Atmospheric deposition occurs when airborne chemical compounds settle onto the land or water surface. Some of the most important chemical pollutants are those containing nitrogen or phosphorus. Nitrogen compounds can be deposited onto water and land surfaces through both wet and dry deposition mechanisms. Wet deposition occurs through the absorption of compounds by precipitation as it falls carrying mainly nitrate ($NO_3^-$) and ammonium ($NH_4^+$). Dry deposition is the direct adsorption of compounds to water or land surfaces and involves complex interactions between airborne nitrogen compounds and plant, water, soil, rock, or building surfaces.

The relative contribution of atmospheric deposition to total nutrient loading is difficult to measure or indirectly assess and many deposition mechanisms are not fully understood. Most studies and relatively extended data sets are available on wet deposition of nitrogen, while dry deposition rates are not well defined. Phosphorus loadings due to atmospheric deposition have not been extensively studied and nation-wide extended data set were unavailable at the time of data preparation for the CEAP project. While research continues in these areas, data records generated by modeling approaches appear to be still under scrutiny.

A number of regional and local monitoring networks are operating in the U.S. mainly to address information regarding regional environmental issues. For example, the Integrated Atmospheric Deposition Network (IADN) (Galarneau et al., 2006) that estimates deposition of toxic organic substances to the Great Lakes. Over the CONUS (conterminous United States), the National Atmospheric Deposition Program (NADP) National Trends Network (NTN) (NADP/NTN, 1995; NADP/NTN, 2000; Lamb and Van Bowersox, 2000) measures and ammonium in one-week rain and snow samples at nearly 240 regionally representative sites in the CONUS and is considered the nation’s primary source for wet deposition data.

The U.S. EPA Clean Air Status and Trends Network (CASTNET), developed form the National Dry Deposition Network (NDDN), operates a total of 86 operational sites (as of December 2007) located in or near rural areas and sensitive ecosystems collecting data on ambient levels of pollutants where urban influences are minimal (CASTNET, 2007). As part of an interagency agreement, the National Park Service (NPS) sponsors 27 sites which are located in national parks and other Class-I areas designated as deserving special protection from air pollution.

Dry deposition of nitrate and ammonia is input to the model for each subbasin. Average daily deposition is added to the appropriate surface soil pool.

Table 3:1-5: SWAT+ input variables that pertain to dry deposition.

Lightning discharge converts atmospheric $N_2$ to nitric acid which can then be transferred to the soil with precipitation. The chemical steps involved are:

step 1: (monoxide)

step 2: (dioxide)

step 3: (nitric acid and monoxide)

More nitrogen will be added to the soil with rainfall in areas with a high amount of lightning activity than in areas with little lightning.

The amount of nitrate added to the soil in rainfall is calculated:

$NO_{3rain}=0.01*R_{NO3}*R_{day}$ 3:1.5.1

where $NO_{3rain}$ is nitrate added by rainfall (kg N/ha), $R_{NO3}$ is the concentration of nitrate in the rain (mg N/L), and $R_{day}$ is the amount of precipitation on a given day (mm H$_2$O). The nitrogen in rainfall is added to the nitrate pool in the top 10 mm of soil.

The amount of ammonia added to the soil in rainfall is calculated:

$NH_{4rain}=0.01*R_{NH4}*R_{day}$ 3:1.5.2

where $NH_{4rain}$ is nitrate added by rainfall (kg N/ha), $R_{NH4}$ is the concentration of ammonia in the rain (mg N/L), and $R_{day}$ is the amount of precipitation on a given day (mm H$_2$O). The nitrogen in rainfall is added to the ammonia pool in the top 10 mm of soil.

Table 3:1-4: SWAT+ input variables that pertain to nitrogen in rainfall.