Critical Loads - Monitoring Strategy - Section 1

Improving Deposition Estimates

The CL Implementation Strategy recommends using the standardized modeled deposition information available on the Portal to determine CL exceedance. These deposition products combine measured and modeled wet and dry deposition to produce estimates of total (wet+dry) deposition across the lower 48 states. For a few forests, regional deposition modeling and mapping efforts may provide enhanced, more accurate estimates by incorporating data from snowpack samples in high elevation areas, nitrogen deposition from throughfall samplers, or cloud and fog deposition from other sources. Regional mapping efforts may also utilize different models such as ClimCalc in the northeastern US or modeled wet deposition by Grimm for the eastern US.

After considering all available options for deposition information, some forests will still seek improvement, given the known deficiencies and/or absence of estimates. The only way to improve the modeled estimates is often obtaining additional measured deposition data. Obtaining measured deposition information would be most important in areas where initial analysis shows that critical loads are likely to be exceeded and deposition is underestimated.

There are several methods available for measuring deposition:

  • Establish a long-term monitoring site as part of an existing network (NTN monitoring for measurement of wet deposition, CASTNET for dry deposition). This requires an initial investment in equipment and a long-term commitment to fund sample analysis, site operations, and maintenance. The benefits include a well-established structure of protocols and procedures, central laboratory analysis of samples, and centralized data analysis and reporting.
  • Bulk or throughfall measurements, especially using “passive” collectors, allow for shorter-term deposition measurements. Passive throughfall collectors based on ion exchange resin (IER) columns were developed to provide a lower cost method of obtaining deposition measurements in remote areas.
  • Lichen tissue can be collected and analyzed for nitrogen, sulfur, and other elements, and these results provide an indirect avenue for obtaining deposition information. Forest Service research shows how annual N deposition can be estimated from the nitrogen content of lichen tissue, once the relationship has been established for a region. This technique is available for use in the western states and Alaska.

Forests are advised to enlist the assistance of the Regional Air Program Manager and local research scientists to determine the most appropriate approach for deposition monitoring.