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Load Duration Curve Methodology for Assessment and TMDL Development Nevada Division of Environmental Protection

The major streams in Nevada have had TMDLs (Total Maximum Daily Loads) established for several years. However for some of these streams, the TMDLs are expressed as an average daily load based upon average long term flow conditions. These TMDLs have been dubbed as "bare bones" TMDLs due to the simplicity of the calculation and their lack of usefulness. While these TMDLs seem to satisfy the requirements of the Clean Water Act, they have contributed little to any watershed/waterbody assessment and restoration plans. These types of TMDLs do little to characterize the problems the TMDLs are intended to address. Without adequate characterizations, appropriate solutions cannot be identified and implemented.

For TMDLs to be more beneficial in the assessment and implementation process, TMDLs should reflect adequate water quality across flow conditions rather than at a single flow event such as average daily flow. Many states have begun to use load duration curves as a more robust method for setting TMDL targets. It is also a useful tool for better characterizing the pollutant problems over the entire flow regime. This paper discusses the steps taken to develop load duration curves and how they can be used in the assessment and TMDL process.

A duration curve is a graph representing the percentage of time during which the value of a given parameter (e.g. flow, load) is equaled or exceeded. Such a graph can be easily generated using a spreadsheet computer program. The following attachment presents the steps involved in developing a load duration curve.

truckee_loadcurv.pdf246.36 KB