TMDL DEVELOPMENT FROM THE “BOTTOM UP” – PART II: USING DURATION CURVES TO CONNECT THE PIECES

Abstract

Reasonable assurance, trading, and adaptive management are key parts of the TMDL process where technical considerations intersect policy issues. Tools are needed which promote effective communication between TMDL developers and those responsible for implementing actions that will lead to measurable water quality improvements. With the large number of TMDLs that must be completed, limited resources, and the complex, inter-related nature of water programs – the “two Ps” are critical to success – practical approaches and partnerships. Watershed analysis has been used to build a “bottom up” approach towards TMDL development as one way to establish a meaningful, value-added framework which links water quality concerns to proposed solutions. A “bottom up” approach takes advantage of networks of programs and authorities across jurisdictional lines. Information on management measures related to both source control and delivery reduction methods are incorporated into the allocation part of TMDL development. Duration curves can support a “bottom up” approach through enhanced targeting. Kansas has been utilizing load duration curves for the past several years as a key part of their TMDL development process. The expanded use of flow duration curves offers an opportunity for enhanced targeting, both in TMDL development and in water quality restoration efforts. In particular, duration curves can add value to the TMDL process by identifying targeted areas, targeted programs, targeted activities, and targeted participants. Flow duration curve analysis identifies intervals, which can be used as a general indicator of hydrologic condition (i.e. wet versus dry and to what degree). This indicator can help point problem solution discussions towards relevant watershed processes, important contributing areas, and key delivery mechanisms. These are all important considerations when identifying those controls that might be most appropriate and under what conditions. In addition, duration curves also provide a context for evaluating both monitoring data and modeling information. This offers another way to look at identifying data needs where adaptive management is being considered or utilized.