2.2 Where, What, and When to Sample
47
survey during a period of stable flows in such regions is therefore more easily
accomplished during the summer as compared with other seasons.
The choice of sampling season may necessitate collection of samples when
flow variations are relatively frequent and substantial. Extreme chemical conditions are often associated with extreme flows. This is the case with some
acidic deposition effects, which tend to be most severe in some regions during
the high, fluctuating flows of spring snowmelt or the large rainstorms associated with hurricanes or other major storm systems. Although these are the
conditions most difficult to characterize, they are often highly relevant for
assessing biological impacts.
Two approaches can be used in synoptic studies to address flow variations
within a season, but each requires stream flow gauges within the sampling
region to monitor flow conditions during the sampling period. In the first,
sampling of each stream is repeated multiple times during the season under
a variety of flow conditions. If the mean and distribution of flows at the collection times are similar (not statistically different) among the streams, the mean
or median chemical concentration at each site can be used as the representative value for comparing streams. This approach was used successfully in the
first large-scale stream survey to assess acidic deposition effects in the United
States (Colquhoun et al. 1984).
The second approach involves collection of samples at all sites during a
period of time that has limited variation in flow. This requires close monitoring of weather conditions coupled with the ability to initiate or interrupt the
sampling on short notice. This approach was successfully used in two snowmelt surveys by Lawrence et al. (2008) in the Adirondack region of New York.
Collection of all stream samples was done over 3 days when flows were elevated
but stable (higher than 90% of the year in one survey and higher than 84% of the
year in a second survey). This approach can be challenging to implement with
stream-sampling sites that are difficult to access or far apart.
Neither of these approaches is likely to collect samples during the most
extreme conditions. However, the stream gauges used to monitor flow in the
region can be used to determine how the flow conditions during the sampling
window related to conditions for the overall year (and previous years). If information is available to show that the chemical measurement of interest is statistically related to flow, then it is possible to estimate the measurement for more
extreme flow conditions based on this relationship and available flow data. For
streams where flow measurements are not available, chemical concentrations
for flows higher than those at the time of sampling can be approximated from
an index stream where flow and chemical concentrations are monitored on a
regular basis throughout the year. Examples of this approach were given by
Eshleman (1988) and Lawrence et al. (2008).
47
survey during a period of stable flows in such regions is therefore more easily
accomplished during the summer as compared with other seasons.
The choice of sampling season may necessitate collection of samples when
flow variations are relatively frequent and substantial. Extreme chemical conditions are often associated with extreme flows. This is the case with some
acidic deposition effects, which tend to be most severe in some regions during
the high, fluctuating flows of spring snowmelt or the large rainstorms associated with hurricanes or other major storm systems. Although these are the
conditions most difficult to characterize, they are often highly relevant for
assessing biological impacts.
Two approaches can be used in synoptic studies to address flow variations
within a season, but each requires stream flow gauges within the sampling
region to monitor flow conditions during the sampling period. In the first,
sampling of each stream is repeated multiple times during the season under
a variety of flow conditions. If the mean and distribution of flows at the collection times are similar (not statistically different) among the streams, the mean
or median chemical concentration at each site can be used as the representative value for comparing streams. This approach was used successfully in the
first large-scale stream survey to assess acidic deposition effects in the United
States (Colquhoun et al. 1984).
The second approach involves collection of samples at all sites during a
period of time that has limited variation in flow. This requires close monitoring of weather conditions coupled with the ability to initiate or interrupt the
sampling on short notice. This approach was successfully used in two snowmelt surveys by Lawrence et al. (2008) in the Adirondack region of New York.
Collection of all stream samples was done over 3 days when flows were elevated
but stable (higher than 90% of the year in one survey and higher than 84% of the
year in a second survey). This approach can be challenging to implement with
stream-sampling sites that are difficult to access or far apart.
Neither of these approaches is likely to collect samples during the most
extreme conditions. However, the stream gauges used to monitor flow in the
region can be used to determine how the flow conditions during the sampling
window related to conditions for the overall year (and previous years). If information is available to show that the chemical measurement of interest is statistically related to flow, then it is possible to estimate the measurement for more
extreme flow conditions based on this relationship and available flow data. For
streams where flow measurements are not available, chemical concentrations
for flows higher than those at the time of sampling can be approximated from
an index stream where flow and chemical concentrations are monitored on a
regular basis throughout the year. Examples of this approach were given by
Eshleman (1988) and Lawrence et al. (2008).