F.6 Wet deposition of inorganic nitrogen

Wet deposition of inorganic nitrogen includes the pluvial delivery of both ammonium and nitrate to the water column. A global total inorganic nitrogen concentration in rainfall is specifiable, together with the proportion of that total that is nitrate. From this, the WQ Module computes ammonium and nitrate concentrations (Equations (F.13) and (F.14), respectively).

\[\begin{equation} \left[{NH_4}\right]_{rain} = \left[{TN}\right]_{rain} \times (1-f_{TN}^{NO_3}) \tag{F.13} \end{equation}\]

\[\begin{equation} \left[{NO_3}\right]_{rain} = \left[{TN}\right]_{rain} \times f_{TN}^{NO_3} \tag{F.14} \end{equation}\]

\(\left[{TN}\right]_{rain}\) and \(f_{TN}^{NO_3}\) are the concentration of total nitrogen in rain and the fraction of that total that is nitrate, respectively. The WQ Module separately multiplies \(\left[{NH_4}\right]_{rain}\) and \(\left[{NO_3}\right]_{rain}\) by each timestep’s rainfall depth and corresponding model surface area(s) to compute the wet mass flux of ammonium and nitrate (Equations (F.15) and (F.16), respectively). These masses are added to the uppermost model layer cells and corresponding changes to ambient concentrations computed.

\[\begin{equation} F_{atm-wet}^{NH_4} = \left[{NH_4}\right]_{rain} \times \text{Timestep rainfall depth} \times \text{Model area(s)} \tag{F.15} \end{equation}\]

\[\begin{equation} F_{atm-wet}^{NO_3} = \left[{NO_3}\right]_{rain} \times \text{Timestep rainfall depth} \times \text{Model area(s)} \tag{F.16} \end{equation}\]

If rainfall depth is specified as being spatially variable (i.e. by application of multiple atmospheric boundary conditions within TUFLOW FV), then \(F_{atm-wet}^{NH_4}\) and \(F_{atm-wet}^{NO_3}\) will correspondingly vary spatially.