Improving land-based total nitrogen load allocation using a variable response matrix-based simulation-optimization approach: A case study

Human activities have increased land-based pollutant loads, which have caused a significant variation in unit source concentration response matrices (USCRMs) of the coastal ocean. A variable response matrix approach (VRMA) for simulation-optimization load allocation is proposed and applied to solve the uncertainty caused by variations in USCRMs in calculating allocated load (AL). In contrast to the traditional fixed response matrix approach (FRMA), the USCRMs are iteratively updated by a simulation model using iterative solutions from an optimization model. The ALs of 48 main dischargers in Shandong Province, a typical coastal province in China, are calculated via the VRMA and FRMA. The water quality under each AL emission is simulated via a three-dimensional water quality simulation model. The advantages of applying the VRMA over the FRMA in waste load allocation are demonstrated by comparing the ALs and the correspondence between the simulated water quality and the water quality standards of the two methods. Results show that the VRMA can maintain better water quality than the FRMA when higher total nitrogen loads are allowed.