Climatic reasons

Due to the alternating influence of winter monsoon and summer monsoon, the seasonal variation of precipitation in Guangxi is uneven, and the dry season and rainy season are distinct. April to September is the rainy season, and its precipitation accounts for 70% ~ 85% of the annual precipitation, which is prone to floods; The dry season is from 10 to March of the following year, and the precipitation only accounts for 15% ~ 30% of the annual precipitation, with little rain.

Influence of monsoon circulation

In normal years, the summer monsoon begins to enter Guangxi in late April and early May. The monthly precipitation has doubled. From mid-May to mid-June, the front belt lingered in South China, and the first precipitation peak was formed in various places. By mid-September, the summer monsoon retreated southward, cold air invaded southward, and precipitation decreased rapidly. Due to the great difference between the intensity of the subtropical high in the western Pacific and the time of westward extension, advance and retreat, the precipitation in Guangxi varies greatly from year to year and is unevenly distributed.

Figure 3- 1 contour map of average precipitation distribution in Guangxi for many years

Figure 3- 1 isoline of average precipitation in Guangxi over the years

Figure 3-2 Relationship between Precipitation and Grain Output in Xingbin District

(According to Zhang Zhigan, 2005)

Figure 3-2 Correlation between Precipitation and Grain Output in Xingbin District

Every year from May to June165438+1October, typhoons or tropical storms invade Guangxi. In the years with fewer tropical cyclones, summer and autumn droughts are prone to occur. In addition, the South China Sea high and the Indian low also have great influence on the weather in Guangxi in winter and spring. The intensity and location of the South China Sea High over the South China Sea or Indo-China Peninsula in winter and half a year vary greatly every year. If the intensity of the South China Sea high is weak, its center is over Indochina Peninsula, and Guangxi is affected by the northwest airflow, there will be a situation of drought and little rain in winter and spring.

El Nino and La Nina also have obvious effects. In the years affected by El Nino, there is little rain in spring in central Guangxi; In the years affected by La Nina, there was less rain in autumn and winter.

(2) High temperature and large evaporation aggravate the drought.

Comparative analysis shows (Table 3-5) that the rainfall in central Guangxi is generally less than that in neighboring Guiping and Guangzhou at the same latitude, but the evaporation is larger. Moreover, in years with less precipitation, evaporation is greater. In the statistical 10 year, the evaporation of Liuzhou Station is greater than the precipitation in 6 years, and that of Nanning Station is greater than the precipitation in 8 years. In dry years, the evaporation of Liuzhou Station is 1. 1 ~ 1.76 times of the precipitation. Nanning station 1. 12 ~ 1.33 times. The lack of rain and high temperature, accompanied by the increase of evaporation, aggravated the development of drought, resulting in a sharp drop in the water volume of reservoirs, the drying up of many streams and ponds, and a sharp drop in the groundwater level, resulting in a serious shortage of production water. At the same time, the drought caused water loss in cultivated land and reduced crop production. According to statistics, from July to September, 2000, the monthly evaporation in Guigang City was above 265438±00mm, which was 20 ~ 40 mm more than the same period in previous years. For three consecutive months, the evaporation was above 2 10mm, which was the first time since meteorological records were recorded. Due to the continuous drought, the affected area of crops in various places has expanded rapidly. The affected area of late rice, late corn, sugarcane and other crops in Guigang City reached 12.3× 104 hm2, and the drought-affected area exceeded 70%.

(C) uneven spatial and temporal distribution of rainfall, seasonal drought is serious

Central Guangxi is located in the south subtropical zone. Although the annual precipitation is mostly between 1, 300 ~ 1, 600 mm, and even as high as 1, 900 ~ 2000mm in wet years, the temporal and spatial distribution of precipitation is uneven, and rainfall is generally concentrated in the rainy season of 4 ~ 5 months (Figure 3-3 and Table 3-6), spring drought or autumn drought. Therefore, the average multiple cropping index for many years is only 1.84, but at present, there is almost annual drought, which seriously affects the development of agricultural production. The data analysis shows that according to the meteorological data of Liuzhou and Nanning, the distribution of precipitation in this area is extremely uneven, and the precipitation is mainly concentrated in March-April, such as 199 1 year and 1992, which accounts for more than 60% of the annual precipitation. Even in years with abundant precipitation, such as 1994 and 1998, the precipitation in the six months from March to August accounts for more than 87% of the annual precipitation, while the precipitation in the other six months is less than 13% of the annual precipitation. It can be seen that the uneven distribution of rainfall during the year is one of the main reasons for seasonal drought.

Table 3-5 Comparison Table of Precipitation and Evaporation in the Areas Adjacent to Similar Latitude in Central Guangxi Table 3-5 Comparison Table of Precision and Evaporation in the Areas Adjacent to Similar Latitude Unit:10-1mm.

Figure 3-3 Typical Precipitation Curve in Central Guangxi

Figure 3-3 Typical Precipitation Curve in Central Guangxi

Table 3-6 Analysis Table of Precipitation Characteristics of Typical Weather Stations in Central Guangxi Table 3-6 Precision Characteristics in Central Guangxi