Nitrate

The amount of nitrate ($NO_3^-$) in the stream may be increased by the oxidation of $NO_2^-$. The nitrate concentration in the stream may be decreased by the uptake of $NO_3^-$ by algae. The change in nitrate for a given day is:

$\Delta NO3_{str}=(\beta_{N,2}*NO2_{str}-(1-fr_{NH4})*\alpha_1*\mu_a*algae)*TT$ 7:3.2.10

where $\Delta NO3_{str}$ is the change in nitrate concentration (mg N/L), $\beta_{N,2}$ is the rate constant for biological oxidation of nitrite to nitrate (day$^{-1}$ or hr$^{-1}$), $NO2_{str}$ is the nitrite concentration at the beginning of the day (mg N/L), $fr_{NH4}$ is the fraction of algal nitrogen uptake from ammonium pool, $\alpha_1$ is the fraction of algal biomass that is nitrogen (mg N/mg alg biomass), $\mu _a$ is the local growth rate of algae (day$^{-1}$ or hr$^{-1}$), $algae$ is the algal biomass concentration at the beginning of the day (mg alg/L), and $TT$ is the flow travel time in the reach segment (day or hr). The local rate constant for biological oxidation of nitrite to nitrate is calculated with equation 7:3.2.9 while the fraction of algal nitrogen uptake from ammonium pool is calculated with equation 7:3.2.7. Section 7:3.1.2.1 describes the calculation of the local growth rate of algae. The calculation of travel time is reviewed in Chapter 7:1.

Table 7:3-2: SWAT+ input variables used in in-stream nitrogen calculations.

Variable Name	Definition	File Name
AI1	$\alpha_1$: Fraction of algal biomass that is nitrogen (mg N/mg alg biomass)	.wwq
RHOQ	$\rho_{a,20}$: Local algal respiration rate at 20$\degree$C (day$^{-1}$)	.wwq
BC3	$\beta_{N,3,20}$: Local rate constant for hydrolysis of organic nitrogen to NH$^+_4$ at 20$\degree$C (day$^{-1}$ or hr$^{-1}$)	.swq
RS4	$\sigma_{4,20}$: Local settling rate for organic nitrogen at 20$\degree$C (day$^{-1}$)	.swq
BC1	$\beta_{N,1,20}$: Rate constant for biological oxidation of ammonia nitrogen at 20$\degree$C (day$^{-1}$)	.swq
RS3	$\sigma_{3,20}$: Benthos (sediment) source rate for ammonium nitrogen at 20$\degree$C (mg N/m$^2$-day or mg N/m$^2$-hr)	.swq
P_N	$f_{NH4}$: Preference factor for ammonia nitrogen	.wwq
BC2	$\beta_{N,2,20}$: Rate constant for biological oxidation of nitrite to nitrate at 20$\degree$C (day$^{-1}$ or hr$^{-1}$)	.swq