TMDL DEVELOPMENT FROM THE “BOTTOM UP” – PART III: DURATION CURVES AND WET-WEATHER ASSESSMENTS

With the large number of TMDLs to be completed, limited resources, and the complex, inter-related nature of water programs – the “two Ps”: practical approaches and partnerships – are critical to success. Dependable tools are needed to promote effective communication between TMDL developers and implementers, so that actions will lead to measurable water quality improvements. A “bottom up” approach is one way to establish a meaningful, value-added framework linking water quality concerns to proposed solutions. A “bottom up” approach capitalizes on the networks of programs and authorities across jurisdictional lines.Kansas has been utilizing load duration curves for the past several years as a key part of its TMDL development process. The increased use of duration curves supports the “bottom up” approach by offering an opportunity for enhanced targeting, both in TMDL development and in water quality restoration efforts. Duration curves can also add value to the TMDL process by expanding the characterization of water quality concerns, linking concerns to key watershed processes, prioritizing source assessment efforts, and identifying potential solutions.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, when combined with other basic elements of watershed planning, can help point problem solution discussions towards relevant watershed processes, important contributing areas, and key delivery mechanisms.Duration curves are not a substitute for field reconnaissance work. Grounded, fact- finding is needed to examine what is actually going on in the watershed. Duration curves do, however, offer a framework that can help refine water quality assessments. Refined analyses using duration curves can be important considerations when identifying those controls that might be most appropriate and under what conditions. This paper uses several examples to illustrate opportunities where duration curves can strengthen watershed assessments and enhance the TMDL development process.