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TOTAL MAXIMUM DAILY LOADS AND CRITICAL LOADS; DIFFERENT MEANS TO ACHIEVE THE SAME END?
Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org
Citation: Pp. 501-501 in Total Maximum Daily Load (TMDL) Environmental Regulations–II Proceedings of the 8-12 November 2003 Conference (Albuquerque, New Mexico USA), Publication Date 8 November 2003. .(doi:10.13031/2013.15605)
Authors: M. A. Backx, and C.T. Driscoll Jr.
Keywords: acid rain, forest ecosystems, TMDL, critical loads, dynamic modeling
Over the past 30 years research has indicated that effects of acidic deposition, especially from
sulfate ( ) and nitrate ( ), cause serious harm to sensitive ecosystems in North America, Europe
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In this study Total Daily Maximum Loads (TMDLs) for sulfur and nitrogen from atmospheric
deposition were calculated for two watersheds in the United States. The Hubbard Brook
Experimental Forest, New Hampshire, is situated in a northern hardwood forest. Acidic
deposition at this site is high and mainly due to emissions of and from industrial facilities in the
Midwest. Loch Vale, a watershed in the Colorado Front Range, has (sub-) alpine conifer
vegetation and receives less inputs of acidic deposition.
To calculate TMDLs the model PnET-BGC was used. PnET-BGC is an integrated dynamic
biogeochemical model that simulates chemical transformations of vegetation, soil and drainage
water. Using a dynamic model instead of a steady-state model allows estimation of the time
needed for an ecosystem to recover after emission reduction strategies are in place and acidic
deposition has decreased. Calculations of TMDL values for the two watersheds were conducted
to help gain insight in the variation of TMDLs based on differences in vegetation types and
levels of acidic deposition.
In comparison, the Critical Loads concept was introduced more than a decade ago, as a basis for
the development of air pollution control strategies in Europe. Critical loads are defined as: 'the
estimate of exposure to pollutants below which harmful effects on specified sensitive elements of
the environment do not occur according to present knowledge'. > Calculated critical load values
for sulfur and nitrogen in the two watersheds were used to analyze the differences and
similarities between this concept and the TMDL approach.