Water environmental capacity calculation based on uncertainty analysis: a case study in the Baixi watershed area, China

Abstract

Abstract Water environmental capacity (WEC) is an important concept in environmental science. A basic theory applied in EIA (Environmental Impact Assessment), WEC is also an indispensable factor in District Environmental Planning and total water pollutant control planning. The results of most calculating methods for WECs are fixed values that correspond to designed conditions and are not practical WECs. Instead of a fixed value in calculating WEC, confidence probability and uncertainty analysis should be combined with the river model. Consequently, the feasible WEC in different hydrological periods can ensure the precision and reliability of the load allotment. In this study, a useful method, Monte Carlo simulation, was combined with a water quality model to calculate the WEC and quantify the impact of a range of input values on the WEC. A case study of nitrogen and phosphorus WEC calculation was conducted in the Baixi Reservoir watershed area in Ningbo City, Zhejiang Province, China. The results show that the WEC values of TN under 90% confidence probability were 5.30 T/period during the wet period, 8.35 during the dry period, and 9.83 during the middle period. In comparison, the values under 80% and 70% were 10.80, 16.39, 18.79, and 15.94, 23.99, 27.71 T/period, respectively. The WEC values of TP were also obtained. The uncertainty analysis- based WEC could help policy makers to set better goals for different hydrological periods, especially in an area dominated by non-point source pollutants.