## Critical Loads Frequently Asked Questions (FAQs)

- What is a critical load?
- The critical load is the level of deposition below which no harmful ecological effects occur.
- What is a target load?
- The target load is the level of deposition set by policy makers to protect a given area of sensitive ecosystem components. The target load may be higher or lower than the critical load based on considerations of economic cost of emissions reductions, timeframe, and other matters.
- How are critical loads calculated?
- There are three main approaches for calculating critical loads: empirical, simple mass balance, and dynamic models.
**Empirical**critical loads are estimated based on observations of the response of an ecosystem or ecosystem component (e.g., foliage, lichens, soil) to a given, observed deposition level.**Simple mass balance**critical loads are calculated based on inputs and outputs of the nutrient of concern (e.g., base cation, nitrogen). Simple mass balance methods are steady-state models that calculate the critical load of deposition to an ecosystem over the long term.**Dynamic models**use a mass balance approach expanded by incorporating internal feedbacks—such as accumulation of N in the system, or exchange of base cations between soil and soil solution from year to year. Dynamic models allow the prediction of time to damage and time to recovery.- Simple mass balance

- The simple mass balance approach is also called the steady-state approach. It is based, very simply, on the concept that if outputs from an ecosystem exceed inputs, you will have a net loss from the ecosystem. On the other hand, if inputs to the ecosystem exceed outputs, you will have a net accumulation in the ecosystem. Simple mass balance methods are
*steady-state*models that calculate the critical load of deposition to an ecosystem over the long term (i.e., one rotation or several cutting cycles, depending on the management, or 100+ years). The main inputs to the ecosystem include atmospheric deposition and weathering of mineral soil; the outputs from the ecosystem include leaching losses and biomass removal .

The basic equation for the**Simple mass balance**approach is given below.

The Critical Load for acidity = CL(S)+CL(N)

**CL(S)+CL(N) = BC**_{dep}**- Cl + BC**_{w}**– BC**_{u}**+ N**_{a}**+ N**_{u}**+ N**_{de}**– ANC**_{le,crit }where

Bc_{dep}- sum of Ca + Mg + Na+ K deposition rate (eq ha-1 yr-1)

BC_{w}- soil weathering rate of Ca + Mg + K + Na (eq ha-1 yr-1)

B_{Cu}- net Ca + Mg + K uptake rate (eq ha-1 yr-1) ultimately removed by harvest

or disturbance

ANC_{le,crit}- acceptable acid neutralizing capacity (ANC) leaching rate (eq ha-1 yr-1).

N_{a}- acceptable net N accumulation rate in the soil (eq ha-1 yr-1)

N_{u}- net N uptake rate (i.e., increment of nutrient in biomass; eq ha-1 yr-1)

N_{de}- soil denitrification rate (eq ha-1 yr-1)

For more details on critical loads calculations, see the ICP Manual on Methodologies and Criteria for Mapping Critical Levels/Loads on the Links page, or the Protocol for calculating critical loads of Nitrogen and Sulfur deposition for forest ecosystems in Forest Service class I areas on the Products page. - What data do I need to calculate critical loads?
- The data required for calculating critical loads depends on the method description for calculation. Generally, you will need deposition, soil, vegetation, and site data. For a list of required data, please look at Data Required for Critical Loads Calculations or European Data Requirements on the Products page.
- How are critical loads used?
- Critical loads can be used to identify and regulate levels of acidic deposition to prevent harmful ecological effects.
- Who uses critical loads?
- Critical loads have been used to regulate emissions in Europe through the 1999 Gothenburg Protocol via the UNECE Convention on LRTAP.

Critical loads can also be used by Federal Land Managers. For an example, read this article. - Where have critical loads been calculated in the US?
- Critical loads have been calculated for many ecosystems in the United States, including Northeastern forests and high elevation lakes in the Mountain West. For a list of studies concerning critical loads, please look at the Summary of Critical Loads in the United States on the Products page.
- Do critical loads tell you when an ecosystem will be damaged?
- No. The critical load is the level of deposition below which no harmful ecological effects occur. When deposition is below the critical load, ecosystems should not sustain damage from acidic deposition. Steady state models do not tell you the time until damage occurs. Dynamic modeling, however, allows you to estimate the time to damage and time to recovery from acidic deposition.

For more information on time and critical loads, see**Box 3**in Approaches for Estimating Critical Loads of N and S Deposition for Forest Ecosystems on US Federal Lands on the Products page. - What’s the difference between a critical load and a target load?
- The critical load is the level of deposition below which no harmful ecological effects occur. The target load is the level of deposition set by policy makers to protect a given area of sensitive ecosystem components. The target load may be higher or lower than the critical load based on considerations of economic cost of emissions reductions, timeframe, and other matters.

The critical load is a property of the ecosystem and is independent of time. A target load can take time into consideration (for example, the time to achieve a certain condition). The critical load does not depend on the current condition of the ecosystem; the target load must take that into account. When “critical loads” are presented for different time periods for an ecosystem, note that these are really target loads. - What’s the difference between the critical load for N nutrient and the critical load for N (acidity)?
- The critical load for nutrient N is the deposition of N to an ecosystem below which no harmful effects of N saturation occur.
The critical load for N is the level of deposition below which no harmful ecological effects caused by acidity occur.

- Is the critical load for S+N the same as the critical load for acidity?
- Yes. The critical load for acidity is the total deposition of acidity inputs to the ecosystem (S and N combined); it is sometimes referred to as critical load for S+N.
- What’s the difference between steady-state and dynamic modeling?
- The concept that the critical load is a steady-state property can be confusing. This means that the critical load is based on the capacity of the ecosystem to buffer deposition inputs. Inherent in the critical load calculation is the assumption that all the processes in the ecosystem are at steady-state, and that the critical load is, therefore, not a function of time. An example of steady-state is a bath-tub with the water running and the drain plug open, with the flow set so that the level of the water is unchanging.

The steady-state assumption means that fewer data are necessary to calculate the critical load. Alternatively, if we have enough information to describe the rates of the physical, chemical and biological processes, we can estimate the time until a given condition is reached. This process is dynamic modeling, which can yield valuable information about time to damage and time to recovery, if the necessary input data are available.

Last Modified: 08/19/2008