A STUDY ON REGULARITY OF DROUGHT AND FLOOD IN THE CHANGJIANG RIVER VALLEY DURING LAST 10 CENTURIES

According to historical records, there are 264 drought and flood years, occurred in the upper and middle reaches of the Changjiang (Yangtze) River during last 1020 years from 961 to 1980. The evolutionary law and developing trend of drought and flood years are studied. The distribution of drought and flood years are non-uniform and the dry and flood seasons in a year are concentrated. At the angle of monsoon circulation, at present the climate in the upper and middle reaches of the Changjiang River is just in the late stage of frequent drought period and the early stage of least flood period. In addition, the cycle of drought and flood and the feature of drought and flood occurred in the upper, middle and lower reaches of the Changjiang River are analyzed. It shows that the short period less than 10 years is in the majority, and the drought and flood occurred most frequently in the middle and lower reaches of the Changjiang River.     

Observed residual currents off the Changjiang （Yangtze） Rivermouth in summertime of 1959 and 1982

Data taken in two large scale ocean observations in China in summer 1959 and 1982 were used to analyze the residual current off the Changjiang (Yangtze) River mouth. The currents at surface off the mouth in July 1959 and 1982 flow northeastward and eastward due to the river discharge, the current speed was larger in 1982 than in 1959. All the bottom currents flow landward due to baroclinic effect. The surface current was controlled by the river runoff and the Taiwan Warm Current (TWC). A return current at surface off the mouth was observed in September 1959. In general, the bottom currents were controlled by the TWC in most study area in addition to the runoff near the mouth. Although driven by 3-D model with the monthly averaged forces (river discharge, wind stress, baroclinic effect, open boundary water volume flux and tidal mixing) in August, the simulated circulations were basically consistent with the observed ones with episodic time manner.     

Fuzzy synthetic evaluation of water quality of Naoli River using parameter correlation analysis

In order to improve the effectiveness of Fuzzy Synthetic Evaluation （FSE） models, a Parameter Correlation Analysis （PCA） was introduced into the FSE and a case study was carried out in the Naoli River in the Sanjiang Plain, Northeast China. The basic principle of the PCA is that the pairs of parameters which are highly correlated and linear with each other would contribute the same information to an assessment and one of them should be eliminated. The method of the PCA is that a correlation relationship among candidate parameters is examined before the FSE. If there is an apparent nonlinear or curvilinear relationship between two parameters, then both will be retained; if the correlation is significant （p〈0.01）, and the scatter plot suggests a linear relationship, then one of them will be deleted. However, which one will be deleted？ For solving this problem, a sensitivity test was conducted and the higher sensitivity parameters remained. The results indicate that the original data should be preprocessed through the PCA for redundancy and variability. The study shows that introducing the PCA into the FSE can simplify the FSE calculation process greatly, while the results have not been changed much.     

Spatial distribution and diurnal variation of chemical oxygen demand at the beginning of the rainy season in the Changjiang （Yangtze） River Estuary

A field observation was carried out in the Changjiang （Yangtze） River Estuary from May 19 to 26, 2003. A total of 29 stations, including 2 anchored stations, were occupied through almost the whole salinity gradient. Based on the observation data, biogeochemistry of chemical oxygen demand （COD） was examined. Spatial distribution pattern of COD shows that it decreased downstream. The COD concentration varied generally within a narrow range of 1.24-1.60 mg/L in the zone around the river mouth, beyond which it decreased rapidly to 0.20 mg/L. In the mixed water zone, the fluctuation in COD was smaller at 2 m above the bottom layer than at the surface layer in 48 h. In the seawater zone, the 48-h fluctuation at the surface was the largest, followed by that of 5 m below the surface and 2 m above the bottom layers in a range of from 2.50 to 0.55 mg/L. Freshwater discharge was the dominant source of COD in the estuary. The average COD beyond the river mouth was 2.7 mg/L, which accorded with the Chinese seawater quality Grade I. Relationships between dissolved oxygen and biogeochemical parameters such as suspended particulate matter, dissolved organic matter and chlorophyll-a were also discussed.     

Seasonal variation of the barrier layer in the PN section

In this paper, we use the conductivity-temperature-depth (CTD) observation data and a three-dimensional ocean model in a seasonally-varying forcing field to study the barrier layer (BL) in the PN section in the East China Sea (ECS). The BL can be found along the PN section with obviously seasonal variability. In winter, spring and autumn, the BL occurs around the slope where the cold shelf water meets with the warm Kuroshio water. In summer, the BL can also be found in the shelf area near salinity front of the Changjiang (Yangtze) River Dilution Water (YRDW). Seasonal variations of BL in the PN section are caused by local hydrological characteristics and seasonal variations of atmospheric forcing. Strong vertical convection caused by sea surface cooling thickens the BL in winter and spring in the slope area. Due to the large discharge of Changjiang River in summer, the BL occurs extensively in the shelf region where the fresh YRDW and the salty bottom water meet and form a strong halocline above the seasonal thermocline. The formation mechanism of BL in the PN section can be explained by the vertical shear of different water masses, which is called the advection mechanism. The interannual variation of BL in summer is greatly affected by the YRDW. In the larger YRDW year (such as 1998), a shallow but much thicker BL existed on the shelf area. Supported by National Basic Research Program of China (973 Program, No. 2005CB422303 and 2007CB411804), the Key Project of the International Science and Technology Cooperation Program of China (No. 2006DFB21250), the “111 Project” of the Ministry of Education (No. B07036), the Program for New Century Excellent Talents in University, China (No. NECT-07-0781)