Is There More to Total Deposition Than Wet and Dry: Lessons From and Utility of Throughfall

Kathleen C. Weathers
Institute of Ecosystem Studies
Box AB, Millbrook, NY 12545

There are two methods that have commonly been used to estimate total deposition: 1) measured (wet) combined with modeled (dry and cloud) deposition from monitoring networks, and 2) the measurement of throughfall flux (TF), which is a measure of total deposition (wet + dry + fog or cloud) as well as the net result of uptake and leaching to the forest floor. The TF method has the advantages of being inexpensive and integrating deposition over complex forest canopies and/or
complex terrain, which are the landscapes where model assumptions for calculating dry and fog deposition fluxes are tenuous, at best. The monitoring data allow site-to-site comparisons and accurate flux numbers for regions adjacent to monitoring station locations.

Throughfall compares well to CASTNET + NADP sulfur (S) and nitrogen (N) deposition for forests adjacent to CASTNET sites. It also compares well to cloud+wet+dry deposition as measured by MADPRo, CASTNET and NADP. Over the past several years, complementary uses of throughfall and monitoring network data have emerged for estimating total deposition to complex terrain. Here, I illustrate the upsides and downsides of throughfall measurements and how they have been used for estimating the deposition of N and S over complex terrain and across regions.

The addition of throughfall measurements to routine monitoring may enhance the utility of NADP, CASTNET and MADPro monitoring, for pollutants, including metals, and nutrients.