Geographical characteristics of Hunan province

Terrain Hunan is surrounded by mountains in the east, west and south, and the middle and north are hoof-shaped. Huping Mountain is located in Shimen County, with an altitude of 2099 meters, which is the highest point in the province. The central part is mostly hilly, with many wide basins and valleys. Dongting Lake Plain is in the north, with low terrain, and the sea level is mostly below 50 meters.

Hydrological characteristics of Dongting Lake water system

(A) the hydrological characteristics of Xiangjiang River Basin

Xiangjiang River is 856 kilometers long, with a drainage area of 94,660 square kilometers, and the whole province has a drainage area of 85,383 square kilometers, accounting for 89. 2% of the basin area. It is the largest river in Dongting Lake system.

1. Water level change.

The change of water level in Xiangjiang River and its tributaries is closely related to the annual distribution of precipitation in the basin. The rainy season begins in March every year, and the river rises. Entering the flood season after April, the precipitation continues to increase and the water level continues to rise. April-July is the season with the highest water level.

2. decisive victory.

Xiangjiang river basin is the first of the "four waters", but its annual runoff is second only to running water.

The annual distribution of Xiangjiang River runoff mainly depends on the annual change of precipitation. It is characterized by long flood season, large flow change and short and stable dry season. The flow began to increase in late March, and rose sharply in April, entering the flood season. May and June saw the largest flow, reaching the peak of the whole year. From April to July, the maximum four-month continuous average runoff accounts for more than 60% of the annual runoff, and after July, the runoff begins to decline, but it is 8. In September, sometimes due to typhoon interference or rainstorm, sometimes the water rises, which is still the flood season, and the runoff in flood season accounts for more than 70% of the annual runoff. 10 months later, the flood season ends and enters the normal period; The dry season is from 65438+February to the following February, and the runoff is the smallest with little change, accounting for about 3% of the annual runoff of 65438+ 10.

3. Sediment concentration.

The Xiangjiang River Basin is characterized by complex rock strata, different natural and geographical conditions and different human factors, which makes the sediment concentration in the basin change with time 106. Although the mountain areas where the tributaries flow upstream are steep, they are mostly hard rock layers with good vegetation, less erosion and siltation and less sediment concentration. The middle and lower reaches and some tributaries flow through Quaternary laterite and loose deposits, and Huagang rocks are distributed in some areas. Weathered shell is thick and the soil is loose. Whenever it encounters heavy rain, it is easy to be washed away, resulting in soil erosion, especially in some small watersheds with lack of vegetation, the erosion is more intense and the sediment concentration along the way is gradually increasing.

(2) Hydrological characteristics of Zijiang River Basin

The main stream of Zijiang River is 7 13 kilometers long, with a natural drop of 280 meters, with a basin area of 28 142 square kilometers and a provincial boundary area of 26,738 square kilometers, accounting for 95% of the basin area.

1. Water level change.

Zijiang River flows through the mountainous area, with steep slope and heavy rain in the basin. The hilly area downstream is also a high value area of precipitation. Whenever flash floods occur in the rainy season, the river surges and plummets, becoming a mountain river. Generally, it enters the flood season in April, and the water level keeps rising. The water level drops in April-June, and reaches the highest level in May-June. In August, the high water level began to decline, and the change was relatively stable, but sometimes there was heavy rain, which also formed a large flood peak. At the end of the flood season in September, it enters the normal season, and the water level after 1 1 is low, and the water level from 12 to 1 in the following year is the lowest, which is the dry season. Zijiang River flows between hills, and the water level changes greatly, generally between 10 and 20 meters.

2. decisive victory.

There are many mountains and hills in Zijiang River Basin, which speeds up the process of rainwater collection and is rich in runoff. Taking Taojiang River as an example, the total annual runoff is 22.7 billion cubic meters, the runoff modulus is 850,000 cubic meters per square kilometer, and the runoff depth is 860.3 millimeters. The regional distribution of runoff is uneven, forming a number of high-value centers with Xuefeng Mountain as the main body, and the depth of runoff is the lowest in Shaoyang Basin.

3. Sediment concentration.

The river sediment concentration in Zijiang River Basin is relatively small, mostly below 0.20kg/m3. Shangqiu Basin is a major agricultural area with soft rocks and loose soil. Whenever the rainy season, the rainstorm is more intense, and the river sediment concentration is higher. The middle reaches are mostly mountainous areas, with good vegetation and low river sediment concentration; Because most of the downstream sediment is deposited in Tuoxi Reservoir, the sediment concentration in the river channel is greatly reduced, and the average sediment concentration in Taojiang Station for many years is only 0. 143 kg/m3. This shows that the sediment concentration of Zijiang River decreases from upstream to downstream.

(3) Hydrological characteristics of Yuanjiang River Basin

The length of Yuanjiang River is1033km, 568km in the province, the natural drop is180m, the drainage area is 89163km2, and the province is 51066km2, accounting for 57%. 3% of the basin area.

1. Water level change.

Yuanjiang River flows between Xuefeng Mountain and Wuling Mountain, with a large riverbed gradient and a water level rise of 108.

2. decisive victory.

The Yuanjiang River basin is rich in precipitation, numerous tributaries and abundant in runoff.

3. Sediment concentration.

The sediment concentration in Yuanjiang River has the characteristics of regional distribution difference and time variation. In the upper reaches of Guizhou Province, it flows through hard rock areas. According to the determination of Guizhou Province, the sediment concentration in rivers is below 0.20kg/m3. The middle reaches are interlaced areas of soft and hard rocks, where hills and basins are scattered, with steep slopes and strong erosion. In addition, due to the influence of human activities, soil erosion is serious, and the sediment concentration in tributaries is above 0.229 kg/m3.

2. decisive victory.

The Lishui River basin is rich in precipitation, and the upper reaches are mountainous and steep, which is easy to collect precipitation and rich in runoff. The average runoff modulus for many years can reach 6.5438+100,000 cubic meters/square kilometer, and the runoff is the first in the province. The measured annual average runoff of Sanjiangkou, the control station, is 65.438+0.48 billion cubic meters, with an annual runoff depth of 997 mm. The upper reaches of Lishui River are rich in precipitation, making it one of the high-value areas in the province.

3. Sediment concentration.

Lishui river basin is one of the rainy and rainstorm areas in Hunan province, with many rainstorms. High strength. There are various rock formations in the basin, with high mountains and steep slopes in the upper reaches, strong erosion and denudation, unreasonable human activities and serious soil erosion. The difference between the maximum and minimum sediment discharge is more than 10 times.

6. Hydrological characteristics of four major rivers in Jingjiang River

After the runoff from Songzi, Ouchi, Taiping and Tiao County (1958 blocked) flows into Dongting Lake, the fluctuation of water level slows down through the regulation of the lake, especially at high water level, the lake is vast, the capacity increases and the fluctuation slows down. The annual water level changes from low to high, and then from high to low, forming a chubby flood peak. Above the big flood peak, there are often small flood peaks, but the fluctuation is not big. From the analysis of the main sources of flood, the four flood peaks are not high, but they are all very large and long, similar to the main stream of the Yangtze River below Yichang.

The highest flood level in Jingjiang usually appears from July to September every year, and the lowest flood level usually appears from February to March. .

The main stream of the Yangtze River flows into Dongting Lake through four tributaries, with an average annual runoff of11800 million cubic meters.

Seven, Dongting Lake water level changes

The water level of Dongting Lake is mainly restricted by precipitation and river regime change, which is mainly manifested in the dual influence of incoming water composition and river and lake water flow jacking. However, for a long time, the sediment deposition of the river and the boundary characteristics of the river bed have also brought certain influence to the change of lake water level.

1. Seasonal variation of water level.

Dongting Lake has a vast water area and a large throughput, which has a certain function of flood storage, so the water level is gradual. Since April, the Surabaya Basin has entered the rainy season, and the river has been rising. Precipitation is concentrated in March-June or April-July, and the maximum rainfall occurs in May or June. During the flood season, many large floods occurred continuously, forming a single or compound flood peak with a steep and thin peak shape. After the flood enters the lake area, the river surface widens, and influenced by many lakes, the peak shape is fat, and the water level rises less violently than the river. June-August is the rainy season in the Yangtze River basin, and the rainfall is concentrated at the same time.

2. Water level fluctuation degree and multi-year change.

Because the topography of the lake area decreases from northwest to southeast, the highest water level in different parts of the lake area is different, about 35-36 meters in the west Dongting Lake, 34-35 meters in the south Dongting Lake and 33-34 meters in the east Dongting Lake, and the outlet of Chenglingji is generally more than 33 meters.

Eight. Tinghu lake system runoff

1.

The runoff of Dongting Lake mainly comes from Hunan, Guizhou, Ruan Cheng and Songzi. The water from the Yangtze River is discharged into the lake from Lotus Pond and Taiping Tune. In addition, Miluo River and Xinqiang River are directly injected into the lake area. .

2. Annual and interannual variation of runoff.

The flow of four waters into the lake is the largest from April to August, accounting for 67. The total inflow of the four waters accounts for 9% of the whole year, and only accounts for 32% from September to March of the following year. Among the monthly inflow, May is the largest, followed by June, and 65438+February is the smallest. The amount of water entering the lake from June to 10 accounts for 90% of the total amount of water entering the lake, with July being the largest.

Geology South China is the most widely divided area of granite crust and the most distinctive geological unit in the world. Most of northern Hunan and southern Hunan belong to Yanshanian or late granite, which is rich in rare earth elements. Among them, around Dongting Lake in northern Hunan, there are abundant alluvial monazite placers, which are mainly produced in the Quaternary alluvium of rivers, such as the weathered granite in Xiang Si Mountain and Dayunshan Mountain in Yueyang, which is 80 kilometers along the whole basin after being washed by hot and humid rain. On the whole, Hunan's rare earth resources have the following characteristics: (1) Rare earth resources are widely distributed; (2) The monazite reserves are large; (3) The variety of light and heavy rare earths is complete; (4) The comprehensive utilization value is high. (Keywords: rare earth resources-occurrence state-Hunan, rare earth resources-characteristics-Hunan, genesis of the deposit)