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Using a Geographic Information System to Determine Wet Deposition Trends of Ammonium in the Central U.S.

Gregory Wetherbee
U.S. Geological Survey,
Branch of Quality Systems,
Denver, CO USA

The U.S. Geological Survey evaluated trends in wet deposition of ammonium measured by the National Atmospheric Deposition Program (NADP) between 1994 – 2006 for the central U.S. region including: North Dakota, South Dakota, Minnesota, Wisconsin, Nebraska, Iowa, Illinois, Missouri, Kansas, Oklahoma, and Arkansas. Results from this study indicate that even though agriculture is the largest emissions source of ammonia in the central U.S., trends in agricultural emissions might have less influence on changes in nitrogen wet deposition than portrayed by NADP in recent years. No temporal trend is apparent in ammonium deposition in the central U.S. between 1994 – 2006, despite increasing wet-deposition ammonium concentrations depicted by NADP in annual concentration isopleth maps (http://nadp.sws.uiuc.edu/amaps2/). Using the ESRI ArcMap Spatial Analyst® geographic information system (GIS), annual ammonium deposition grids for the central U.S. were generated by multiplying the Parameter-elevation Regressions on Independent Slopes Model (PRISM) precipitation grids (http://www.prism.oregonstate.edu) by the NADP precipitation-weighted concentration grids (http://nadp.sws.uiuc.edu/isopleths/grids.asp). PRISM uses a more extensive and denser network of point measurements than the NADP, which may yield more detailed precipitation patterns for the central U.S. To evaluate changes in ammonium deposition, the annual ammonium deposition grid values were classified into four deposition ranges: <2.0, 2-3, 3-4, and >4.0 kilograms per hectare (kg/ha), and the percentages of grid cells within each of these ammonium deposition categories were plotted (figure 1). Regression analysis and the Seasonal Kendall Test of the annual PRISM data indicate a weak but decreasing trend in annual precipitation during 1994-2006. The decreasing precipitation offsets increases in ammonium concentration resulting in no net change in ammonium wet deposition. This study shows the potential utility of GIS in studying temporal wet deposition across the nation.