Definite setting of water quality target is a challenge when considering non-point source (NPS) pollution which demands reasonable allocation of reduction requirements. However, research on the dynamic water environmental capacity (WEC) allocation aimed at reflecting varying rainfall conditions has rarely been performed. In this study, the relationship between rainfall and WEC was explored by a stochastic algorithm, while then reduction target was allocated in a small catchment, the Three Gorges Reservoir Region (TGRR), China. Combined with the distributed model, three commonly adopted best management practices (BMPs) were located precisely. The results indicate that a specific rainfall threshold exists for water quality (84 mm/month for the study area) which reveals the comprehensive effect of rainfall flushing and dilution effect. At 90% confidence, the total phosphorus (TP) load was 1518 kg/yr, and the ideal WEC was 1194 kg/yr, with excess of 324 kg/yr. In addition, the highest amount of inflow phosphorus can be reduced by fertilizer reduction (FR) with 303.8 kg/yr. Besides, slope-to-terrace (ST) measures should be implemented mainly in the upper reaches, while FR should be preferentially considered in the lower reaches. For the small watershed dominated by agricultural NPS, this study can provide accurate water quality target settled by WEC calculation impacted by uneven rainfall and a feasible watershed management.
