream
ZhamashenkeZM99.9838.237161957–2010Upstream
YingluoxiaYL100.1838.8215841945–2012limit
HeiHe BridgeHB100.3839.02n/an/aMidstream
Gao'aiGA100.4039.1310341977–2010Midstream
PingchuanPC100.1039.99n/an/aMidstream
ZhengyixiaZY99.4239.7910171957–2012limit
ShaomayingSM99.9640.75n/an/aDownstream
LangxinshanLX100.3641.08n/an/aDownstream
Juyan LakeJY101.1142.21n/an/aDownstream

Following the available discharge data, a flow increase is present in the upstream area, 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.