Addressing Uncertainty in Watershed Management.

The United States Environmental Protection Agency (USEPA) is promoting through regulation and guidance a watershed approach for not only addressing water quality impairments, but also for protection of high quality water resources. The watershed approach discussed in this paper is limited to the nine-element watershed management plan requirements under Section 319 of the Clean Water Act (CWA) and in the Total Maximum Daily Load (TMDLs) Program.  Two major limitations associated with using the watershed approach in these two programs are the availability of suitable water quality data and the reliance on models. The limitation associated with suitable water quality data is governed by both the volume of data available and its quality. Addressing both the complexity of Nonpoint Source (NPS) load assessment (volume, timing, source, form) and linkage of load assessment to observed water quality conditions requires collection of robust datasets, and the failure to do so creates uncertainty in watershed decision making.  It is well known that our inability to adequately account (measure/estimate) for the various components of NPS loading at a particular point in time and location is a result of natural system variability and sampling error involved in the collection of water quality data. The use of these data in modeling performed to guide decision making further compounds the uncertainty associated with the initial data collection. USEPA has developed a two-stage framework to address this uncertainty under the watershed approach. This framework covers data collection, its application to models and the use of modeling results in decision making. USEPA has established through regulation how managers are to address the issue of uncertainty in TMDLs. For other Section 319 watershed planning efforts, USEPA addresses uncertainty  through guidance and associated regulations. This presentation will address how uncertainty is being addressed in the development and implementation of TMDLs and Section 319 watershed management plans.