Sources, Instream Transport, and Trends of Nitrogen, Phosphorus, and Sediment in the Lower Tennessee River Basin, 1980-96


Export. Equivalent to yield, and used in place of that term in comparisons with input to a watershed.

Flow-weighted mean concentration. The ratio of instream load of a constituent to the mean discharge during the period of transport (dimensions of mass per volume); and equivalent computationally to the flow-weighted mean of the model estimates of daily concentration. Expressed in units of concentration (milligrams per liter). This quantity is used, in place of load or yield, for evaluating average water-quality conditions at the site, and for comparing water quality among sites with differing discharge characteristics.

Instream delivery processes. Processes such as channel storage and aquatic biological assimilation that control how much of the stream input is transported along the channel and is exported from the watershed.

Instream load. The mass of a constituent moving past a specified point in a channel (for example, the mouth of a river basin) during a specified period of time. The instream load can be estimated by monitoring the concentration of the constituent periodically, and streamflow continuously, at the specified point.

Land-water delivery processes. Transport of a portion of land-phase inputs, overland or in the subsurface, from the point of deposition or application on the land surface to the stream channel (Smith and others, 1997). Some of the factors influencing land-water delivery include distance to the channel, land slope and runoff characteristics, soil-drainage characteristics, biological processing or storage within the vegetative cover or soil, and hydrogeology.

Land-phase input. The mass of a constituent deposited on (through natural processes or human activities) or derived from (through erosion of natural materials) the land surface in the watershed. Land-phase inputs are the basis for estimating the contribution from most nonpoint sources of constituents, inputs from which cannot be otherwise quantified. Only a portion of the land-phase input reaches the stream channel by overland or subsurface transport processes, referred to as land-water delivery processes.

Nonpoint source. A source of a water-quality constituent that is not discharged directly to the stream channel at a discrete location, but rather originates as land-phase inputs to broad source areas. An unknown percentage of a nonpoint-source input is transported overland or subsurface and reaches the stream channel as diffuse input.

Point source. A source of a water-quality constituent that is discharged directly to the stream channel from a discrete location (for example a pipe, tank, or pit).

Stream input. The mass of a constituent delivered to the stream channel from sources in the watershed. Stream inputs include inputs discharged directly from sources to the stream channel (such as wastewater discharges), as well as the portion of land-phase input that reaches the stream channel.

Trend. The change in the concentration of a water-quality constituent over time.

Yield. The ratio of instream load of a constituent to the area of the watershed (dimensions of mass per time per area). This area-normalized load is used, in place of load, to compare instream loads among watersheds with different drainage areas, and to compare with inputs to the watershed.


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