Combined inverse modeling approach and load duration curve method for variable nitrogen total maximum daily load...

Nonpoint sources (NPS) pollution has been an important cause for water quality impairment worldwide. To take the temporal variations of both NPS pollution and in-stream attenuation into consideration, an inverse modeling approach and the load duration curve (LDC) method were combined for variable nutrient total maximum daily load (TMDL) development.

Water quality and hydrological parameters were monitored monthly along the ChangLe River system in 2004-2008. The catchment NPS export load (EL) and TMDL for total nitrogen (TN) were estimated by the inverse format of an existing stream nutrient transport equation. The LDC method was used to describe the variability of EL, TMDL, requiring load (RLR) and percent (the ratio between the RLR and the EL, RPR) reduction, and then to set the variable requiring reductions under different uncertainties.

Although both EL and TMDL for TN increased with stream flow, the increments of EL became larger than that of TMDL with increasing stream flow. Thus, RLR also increased with stream flow. The contribution of in-stream attenuation capacity for TN TMDL, which decreased with stream flow, occupied 37.3 ± 10.4% of the TMDL for the entire river system. To assure 90% compliance with the target in-stream TN level, the RLR and RPR was 1.16 × 10(3)-19.02 × 10(3) kg day(-1) and 53.6-59.9% for different flow regimes, respectively.

For the NPS pollution-dominated watershed, temporal variable expressions of TMDL and requiring reduction are both necessary. This combined approach provides researchers and managers with a simple but efficient tool for variable TMDL development.