Nitrogen in Rainfall

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.

Variable Name	Definition	Input File
RCN	$R_{NO3}$: Concentration of nitrogen in the rain (mg/L)	.bsn
rammo_sub	Atmospheric deposition of ammonium (mg/L) values for entire watershed	.atm
Rcn_sub	Atmospheric deposition of nitrate (mg/L) for entire watershed	.atm