Figure 1. Possible sources of heterogeneities in flowing water
To obtain mean values of water quality parameters in flowing water, the analyst must have
some knowledge of the mixing processes that are present. In situ data are needed for the
verification. If the stream is turbulent and well mixed, it may be the case that any location can
accurately represent the quality of the water. If the stream is not well mixed and has
heterogeneities in water quality, the data must be flow weighted.
Flow-weighted data allow one to calculate the mass transport of parameters through the cross
section of the stream in time. Some examples of flow-weighting include temporal quantification
of dissolved oxygen mass or average dissolved oxygen concentration moving down a river, a
record of average total dissolved gas saturation, mass transport of nutrients, or a record of
average temperature. The important aspect is that the value of the parameter of interest is
averaged across the area of the channel cross section with respect to velocity.
Any verification must be both qualitative and quantitative. This technical note describes
approaches for statistically quantifying and verifying the adequacy of monitoring sites for
measuring the average water quality at river transect. Total dissolved gas data collected from the
Columbia and Snake Rivers are used to illustrate these techniques. The statistical methods
provided will allow users with a basic knowledge of statistics to design and implement studies to
verify the representativeness of their own monitor locations. A review of statistics with water
quality applications can be found in Gaugush (1986). It should be noted that, although this
technical note is based on the use of automated fixed water quality monitors, the procedure
described can be applied to manual monitoring as well.
Water Quality Technical Note AM-03 (January 1998)