Defining the Critical Condition in the TMDL Development Process: Continuous, Statistical or Event-Based Approach
One of the key challenges in the TMDL development process is how to define the critical condition for a receiving waterbody impacted by both point and nonpoint sources. Knowledge of the critical condition could help identify the feasible allocation scenarios needed to be taken to meet water quality standards. However, EPA guidelines thus far do not specify how to define the so-called design or critical conditions. When point sources and nonpoint sources are considered an “integrated” manner, the worst-case scenario is not obviously known because nonpoint sources are mostly storm-driven. Therefore, the objective of this paper is to illustrate the strength and weakness of available methods in defining critical condition during the TMDL development process and highlights the applicability of a practical event-based approach.