Critical Loads - Spatial Data

This page hosts calculated critical load exceedances for national published critical loads data sets (critical loads exceedancesfor the El Yunque National Forest in Puerto Rico, and the Chugach and Tongass National Forests in Alaska are not available as part of the spatial data hosted below – these National Forests should reference the “User’s Guide for Setting Empirical Critical Loads for Nutrient Nitrogen”).  These national GIS shapefiles contain CL exceedance calculations performed by the Forest Service Air Program.  When referencing this data, cite the relevant original references from the footnotes below and include the proper CLAD acknowledgment1.

  1. Nationwide CLs of acidity for surface waters2
  2. Nationwide CLs of acidity for forested ecosystems3, and
  3. Nationwide empirical CLs of nutrient nitrogen4 for ecosystem receptors and responses (fungi, lichens, herbaceous vegetation, forests, and nitrogen leaching)

Click here to download a zip file to view and evaluate all of the above described CL exceedances, along with additional spatial data that might be useful during analysis (National Forest proclamation boundaries, Class I and Class II Wilderness boundaries, 6th level hydrologic unit code (HUC) boundaries used in the FS Watershed Condition Framework, and NADP and CASTNET deposition monitoring stations).  Follow these instructions to download and save the CL_Exceedances.zip file, open the Nationwide_Critical_Load_Exceedances.mxd map, and clip national data to your area of interest and run analyses recommended in the Critical Loads Implementation Strategy.  JPG images of the national CL exceedances can be viewed and downloaded here.

The Portal hosts an explanation of the multiple CLs concept (how a given area can have multiple associated CLs), and how to interpret multiple CLs simultaneously. There is also a description of the different approaches for calculating CLs, including advantages and disadvantages associated with the different methods.

*Please reference the Critical Load Exceedance Documentation for Nationwide CL Efforts for documentation on how CL exceedances were calculated for the data hosted on the portal, as well as the CL Exceedance Calculation Protocol for instructions to calculate CL exceedances using site-specific CL or deposition information.

Critical Load Spatial Data

Critical load values can be downloaded directly from the Critical Loads of Atmospheric Deposition Science Committee (CLAD) National Critical Loads Database. Forest Service Personnel can access this data on the Forest Service T-drive at T:\FS\NFS\WOWatershedFishWildlife\Project\AirQualityPortal\GIS\CriticalLoads\CLs.zip.

Atmospheric Deposition Spatial Data

Atmospheric deposition is measured and modeled by a variety of sources. The national critical load exceedances described above were calculated using the 2010-2012 three-year average estimates produced by the NADP Total Deposition Science Committee (TDEP) v2014.01 5. This deposition output was produced using a method that combines monitoring data with output from the Community Multiscale Air Quality (CMAQ) modeling system, giving priority to measurement data near the location of the monitor and priority to CMAQ data in areas where monitoring data are not available.  This TDEP output represents the best and most recent estimate of total deposition currently available for a national assessment. Although using the most recent deposition data will provide the best estimate of current critical load exceedance conditions, a three year average from 2010-2012 was used to minimize the effect of any one outlier year on the overall results.  Metadata for this 2010-2012 TDEP deposition model run is available and should be consulted prior to use. Caution: the CMAQ deposition model is known to underestimate deposition in certain situations (high elevations and complex terrain, as well as areas with high cloud and fog deposition, and areas in the arid south and west). Until methods are developed to correct these underestimates, be aware that deposition could be higher than the model predicts and CL exceedances are possible. 

TDEP total nitrogen (n_td) and total sulfur (s_td) estimates for the years 2000-2012 and the 3-year average of 2000-2002 and 2010-2012 can be downloaded directly from the EPA ftp site.  Only N deposition is relevant to the empirical CLs of nutrient N (because these are N-only CLs), while both S and N deposition are relevant to the CLs of acidity (because both S and N are acidifying).  Absent specific rationale to the contrary, we recommend using this deposition layer to calculate CL exceedances for site-specific CLs to maintain nationwide consistency. Regionally-refined deposition modeling efforts that have documented QA/QC methods can be substituted where appropriate.

Comparisons between these deposition estimates and other methodologies previously used to estimate atmospheric deposition have been performed.  The attached report documents how the results of different methodologies vary, and provides estimates of nitrogen and sulfur deposition using each methodology within every National Forest System Forest and Wilderness.

References

1Acknowledgment when using CLs data: "We acknowledge Critical Load Atmospheric Deposition (CLAD) Science Committee of National Atmospheric Deposition Program (NADP) for their role in making available CLAD_CL_ACID_v2.accdb and CLAD_CL_N_v2.accdb datasets.”
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2Lynch, J, LH Pardo and C Huber.  2013.  Detailed documentation of the CLAD U.S. critical loads of sulfur and nitrogen Access database, version 2.0. Created for the Critical Loads of Atmospheric Deposition (CLAD) Science Subcommittee of the National Atmospheric Deposition Program (NADP).

McDonnell, TC, BJ Cosby, TJ Sullivan. 2012. Regionalization of soil base cation weathering for evaluating stream water acidification in the Appalachian Mountains, USA. Environmental Pollution, 162: 338-344.

Miller, E. 2011. Steady-State Critical Loads and Exceedance for Terrestrial and Aquatic Ecosystems in the Northeastern United States. NPS/Multi-Agency Critical Loads Project Technical Report.
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3McNulty, SG, EC Cohen, JA Moore Myers, TJ Sullivan, and H Li. 2007. Estimates of critical acid loads and exceedances for forest soils across the conterminous United States. Environmental Pollution 149: 281-292.
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4Pardo, LH, MJ Robin-Abbott, and CT Driscoll. 2011. Assessment of nitrogen deposition effect and empirical critical loads of nitrogen for ecoregions of the United States. Gen. Tech. Rep. NRS-80. U.S. Department of Agriculture, Forest Service Northern Research Station.  291 pp. 

Geiser, LH, SE Jovan, DA Glavich, and MK Porter. 2010. Lichen-based critical loads for atmospheric nitrogen deposition in Western Oregon and Washington Forests U.S.A. Environmental Pollution 158: 2412-2421. ..................................................................................................................................................................

5Schwede, DB, and GG Lear. 2014. A novel hybrid approach for estimating total deposition in the United States. Atmospheric Environment 92:207-220.