Selecting a Watershed Definition

Nearly all aquatic resource monitoring programs involve watersheds as part of the strategy, especially during diagnostics and assessment activities. Conceptually, the conditions at the monitoring site are influenced by the conditions in the surrounding area, often delineated by the drainage basin or watershed. Monitoring objectives and designs benefit from well reasoned watershed definition choices.

Watershed based alternatives as elements of target population (Streams)

All watersheds associated with every location on stream/river linear network ("watersheds", infinite #)
All hydrologic units at a specified field level of HUCs ("HUCs", finite #)
All watersheds associated with streams defined at confluence ("confluence watersheds", finite #)
Illustration

Monitoring Objectives Determine Element Definition Choice

How many stream/river km in the US meet their designated use?
What is the condition of all 6th field HUCs within Region?
What proportion of headwater watersheds in Region exceed total nitrogen criteria at outflow of watershed?

Implications of choice

Survey designs possible regardless of choice; hence not a deciding factor
Watersheds include all possible true watersheds, at cost of variable size
Confluence Watersheds are a subset of watersheds
HUCs tile the landscape into elements at the same approximate scale, at cost of introducing "incomplete" watersheds
Defensible scientific inference from the sample to the target population critical

In addition, Program objectives for diagnostic analyses, model forecasting, and potential management actions should be taken into consideration. Although each definition has its proponents, and in some situations the choice is more apparent, the general weight of evidence supports using a "watershed" definition. It appears that, in general, the integrated 305(b) & 303(d) guidance will be best supported by a "watershed" definition, e.g., assessment units and TMDL processes.

Selecting a Watershed Definition

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