Water Chemistry Field Sampling
48
2.2.3.3 Characterization of Episodic Chemistry
during Hydrologic Events
Characterization of episodic chemistry (generally of streams, occasionally
also of lakes) during hydrologic events is challenging under the best of circumstances. Given the additional complications of access difficulties for remote
sites, often long travel distances, and complications and safety concerns introduced by seasonal snowpack, episodic sampling in backcountry settings is seldom attempted. Nevertheless, such sampling can yield important information
to aid in interpretation of surface water acid-base and nutrient chemistry.
Because of the transient and unpredictable nature of hydrologic events, precise timing of sample collection occasions is generally not possible. In particular,
it cannot be assumed that such samples necessarily capture the most extreme
chemical conditions. For that reason, multiple high-flow samples should be collected, if possible, and they should ideally be distributed across multiple years.
2.2.3.4 Long-Term Monitoring
If an LTM program for lakes is intended to represent a particular time of year,
interannual variation in lake hydroperiod (periodicity in lake conditions that
reflect the changes of the seasons, including water and air temperature, snowmelt, and vegetative development) can introduce substantial variability. This can
be especially problematic for high-elevation lakes. The chemistry of such lakes
can change gradually or abruptly in response to spring snowmelt, large rainstorms, or fall overturn. Such changes are largely governed by the depth of the
snowpack, patterns of rainfall, and temperature. Thus, a program that entails, for
example, sampling the first week of July each year, while reducing some aspects
of interannual variability, may still yield considerable year-to-year variability
as a consequence of interannual differences in snowmelt hydrology. One potential solution is to target sampling to a specific degree day, which is calculated
based on maximum and minimum daily temperature (cf. http://pnwpest.org/
wy/index.html). Standardization of sampling timing on the basis of degree day
can partially adjust for interannual differences in snowmelt and the transition to
summer weather. It therefore should eliminate some, but not all, of the variability
associated with standardization based on the calendar for certain types of lakes.
Such an approach should not, however, ignore the influence of flow conditions.
One can quantify changes over time in the concentration-discharge
relationship. Using this approach, one compares differences between the
concentration-discharge relationship determined during one period of time
(several months to several years) and the same relationship determined during a later period of time of similar length. If, for example, the concentration
of stream ANC is higher at a given flow condition this year than it was several years ago, this pattern may suggest that ANC may be increasing at times
represented by that flow regime, independent of any changes in flow.
48
2.2.3.3 Characterization of Episodic Chemistry
during Hydrologic Events
Characterization of episodic chemistry (generally of streams, occasionally
also of lakes) during hydrologic events is challenging under the best of circumstances. Given the additional complications of access difficulties for remote
sites, often long travel distances, and complications and safety concerns introduced by seasonal snowpack, episodic sampling in backcountry settings is seldom attempted. Nevertheless, such sampling can yield important information
to aid in interpretation of surface water acid-base and nutrient chemistry.
Because of the transient and unpredictable nature of hydrologic events, precise timing of sample collection occasions is generally not possible. In particular,
it cannot be assumed that such samples necessarily capture the most extreme
chemical conditions. For that reason, multiple high-flow samples should be collected, if possible, and they should ideally be distributed across multiple years.
2.2.3.4 Long-Term Monitoring
If an LTM program for lakes is intended to represent a particular time of year,
interannual variation in lake hydroperiod (periodicity in lake conditions that
reflect the changes of the seasons, including water and air temperature, snowmelt, and vegetative development) can introduce substantial variability. This can
be especially problematic for high-elevation lakes. The chemistry of such lakes
can change gradually or abruptly in response to spring snowmelt, large rainstorms, or fall overturn. Such changes are largely governed by the depth of the
snowpack, patterns of rainfall, and temperature. Thus, a program that entails, for
example, sampling the first week of July each year, while reducing some aspects
of interannual variability, may still yield considerable year-to-year variability
as a consequence of interannual differences in snowmelt hydrology. One potential solution is to target sampling to a specific degree day, which is calculated
based on maximum and minimum daily temperature (cf. http://pnwpest.org/
wy/index.html). Standardization of sampling timing on the basis of degree day
can partially adjust for interannual differences in snowmelt and the transition to
summer weather. It therefore should eliminate some, but not all, of the variability
associated with standardization based on the calendar for certain types of lakes.
Such an approach should not, however, ignore the influence of flow conditions.
One can quantify changes over time in the concentration-discharge
relationship. Using this approach, one compares differences between the
concentration-discharge relationship determined during one period of time
(several months to several years) and the same relationship determined during a later period of time of similar length. If, for example, the concentration
of stream ANC is higher at a given flow condition this year than it was several years ago, this pattern may suggest that ANC may be increasing at times
represented by that flow regime, independent of any changes in flow.