A B S T R A C TThe main objective of this study was to apply the water quality analysis simulation programme to develop total mass control of pollutant (TMCP) programmes by calculating the water assimilative capacities for chemical oxygen demand (COD) and ammonia nitrogen (NH 3 -N) of a river that connects the Taihu Lake and the Yangtze River. Based on the distribution of water assimilative capacities, the geographical distribution of the pollutant permitted discharge load was validated by calculating the Gini coefficients for the economic, social and environmental efficiencies. According to the validation results, pollutant permitted discharge load was reallocated between townships and the rates of the reductions in pollutants for each township in the watershed were calculated. The water assimilative capacities of COD and NH 3 -N of Xicheng Canal watershed were calculated as 4803 ton/yr and 186 ton/yr, respectively. The results for the reallocation of pollutant permitted discharge load between townships indicated that the reduction in COD was small, with only 11.2% reduction for Qingyang Town and 33.6% for Chengjiang Town. In contrast, the NH 3 -N reductions of Yuecheng Town, Nanzha Town, Qingyang Town and Chengjiang Town were 52.9, 40.0, 61.4 and 54.5%, respectively, while the total reduction for Xicheng Canal watershed reached 53.5%. Based on the case, a new TMCP framework for allocating the pollutant permitted discharge loads in rivers was established, which not only combined the total mass of pollutant with the water quality but also took account for equity and efficiency in the allocation of pollutant permitted discharge loads.

Nonpoint sources are difficult to control because their nutrient contribution from different parts of a watershed can vary substantially. Identifying critical source areas of nutrient loss is an important step in watershed pollution mitigation programs. This study sought to use an integrated index model to differentiate between subbasins that serve as critical source areas of N and P nonpoint sources of pollution in China's Tiaoxi watershed. In contrast with previous N and P indices, multiple sources of pollution (i.e., agronomic activity, domestic wastewater, livestock farming, and aquaculture) were considered. Nitrogen and P source factors (i.e., N and P annual export loads) and transport factors were multiplied to determine the overall risk of nutrient loss in the integrated index model. Critical source areas were identified by a higher nutrient loss index. Of the 92 subbasins within the Tiaoxi watershed, 13 were determined to be critical sources for N, 10 for P, and seven for both N and P. Critical source area identification corresponds well with water quality data from the subbasins. The results show the potential use of the integrated index model for prioritizing and targeting watershed pollution mitigation activities at the subbasin level.