Purpose and Study Design
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extent a given lake or stream has acidified since the Industrial Revolution.
A second general approach is to conduct future scenario modeling to estimate
future changes in water chemistry in response to one or more scenarios of
emissions control and deposition. A third modeling approach is simulation of
the critical or target loads of atmospheric deposition to protect or restore acidsensitive or nutrient-sensitive aquatic resources (see box on critical and target
loads). All of these modeling approaches require compilation of model input
data. These can include, depending on the selected model, data on soil chemistry, water chemistry, estimates of historic and current atmospheric deposition,
hydrology, and vegetative characteristics.
CRITICAL AND TARGET LOADS
Modeling of critical or target loads requires that a number of decisions be made prior to initiating the modeling. These decisions determine what resources are to be protected, at what level, and over what
time period. Sensitive resources to be protected by a given critical
or target load can include fish or other aquatic biota, vegetation,
or soil condition. To protect these resources, one or more chemical
indicators are typically chosen. Often, ANC is used as the indicator
for protecting aquatic resources. In that case, one or more critical
ANC levels must be selected (i.e., ANC = 20 or 50 μeq/L), typically in
association with known or suspected dose/response relationships for
various sensitive species. Different critical ANC levels are expected to
protect different species of aquatic life. Finally, one selects a steadystate approach or specifies the time period over which the sensitive
resources are to be protected, or over which the damaged resources
are expected to recover. Steady-state critical loads are determined
irrespective of time. Dynamic critical loads, or target loads, may be
determined for various endpoint years, for example, 2050 or 2100.
Each of these various decisions that must be made to simulate critical or target loads has an influence on the resulting model simulated
values. A target load can be selected that is higher than the modeled
critical load if the objective is to make some limited progress toward
reaching the critical load. Conversely, a target load can be selected
that is lower than the critical load to ensure that the sensitive ecosystem is fully protected given modeling uncertainty or to attain the
targeted threshold chemistry more quickly in the case of resources
that have already been damaged.
22
extent a given lake or stream has acidified since the Industrial Revolution.
A second general approach is to conduct future scenario modeling to estimate
future changes in water chemistry in response to one or more scenarios of
emissions control and deposition. A third modeling approach is simulation of
the critical or target loads of atmospheric deposition to protect or restore acidsensitive or nutrient-sensitive aquatic resources (see box on critical and target
loads). All of these modeling approaches require compilation of model input
data. These can include, depending on the selected model, data on soil chemistry, water chemistry, estimates of historic and current atmospheric deposition,
hydrology, and vegetative characteristics.
CRITICAL AND TARGET LOADS
Modeling of critical or target loads requires that a number of decisions be made prior to initiating the modeling. These decisions determine what resources are to be protected, at what level, and over what
time period. Sensitive resources to be protected by a given critical
or target load can include fish or other aquatic biota, vegetation,
or soil condition. To protect these resources, one or more chemical
indicators are typically chosen. Often, ANC is used as the indicator
for protecting aquatic resources. In that case, one or more critical
ANC levels must be selected (i.e., ANC = 20 or 50 μeq/L), typically in
association with known or suspected dose/response relationships for
various sensitive species. Different critical ANC levels are expected to
protect different species of aquatic life. Finally, one selects a steadystate approach or specifies the time period over which the sensitive
resources are to be protected, or over which the damaged resources
are expected to recover. Steady-state critical loads are determined
irrespective of time. Dynamic critical loads, or target loads, may be
determined for various endpoint years, for example, 2050 or 2100.
Each of these various decisions that must be made to simulate critical or target loads has an influence on the resulting model simulated
values. A target load can be selected that is higher than the modeled
critical load if the objective is to make some limited progress toward
reaching the critical load. Conversely, a target load can be selected
that is lower than the critical load to ensure that the sensitive ecosystem is fully protected given modeling uncertainty or to attain the
targeted threshold chemistry more quickly in the case of resources
that have already been damaged.