TMDL allocation is a required component for establishing TMDLs and its purpose is to create a technically feasible and reasonably fair division of the allowable load among various sources. Current TMDL guidance is sparse in the area of allocation development, largely because the allocation
of loads is considered a policy decision on how to distribute costs among different stakeholders in order to achieve a water quality goal. Better science is needed for supporting allocation decisions and guidance on the various methods for making these decisions. 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, one objective of this paper is to clearly illustrate the importance of defining critical condition relating to TMDL allocation. As a viable alternative approach to TMDL development
and allocation, the event-based Critical Storm-Flow (CFS) approach explicitly addresses critical condition as a combination of stream flow, magnitude of storm event and initial watershed condition. One of the major advantages is its ability to assess the risk associated with a certain load
reduction scenario and examine impacts of allocation options during TMDL development, which makes nonpoint source control more consistent with the traditional approach to point source management. Case studies in this paper emphasize TMDL allocation for nutrient and pathogen TMDLs. Key factors
influencing critical condition and TMDL allocation were discussed, for example, stream flow, precipitation and antecedent condition. Ongoing research on TMDL allocation techniques was presented, such as explicit incorporation of cost function in TMDL allocation and use of decision support
system in assisting the formulation of consensus-based management plan.
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