Critical Condition Modeling and Analysis in TMDL Development and Implementation

Abstract

The USEPA’s Total Maximum Daily Load (TMDL) program under the Clean Water Act requires taking into account the critical conditions for stream flow, loading, and water quality parameters to develop TMDLs. However, properly defining a waterway’s critical conditions is one of the key challenges in developing a TMDL. Consequently, many current TMDLs address this aspect only in a cursory manner. This paper presents a review of critical condition modeling and analysis approaches and illustrates the strengths and weaknesses of available methodologies to define critical conditions with a view to provide guidance for TMDL development and implementation. Four major approaches for critical condition modeling and analysis are typically employed in TMDL studies: low-flow analysis using steady-state models, continuous simulation using dynamic models, statistically based load-duration curves, and the event-based critical flow-storm approach. By understanding better how to define critical conditions, water quality professionals can develop more-meaningful and improved estimates of TMDLs and formulate appropriate strategies to control pollution from point and nonpoint sources at a watershed level.