Estimating Critical Loads
The DEP module of RAINS-Asia assesses the sensitivity of
various ecosystems (their "critical loads") to acidic deposition, and compares
this information to the deposition data generated within the same module. This
process identifies the most ecologically sensitive areas which are at greatest
risk of damage from present or projected levels of sulfur deposition.
A critical load of an ecosystem is essentially a 'no-effect' level for a pollutant, that is, the level of a substance (acid deposition, as an example) which does not cause long-term damage to an ecosystem. Areas which have a limited natural capacity to absorb or neutralize acid rain have a low critical load. Ecosystems which are more able to buffer acidity (through different soil chemistry, biological tolerances, or other factors) have a correspondingly higher critical load. Assessing the natural capacity of ecosystems to withstand current and projected levels of pollution is a method of measuring ecosystem health, and can serve as a way to assess the environmental benefits of emissions reductions.
Critical loads have been computed and mapped in Southeast Asia, comprising China, Korea, Japan, The Philippines, Indo-China, Indonesia and the Indian subcontinent. The methodology involved the Steady-State Mass Balance (SSMB) method adapted to Asian characteristics. In Asia critical loads have been computed for 31 different vegetation types. The computation of critical loads is based on plant response criteria and soil stability criteria for each of these vegetation types, which include both natural and managed ecosystems. A critical molar ratio of the concentrations of base cations to aluminum in soil solution, (BC/Al), for each plant species is used as indicator of plant response. Using these critical BC/Al ratios in the SSMB method it is possible to compute maximum allowable acidifying deposition, i.e., the critical load. Critical values for BC/Al as thresholds for root growth reduction have been established for many plant species. The soil stability criterion is additionally introduced to avoid the possibility that acid deposition leads to Aluminum leaching in excess of Aluminum produced by weathering and other processes, e.g. in high precipitation areas. Critical loads for each ecosystem are computed by taking the minimum of either the plant response based result or the soil stability based computation of critical loads. Other data, i.e. climate, soil, geology and vegetation data, and their geographical distribution, required to compute and map critical loads have been derived from a variety of sources.
The Steady-State Mass Balance approach was applied in Asia in conjunction with the semi-quantitative method of relative sensitivity in order to compare the areas with stock-at-risk. The uncertainty of the critical load estimates is largely due to the uncertainties of base cation weathering and base cation deposition estimates which require further verification.
Critical load maps have been developed by the Coordination Center for Effects at the National Institute for Public Health and Environment (RIVM) in the Netherlands. The coordination center for the derivation of critical loads for Asian ecosystems was established at the Research Center for Eco-Environmental Sciences in Beijing. The Center serves as a focal point for the collection of national input data, together with a network of environmental researchers from several Asian nations and Western collaborating institutions. A geographic information system (GIS) and critical loads models were installed at the center, and training in the systems was provided through periodic exchanges of personnel.
For further information on critical loads, contact RIVM.
Return to RAINS-Asia/Deposition module
Return to RAINS-Asia