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The concept of critical nutrient amounts (CNA) for a watershed was developed to address eutrophication in surface waters from diffuse (non-point) source pollution. CNA is defined as the maximum allowable applied or generated amount (AGA) of a nutrient from natural and human sources that can be emitted and still allow compliance with water quality standards. The CNA calculation method is based on properties of diffuse source pollution, including (i) estimation and analysis of nutrient input-output balances in terrestrial and riverine systems; (ii) prediction of terrestrial nutrient export loads and AGA using riverine loads; and (iii) calculation of critical AGA to meet different regulatory compliance locations (e.g., end of a reach or for the whole reach). The CNA concept was developed, validated and applied for total nitrogen (TN) and phosphorus (TP) in the ChangLe agriculture-dominated watershed (864km²/27.8km reach) of eastern China. Results indicated that CNA was 7174tNy⁻¹ and 5004tPy⁻¹ for the reach-end control method and 8290tNy⁻¹ and 4425tPy⁻¹ for the whole-reach control method. Annual TN AGA exceeded CNA by 53.2-61.3% and 46.0-55.2% for reach-end and whole-reach control methods in 2004-06, respectively. In contrast, TP AGA values were 90.3-95.9% and 68.3-73.2% below CNA values for reach-end and whole-reach control methods, respectively. These calculations provide a target or permissible nutrient amount that can be used to develop management practices that allow attainment of water quality objectives at the watershed scale.
