The Critical Flow-Storm Approach for Nitrate TMDL Development in the Muddy Creek Watershed, Virginia

One of the key challenges in the TMDL development is how to define the critical condition for a receiving water body. When point sources and nonpoint sources are considered in an “integrated” manner, the worst-case scenario is not obviously known because of the fact that pollutant loads come from both constant flow point sources and storm-driven, time-varying nonpoint sources. The main concern of using continuous simulation approach is that the use of data during specific representative hydrologic period does not necessarily cover the most “critical” condition and it is very data intensive. A practical, event-based Critical Flow-Storm (CFS) approach was developed with application in the Nitrate TMDL Development for Muddy Creek/Dry River in Virginia using BASINS/HSPF. As an alternative method for TMDL development, the CFS approach gives reasonable results and explicitly addresses the critical condition as a combination of stream flow, magnitude of the storm event and initial watershed condition.