Abstract: Non-point source pollution from agricultural watersheds has an increasing contribution to rapid deterioration of downstream water environmental quality in recent years.Two typical agricultural watersheds(Liucha River valley and Qianxi River valley),characterized by high grain yield and multi-pond in the Yangtze-Huaihe region of China,were selected as examples to study effect of inflow rivers in small-scaled catchments on water quality in the convergence area of the river and lake,using grey relationship analysis (GRA).Water quality in the rivers and in the convergence area in the lake was monitored twice a month from September 2004 to September 2005.It was found that not only the concentration of TN and TP,but also dissolved metals,such as manganese,iron and aluminum,were the highest in the lake water in the period between October and December in 2004.Grey correlative analysis showed that the water quality in the inflow rivers of the two catchments was highly related to that in the convergence area of the lake with grey correlativity ranging between 0.687 and 0.896.In the case of a small-scaled catchment,water quality in the lake responded rapidly to the variation of water quality in the inflow river,which suggests the non-point source pollution from an agricultural watershed contributes significantly to the pollution of the waterbody of the lake.

Key words: quality in watershed, nutrient, grey relationship analysis, eutrophication, Chaohu Lake