# 3:1.5.1 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: $$N\_2+O\_2\xrightarrow{arc of electricity}2NO$$                  (monoxide)

step 2: $$2NO+O\_2\rightarrow NO\_2$$                                    (dioxide)

step 3: $$3NO\_2+H\_2O\rightarrow 2HNO\_2+NO$$         (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.

| Definition                                                            | Source Name | Input Name | Input File |
| --------------------------------------------------------------------- | ----------- | ---------- | ---------- |
| $$R\_{NO3}$$: Concentration of nitrogen in the rain (mg/L)            | RCN         |            | .bsn       |
| Atmospheric deposition of ammonium (mg/L) values for entire watershed | rammo\_sub  |            | .atm       |
| Atmospheric deposition of nitrate (mg/L) for entire watershed         | Rcn\_sub    |            | .atm       |