This problem is divided into two aspects. The first is precipitation. From the time point of view, China is in the monsoon region, with more precipitation in summer and autumn and less precipitation in winter and spring, so the old ones are unevenly distributed. Spatially, the humid air flow from the Pacific Ocean can increase the precipitation in China, but many mountains and rivers in China hinder the warm and humid air flow from moving northward, resulting in uneven distribution of precipitation in inland and coastal areas. Secondly, it is related to the flood season of each river. Generally speaking, the water quantity of each basin in China will increase in spring and summer, and the water resources in the southern basin are relatively abundant than those in the north, so the water resources in China are unevenly distributed in time and space. . . . In addition, the spatial and temporal distribution of water resources in China is uneven, and the combination of water and soil resources is uneven. China has a vast territory, spanning three latitudes: high, middle and low. The regional distribution of precipitation and runoff in China is extremely uneven due to the great difference between east and west and the great difference between north and south latitudes, and the general trend is to decrease from the southeast coast to the northwest inland. The distribution trend of surface runoff in China is basically similar to that of precipitation, but the regional distribution is more uneven than that of precipitation. Generally speaking, there is more water, more people and less land in the south; There are many people and little water in the north. The total amount of water resources in the four southern regions accounts for 8 1% of the whole country, the per capita water consumption is about 1.6 times of the national average, and the water consumption per mu is 2.3 times of the national average. Among them, the per capita water resources of southwest rivers reach 15 times of the national average, and the water resources per mu reach 12 times of the national average. The total water resources of the Yellow River, Huaihe River and Haihe River in the north account for only 7.5% of the country, while the population and cultivated land account for 34% and 39% of the country respectively. In particular, the per capita and per mu water consumption of Hailuan River is the lowest in China, with per capita water consumption being 16% of the national average and per mu water consumption being 14% of the national average, making it one of the regions with the most serious water shortage in China. Compared with the north, the per capita water consumption of the former is 4.5 times that of the latter, and the average water consumption per mu is 9. 1 times. Compared with Hailuan River, the per capita water consumption of southwest rivers is 89 times that of the latter, and the average water consumption per mu is 87 times that of the latter.

The time distribution of water resources in China has changed greatly, and floods and droughts are frequent. According to the law of water cycle, the water cycle in nature is unstable in time, so the time distribution of water resources is uneven. In particular, China is located in the world-famous East Asian monsoon region, which belongs to a remarkable continental monsoon climate, so the annual and inter-annual changes of precipitation and runoff are quite large, and the changes in water-deficient areas are generally greater than those in water-rich areas. According to statistics, the maximum continuous precipitation for four months in most parts of the country accounts for about 70% of the whole year. The maximum 4-month runoff in most parts of the south accounts for about 60% of the annual runoff, and it can reach more than 80% in parts of North China and Northeast China.

The ratio of maximum to minimum annual precipitation is 2-4 times in the south and 3-6 times in the north. The ratio of the maximum and minimum annual runoff of the Yangtze River, the Pearl River and the Songhua River is 2-3 times, that of the Yellow River is 4 times, that of the Huaihe River is 15 times and that of the Haihe River is 20 times. Moreover, there is a phenomenon of continuous abundance and drought in successive years. For example, the average annual runoff of the Yellow River 1 1932 1938+0 is 30% less than normal, and the average annual runoff of Haibei River 1980 ~ 1984 is 65438 less than normal. In the past 80 years, Songhua River has experienced a dry season and a wet season of 1 1 year and 13 years. 19 16 ~ 1928 The annual average runoff is 4 1% less than normal, while 1953 ~.

The annual disparity and high concentration of precipitation and runoff not only bring difficulties to the development and utilization of water resources, but also are one of the important reasons for frequent floods and droughts and unstable agricultural output.