Kang Lingling，Wang Jinhua，Wang Yunzhang and Xiao Peiqing

Institute of Hydraulic Research，Yellow River Conservancy Commission，Zhengzhou 450003，China

Abstract:Temporal and spatial characteristics of runoff were analyzed according to natural runoff data at Lanzhou station between the year of 1934 to 2003 and other station’s data from the year of 1955 to 2003. The results showed: ① Runoff of the basin mainly comes from the area between Jimai and Lanzhou, and the reach from Jimai to Maqu is the main sources; ② Runoff in flood season accounted for 69% to 80% of that in whole year; ③ Three much water and three low water periods were experienced during recent 70 years at Lanzhou station and presented period of quasi three year; ④ Recently runoff from each area reduced, especially in Taohe and Matang area; ⑤ Runoff of July to December decreased 20% to 38%.

Key words:  decrease, runoff , above Lanzhou, Yellow River upper reach

Area above Lanzhou in the upper reach of the Yellow River (above Lanzhou or the whole area is shortened) passes through Gansu, Qinghai and Sichuan province. The catchment area is 22.26×10⁴ km², which accounted for 30% of the middle reach. The area is mainly affected by monsoon in the plateau. So the middle and southern part is humid area and semi-humid area, but the northern part is arid and semi-arid area. The area, especially middle and southern part has plenty of rainfall affected by circumfluence of plateau monsoon and topography, which become the main source of runoff in the upper reach even the whole Yellow River. According to the statistics between the year of 1956 to 2003, annual runoff in Lanzhou station reached to 3.28 ×10¹⁰m³, accounted for more than 60% of the total runoff in the upper and middle reach of Yellow River.

Since 1920s, runoff in the upper reach of the Yellow River dramatically decreased which not only aggregated the conflict of Yellow River water resources and deteriorated ecology environment, but also brought about economic loss of the middle and lower reach of the Yellow River.

The amount of water resources in area above Lanzhou not only relate to supply and demand of water resources of the middle and lower reach, but also has direct effects on the benefits of the main hydraulic projects and dry river frequency in the lower reach.

1  Temporal and spatial characteristics of runoff

1.1  Different areas of the catchment

According to the geography and water system of the basin considering the data of gauging station, area above Jimai (Dari) (river source was shortened), Jiman to Maqu area (Jima area was shortened), Maqu to Tangnaihai area (Matang area was shortened), Tangnaihai to Lanzhou area (Tanglan area was shortened), Datonghe watershed of Huangshui (Huangshui was shortened) and Taohe watershed was divided in area above lanzhou.

1.2  General information

Water conservancy projects were built in the main branch above Lanzhou area and store capacities were kept as well as water demand was needed owing to agriculture and population’s increase, which led to runoff was affected by the above population activities. In order to analyze the temporal and spatial characteristics of runoff reasonably, basic data used to analyze was the reductive natural runoff.

The reductive methods were followed as: ① runoff in Jimai, Minhe and Hongqi gauging station denoted that of river source area, Huangshui area and Taohe area. ② The difference value of gauging station runoff of lower and upper reach denoted the interval runoff. At the same time, the data length was considered, so the runoff length was chosen in the year of 1956 to 2003, apart from 1934 to 2003 year of Lanzhou station, which is helpful to analyze the temporal change of runoff.

  Runoff in the whole area was calculated by the runoff of six gauging station under the condition of no special explanations, because the sum of runoff in the above area is almost same with runoff of gauging station in Lanzhou area. According to reference, the beginning period of flood season is between May and September, so the statistics period of flood season runoff is chosen between May and October considering the realistic status of runoff distribution.

1.3  Area distribution

The percentage of area and runoff was seen in Table 1. From the table, the acreage sum of Jima, Matang and Tanglan area accounted for 53.5%, but runoff accounted for 67.4%, so these three areas are the main water resources area. Runoff variation of them can basically affect the more or less of water resources of the catchment. Catchment of 41,044 km² in Jima area accounted for 18.4%, but runoff accounted for 31.6%, which can proven this area is the key water resources area in the upper reach of the Yellow River.

Table 1   Annual runoff of each area and its percentage in the whole area

From relief map, Jima area lies in the biggest bend area of the Yellow River upper reach and has low latitude. The area is wide, so rainfall was formed by the vapor from the southwest and southeast, so its rainfall is plentiful in the upper reach of the Yellow River. In addition, good vegetation including grassland and wetland existing in the area can keep water resources, so it was called “Natural Pool” of the Yellow River.

1.4  Annual runoff distribution

Annual runoff distribution of each area was given in Fig. 1. The results showed average runoff of area above Lanzhou was 3.282×10¹⁰ m³. Runoff in flood season and non-flood season is 2.529×10¹⁰ m³ and 7.53×10⁹ m³, which accounted for 77% and 23% separately. Considering the runoff in each area, runoff in flood season accounted for 69% to 80%, among of them the river source area is the main part and presented a decreased trend; runoff in season of non-flood only accounted for 20% to 31%, which showed the runoff in flood season is very high.

Fig.1   Annual runoff distribution of each area

For monthly runoff, the minimum runoff always occurred in February of each area, but the maximum runoff occurred differently. For example, more runoff occurred in July in River source and Jima and Matang area of the Yellow River upper reach, and reached the subordination peak value in September; while more runoff occurred in August, October and September of Tanglan, Huangshui and Taohe area, and reached the subordination peak value in August in Huangshui area. The results showed the annual runoff distribution has big variations of the catchment.

2  Temporal variation of runoff

2.1  Years variation of runoff

Years values of runoff in Lanzhou gauging station were given in Table 2 since 1934. Considering the characteristics of runoff since 1950s, the results were followed by:

Table 2   Years values of natural runoff in Lanzhou gauging station

Note: (1): 1934 to 1939 year was choose r in 1930s. (2) 2000 to 2003 year was choose since 2000.

⑴Early values of runoff in Lanzhou gauging station was consistent with the runoff values of the main area since 1950s, and is totally the same with the years value of runoff of flood season. The runoff is rich in 1930s, 1940s, 1960s and 1980s, but less in other periods. Especially runoff decreased much since 1990s.

⑵Depart of runoff in Lanzhou gauging station of flood season is almost similar apart from 1970s.

⑶Runoff in flood and non-flood season of other areas keep the same trend with runoff in Lanzhou station apart from Matang area, Tanglan area and Huangshui area.

⑷Even runoff in flood season of Matang area is consistent with runoff in Lanzhou gauging station, but is different with runoff in non-flood season. Among of them, runoff presented negative departure in 1970s and positive departure in 1990s.

⑸Runoff trend of Tanglan area and Huangshui area is almost same. Years value of flood season in 1950s reached beyond the average value, which is different from runoff in Lanzhou station runoff didn’t reach its minimum value since 1990s, but yearly runoff reduced 10% in 1970s and 1980s in these two areas.

2.2  Variations of different phases

Runoff curve was given in Lanzhou gauging station since 1934 (Fig.2). It can be clearly seen, variations of natural runoff in Lanzhou station presented characteristics of different phases. Three much water phases and three low water phases were seen in Table 3, and average phases of much water is 13 years, the longest 18 years and the shortest 6 years, which is 2 times of that of the phases of low water. Durations of low water is relatively lower than that of much water periods of 10.3 yeas average values, the longest 14 years and the shortest 6 years, and its probability is 6.5 times much water phases.

Fig.2   Runoff in Lanzhou station (solid line) and the average values of five years (dashed line)

Table 3  Yearly natural runoff of different phases in Lanzhou gauging station

2.3  Periodic variations

The results showed variations of runoff in the upper and middle reach presented periodic characteristics ^[3]. Are natural runoff variations in areas above Lanzhou of the Yellow River periodic evidently too? This can be explained from the following two aspects.

2.3.1  Analysis of function spectrum

According to theory of function spectrum analysis in statistical diagnosis and prediction of current climate edited by Wei Fengying, the biggest lag is 23 years. So function spectrum of natural runoff was calculated of 70 years from 1934-2003, the results was shown in the below Fig.3.

Fig. 3   Function spectrum of natural runoff in Lanzhou station of the Yellow River upper reach

(T and L denote period and the number of wave separately in horizontal axis; vertical axis denotes spectrum density)

It can be seen from the critical value of ＝0.05, runoff variations in Lanzhou station has the main periods of 2.9 years and 2.7 years(smooth line denotes standard spectrum of red noise), that is period of 3 years is more clear characteristics.

2.3.2  Wavelet analysis

Wavelet analysis not only measured period variation of natural runoff but also showed the location of period variation. Morlet wavelet analysis was used in the paper to calculate natural runoff in Lanzhou station of the Yellow River upper reach. The results were shown in Fig.4.

Fig.4 Wavelet analysis of natural runoff in Lanhzou station of the Yellow River upper reach

(Horizontal axis denotes time (year); Vertical axis denotes the length of period wave (year))

It can be seen from Fig.4 runoff variations of Lanzhou station clearly presented period of three years from the middle of 1930s to the end of 1970s. This characteristic of period variation is not evident since 1980s. But period variations of 9 years and 16 to 20 years are stable, which passed through the whole time length.

3  The prominent characteristic of recent runoff change

3.1  The characteristic of annual runoff change

Table 4 showed recently （1990 to 2003 year）annual runoff coming from each area upwards Lanzhou station and the ratio to the past period（1956 to 1989 year）. Those conclusions can be made according to the table.

Table 4   Recent and he past period annual runoff in each area 

                                                                                     unit: ×10⁸m³