The heavy losses caused by the recent floods in southwest China's Sichuan Province and Chongqing Municipality have attracted wide attention at home and from abroad. It is held that the huge amount of water storage in the Three Gorges reservoir has caused continual downpour in the Sichuan-Chongqing region. To counter this rumor, this correspondent from People's Daily had an exclusive interview with several experts with the Office of State Flood Control and Drought Relief Headquarters, the Ministry of Water Resources and China Meteorological Administration. They expressed the view that the cataclysm in the Sichuan-Chongqing region has nothing to do with the Three Gorges Water Storage Project.

The formation of floods

Simply speaking, the recent severest floods in the Sichuan-Chongqing region is caused by the low vortex shear climatic system, which is fairly common and the main cause of rainfalls in China's southwest region. Although the weather system that caused the recent floods is common, the amount and the location of the rains in southwest China this time are accidental. The natural phenomenon has its periodicity and non-repetitiveness, which will not simply repeat at the same time and in the same place.

The cataclysm this year unlike previous ones

The flood in the upper reaches of the Yangtze River, that is, the flood in the Chuanjiang River, usually happens in July and August, but this year, it comes as late as September. However, there has always been the saying, "autumn rain comes to the western region of China". Autumn floods often occur in the Hanjiang River and Chuanjiang River, the middle reaches of the Yellow River and the Songhuajiang River. China has a vast territory, the Pearl River usually enters the flood season in April each year, and June, July and August are the country's main flooding period. The flood period in the Yellow River Valley does not end until after the Frost's Descent (18th solar term) in October. It was at this time last year that fairly serious floods occurred in the Weihe River and Hanjiang River.

The reason for the heavy losses caused by floods in the Sichuan-Chongqing region is attributed to the downpour in a short span of time, which led to large peak flow, high water level and rapid rising tide. Furthermore, the torrential rains concentrated on such a small, 40,000-sq km area along the Qujiang River, a tributary of the Jialingjiang River, and the Xiaojiang River, a tributary of the Yangtze River. People were caught unprepared by the abrupt rise and fall of floodwater. The water level of the Qujiang River swelled over 20 meters in one day or so, inundating many dikes, and causing serious disasters.

Relationship between the food and water storage of Three Gorges Project

Experts believe that there is no direct relation between the severe rainstorm in the Sichuan-Chongqing region and the increased amount of evaporation caused by the Three Gorges water storage. Despite the increased water retention and the enlarged water surface, the amount of evaporation is limited, so it is impossible to cause such torrential rains.

Rainfalls in the Sichuan-Chongqing region are mostly concentrated in May-September, and there are more rainfalls in autumn, rainstorm may occur in some regions affected by the weather situation. Available material shows that rainstorms have happened in some areas of this region in years with relatively large precipitations. For instance, Bazhong County experienced two rainstorms--a 399 mm rainstorm on September 4-7, 1973 and a 313 mm rainstorm on September 10-20 that same year.

According to an analysis of the historical data collected in the past 50 years after the setting up of the hydrographic station, although the occurrence of such a serious flood is rare in this season and this region, however, the quantity of flow of the Qujiang River, a tributary of the Jiangling River, reached 20,000 cubic meters per second (m3/sec) this year, whereas in October 1975, the quantity of flow had amounted to 24,000 m3/sec, surpassing this year's amount. The quantity of flood in the upper reaches of the Qujiang River this year ranks 4th in history, and that of the lower reaches is placed 2nd, the highest ever in history recorded prior to the building of the Three Gorges Project.

Three Gorges' influence on climatic change in surrounding areas

According to the design, the highest retained water level of the completed Three Gorges Project can reach 175m and increase the water surface by 1,084 sq km. This change may exert some impact on the environmental climate, for instance, the increase in, and the even distribution of, rainfall, but such influence on a large weather system is very weak.

After almost 50 years of climate monitoring data analysis and weather value pattern analog-result analysis of the Three Gorges reservoir region, meteorologists believe that the current climatic variation is a natural change and the climatic changes in the Three Gorges region is identical with that of the entire southwest region. The Three Gorges' stored water only exerts a minor impact on the climate of local areas and the range of the impact is kept within 20 km. Specifically, rainfall in those areas close to the reservoir could be reduced while the annual precipitation in those areas 15 km away from the reservoir could increase around 10 mm. Affected by the reservoir retained water, summer temperature in the reservoir areas would drop 0.5-1.0 degree C. and winner temperature would increase 0.3-1.0 degree C, which might mildly balance the temperature in that area. However, some areas may experience thick fog and strong wind because of the reservoir's large water surface. But the situation is not completely the same in different seasons.

China Meteorological Administration has established an additional automatic meteorological station in the Three Gorges region and launched stereo meteorological observation with the support and help of the State Council Three Gorges Project Construction Committee to accumulate greater and more reliable observational data, by which meteorologists may conduct in-depth research on the Three Gorges Project's impact on the climatic environment and validate the value pattern analog analysis.

By People's Daily Online