Climate and its influence on wetland ecological environment

1. Climatic characteristics of Huixian karst wetland

Huixian karst wetland is located near the Tropic of Cancer and belongs to a typical subtropical monsoon climate. Located in the southern edge of the rainstorm center in northern Guangxi, it is rainy in Guangxi. Influenced by strong solar radiation and monsoon circulation, it has four distinct seasons, mild and humid climate, abundant precipitation and abundant heat. Summer in this area is long and humid, with southerly winds prevailing and rainy; Winter is short and dry, with prevailing northerly winds, low temperature and little rain; The temperature in spring and autumn is similar, the temperature in autumn is slightly higher than that in spring, and the alternating law of winter and summer monsoon is obvious. Because of the complex terrain, sometimes there are frequent cold air activities and more disastrous weather. According to the data of Guilin Meteorological Bureau, the annual average temperature in the study area is 19.5℃, the monthly average minimum temperature is 8.6℃(65438+ 10 month), the monthly average maximum temperature is 28.9℃ (July), the extreme maximum temperature is 38.8℃ and the extreme minimum temperature is -3.3℃. The annual average rainfall is (1951~ 2008)1330.4mm, the annual average maximum rainfall is 2452.7mm, the annual average minimum rainfall is 13 13.3mm, the rainfall peak is in May, and the monthly average rainfall is 330. The maximum rainfall in a day is about 276.4mm, and the annual rainy season lasts on average 130 ~ 140 d, mainly from April to September, especially from May to mid-July, which is a period of frequent rainstorms, with rainfall accounting for about 50% of the whole year, and rainstorms average 7.6 times a year, often causing waterlogging. The main reason is that with the gradual change of spring and summer seasons, cold air invading this area from the north is more frequent. At the same time, the western Pacific subtropical high gradually strengthened and moved northward, and the southerly airflow behind the high affected the sky over South China. The southerly airflow in front of the Bay of Bengal trough often affects the Nanling Mountains, and the southerly airflow continuously transports the water vapor over the South China Sea and the Bay of Bengal to this area. If there is a cold front in the north and the upper trough and vortex move south, there will often be strong convective precipitation and heavy rain. If the cold air in the north and south is weak, the cold air first invades from the Xiang-Gui Corridor, and often forms a stationary front with warm air near the study area to move north and south, and there will be a continuous rainy weather process. In the middle and late July, the rainfall decreased obviously, and high temperature and dry weather appeared. In autumn, affected by the winter wind, cold air in the middle and high latitudes began to affect this area frequently. The weather is sunny and rainy, the autumn is crisp and the climate is pleasant, which is the best season for tourism. Among them, the precipitation is obviously less after September, and the dry season is 10 to March of the following year, and the wetland often has obvious drought; 1~ February is the driest and coldest period in a year, and the temperature is mostly 1 ~ 5℃.

The average annual evaporation in Huixian County is 1569.7 mm, and the maximum evaporation is in July, reaching the frost-free period of 199 mm for 302 days.

Second, climate change in the past half century.

There is no record that the hydrological conditions on the surface (or underground) are changed due to natural conditions such as large-scale natural river diversion, karst collapse and river channel (including underground river pipeline) blockage, thus affecting the ecological environment of the wetland. However, global warming and precipitation reduction are obvious in this area, and the impact of short-scale multi-year climate change on wetland environment is very obvious. The climate of Huixian karst wetland has the following changes:

1) The temperature is rising year by year. From 1962 to 2006, the overall temperature showed an upward trend year by year, with obvious changes (Figure 5- 1). For example, in 1969, the annual average temperature was 18.96℃, and 199 1 year rose to 19.43℃, reaching 19.56℃ in 2006. In less than 40 years, the annual average temperature has risen by more than 0.6℃.

Figure 5- 1 Changes of annual average precipitation and annual average temperature in Guilin (the thick line in the middle is the 5-year moving average)

2) Extreme weather and climate events occur frequently. Since 1962, the rainfall in Huixian karst wetland has fluctuated periodically, with a slight decreasing trend, but the change is not obvious (Figure 5- 1). In recent years, the precipitation has even increased in some years. For example, the average annual precipitation in 1969 is 1300mm. 199 1 year is 1407mm, 1997 is 18 18mm, and in 2006 it was 1934mm. However, the uneven degree of precipitation has increased, and precipitation mainly appears in the form of continuous rainstorm and heavy rainstorm. The frequency and intensity of rainstorm and heavy rainstorm have obviously increased (Figure 5-2). For example, from April to August, the proportion of precipitation in the total annual precipitation increased significantly, especially from 2005 to June 2007, the precipitation has reached more than 65,438+0/4 of the total annual precipitation. The dry season is long, especially in 2007, when the wetland precipitation was the lowest in recent 10 years, but the precipitation in June of that year accounted for 1/3(33%) of the total annual precipitation. In addition, the frequency and duration of other extreme weather and climate events such as high temperature, drought and freezing are also increasing. These unfavorable factors have obvious influence on the ecological process and evolution of wetlands, which are manifested in the increase of floods, droughts and pests.

Figure 5-2 Huixian Wetland Year (Lingui and Yanshan Meteorological Stations)

Thirdly, the impact of climate change on wetland ecological environment.

The impact of climate change on wetland ecological environment can be roughly summarized in two aspects.

1. Drought and water shortage, reduced water area, decreased groundwater level, and seriously damaged wetland ecology.

As can be seen from Figure 5- 1, the general climate change trend is: the temperature has risen rapidly since 1984, while the precipitation has changed periodically, and has decreased year by year since 2000 (1955 ~ 1965 and1982 ~/kloc)

The decrease of precipitation year by year, the most direct impact is the decrease of wetland water supply. According to the 5-year moving average precipitation, the atmospheric precipitation in 200 1, 2003 and 2005 (the multi-year average precipitation is calculated as 1960mm, 1880mm and 1660mm respectively) has a negative impact on the wetland basin (the catchment area is calculated as1860mm). 3.5532× 108m3, 3. 1374× 108m3, the total water resources in 2003 decreased compared with 20051512×104m3.

With the decrease of water resources, the temperature in the wetland area continues to rise, and the dry season is prolonged, which leads to the continuous aggravation of air drying and ground evaporation (Figure 5-3). The emergence of dry and wet climate and frequent droughts (increased frequency and intensity) are not conducive to the accumulation of water in the wetland, which directly leads to the cut-off of rivers, the drying up of springs, the continuous decline of surface and underground water levels and the reduction of water area, which not only directly affects the hydrological process of the wetland surface and underground, but also directly affects the wetland. According to the literature, during the 845 years from the fourth year of Song Chongning (1 105) to 1949, serious drought occurred in the study area for 49 years, especially in the areas where Huixian County and Sitang Town are located. Because it is located in the "Gui Xiang Corridor" with airflow taxiway, the warm and humid airflow is difficult to stagnate, and it is the most frequent drought area. The main drought records are as follows:

1953 June16 ~ July 25th, 40 days without rain and drought; 1954, 1955 and 1958 are all drought.

From August of 1963 to August of 131October 27th, there was no precipitation for three consecutive months, which was a severe drought year.

Figure 5-3 Absolute Humidity of Huixian Wetland (Lingui and Yanshan Meteorological Stations)

1965 from June 20th to July 30th, the national rainfall was only 6 1. 1 mm. ..

1August 29th, 969 to19th1October 3rd, the rainfall was only 0.7mm. ..

1August 20, 974 to 1654381October 30, the rainfall was only 52.8mm, and there was a serious drought.

1August 27, 976 to 10/0/October 6, there was no rain and the drought was serious.

The drought occurred from September 2 1980 to1October 16.

From1August 1986 to 161October 22, there was no rain for 68 consecutive days, which caused a serious drought.

The groundwater level of Huixian karst wetland is generally 0.5 ~ 1.5 m from the surface, and the lowest is about 3 m from the surface in dry season. After the drought of 10 in 2007, most karst springs (underground rivers) in Huixian wetland dried up, the rivers were cut off, the groundwater level dropped below 3m, the lake water area was obviously reduced, the swamping process of wetland lakes and the degradation process of swamps were accelerated, and with the shrinking living area of aquatic plants and the reduction of biological habitats, some wet aquatic plants, including Isoetes sinensis and common wild rice, were on the verge of extinction. Due to the obvious reduction of precipitation, the amount of water in lakes and rivers is small, and most rivers and lakes are in a static state, and the water quality is deteriorating. From June, 5438 to October, 2007, due to drought and water shortage, the water level of Taiping River in the west of the wetland decreased, and the sewage treatment system in Lingui County in the upstream was not perfect. Most of the domestic sewage in the county is discharged to Taiping River, and some industries are polluted and over-bred. From June+10 in 5438, the river turned black and smelled bad. By 65438+February, the Taiping River, Qingshui River and Xiang Si River had nearly dried up (Figure 5-4 to Figure 5-6). On February 20, 2009, due to the drought, the water flow in Bengqiao Village, Dongshan Village Committee, Nanbianshan Township, Lingui County, which is located in the south of the wetland, decreased, and the wastewater from three duck farms (raising 5,000 ducks) behind the river was directly discharged into the river, which also caused the water quality to deteriorate (black and smelly), which seriously affected the lives and production of villagers.

Figure 5-4 Dry Acacia River Bed

More seriously, due to the reduction of water resources, the water storage depth of wetland lakes continues to decline, and the surface water area decreases. Taking Mudong Lake as an example, the average water depth of the lake was 0.8 ~ 1. 1m in 2006, and decreased to 0.5~0.8m in 2008. The persistent drought in 2009 caused most shallow lakes or swamps such as Li Antang Wetland to dry up. In the past, the bird habitat rich in aquatic plants, fish, shrimp and shellfish has become a dry desert. However, the water level of the lake drops, which makes the surface of the shallow lake and most of the marshes dry up, and also provides conditions for people to reclaim land or dig up breeding places (fish ponds, duck farms, etc.). ) and other destructive development (Figure 5-7).

2. Frequent floods.

Due to the special hydrogeological structure, Huixian karst wet area was originally one of the three major waterlogging areas in Guangxi. Every year, a large area of waterlogging is formed in the rainy season. In recent years, extreme weather has further aggravated the degree and scope of waterlogging in this area. As can be seen from Figure 5-2, since 1997, the variation range (precipitation intensity) and frequency of the precipitation curve of Huixian karst wetland have obviously increased, and the rainfall mainly appears in the form of continuous rainstorm and rainstorm. In a short time, a large amount of rainwater is concentrated in the low-lying wetland, but the two main drainage outlets in the east and west of the wetland are narrow and their flood discharge capacity is limited. In addition, the original swamps and lakes in the wetland have been artificially developed into cultivated land in a large area, which makes the flood storage function of the wetland obviously weakened, which is the root cause of large-scale waterlogging in the wetland and has a great impact on the wetland ecology. According to incomplete statistics, from 1 105 to 1949, catastrophic floods occurred in 48 years. After 1950, floods became more frequent, showing a trend of minor disasters in two years and major disasters in five years. For example, in May of 1978, the rainfall in Huixian reached 328.3mm from May of 15 to May of 18, causing serious floods; In the first half of June, 2007, there were continuous rainstorms with a total rainfall of about 340mm (Yanshan Station), especially on June 9 and June 13. After the continuous rainstorm of 100 mm, the water level of Wang Jia Bridge on Xiangsijiang River rose by about 2m in less than 12h, and the water level was close to the elevation of 150.0m above sea level. Huixian karst wetland has become Wang Yang, and the flooded area of Qingshuihe River basin wetland and Jiutoushan floodplain only exceeds 1000 hm2. From June 12 to June 13, 2008, a torrential rain occurred in Guilin, causing a once-in-50-year flood, which caused the water level in the lower reaches of the wetland to soar to 15 1.0 m, and the Liangfeng River flooded Guiyang Highway (Figure 5-8, right), with a total wetland area.

Figure 5-5 The dried-up underground estuary of Qingshuijiang Wetland has cracked the ground and killed a large number of fish.

Figure 5-6 Longshan Beihu Wetland under the influence of drought and water shortage and human activities.

In addition, the frequency of other extreme weather and climate events such as freezing and hail is also increasing. For example, in March of the 17th year of Kangxi (1678), February of the 9th year of Yongzheng (173 1), February 11th of the 26th year of Qianlong (176 1), and January of the 30th year of Guangxu (199).

Figure 5-7 Development Marsh (August 2009)

Figure 5-8 June 2008 14 Huixian Wetland Flood