N.6 Photolysis

The WQ Module conceptualises photolysis as the conversion of refractory dissolved organic carbon, nitrogen and phosphorus to their labile dissolved and inorganic equivalents, under the action of photosynthetically active (PAR), ultraviolet A (UVA) and ultraviolet B (UVB) radiation. These radiation bands are provided to the WQ Module from TUFLOW for each computational cell at each water quality timestep. The WQ Module computes total photolysis of refractory dissolved organic carbon \(F_{photo-tot}^{RDOC}\) as the sum of photolysis due to each of these three bands via Equation (N.20). \[\begin{equation} F_{photo-tot}^{RDOC} = F_{photo-PAR}^{RDOC} + F_{photo-UVA}^{RDOC} + F_{photo-UVB}^{RDOC} \tag{N.20} \end{equation}\] Each band’s photolysis is computed via Equation (N.21), with Radiation\(_{band}\) (where band is PAR, UVA or UVB) being previously converted from \(W/m^2\) (as provided by TUFLOW) to moles of photons. \[\begin{equation} F_{photo-band}^{RDOC} = \text{Radiation$_{band}$} \times \underbrace{c \times 10^{-d \times \lambda_{band}}}_{\text{Apparent quantum yield}} \times \underbrace{CDOM \times e^{S\times(x-\lambda_{band})}}_{\text{Absorption coefficient}} \tag{N.21} \end{equation}\] where \(c\) = 7.52, \(d\) = 0.0122, \(\lambda_{band}\) is each band’s characteristic wavelength, \(S\) = 0.0188 and \(x\) = 440.0 nm. \(CDOM\) is chromomorphic dissolved organic matter and is computed via Equation (N.22). \[\begin{equation} \href{AppDiags.html#WQDiagCDOM}{CDOM_{\langle computed \rangle}} = 0.35 \times e^{0.1922\times \left( \left[DOC\right] + \left[RDOC\right]\right) \times (12/1000)} \tag{N.22} \end{equation}\] \(\left[DOC\right]\) and \(\left[RDOC\right]\) and the ambient concentrations of labile and refractory dissolved organic carbon, respectively.

N.6.1 Consumption

Once total photolysis has been computed, the associated consumptive fluxes (losses) of refractory dissolved organic carbon, nitrogen and phosphorus are computed via Equation (N.23), using ambient refractory dissolved nitrogen and phosphorus concentrations to proportion the computed fluxes. \[\begin{equation} \left.\begin{aligned} \href{AppDiags.html#WQDiagPhoto}{F_{photo\langle computed\rangle}^{RDOC}} =& F_{photo-tot}^{RDOC} \\ \\ F_{photo\langle computed\rangle}^{RDON} =& F_{photo-tot}^{RDOC} \times \frac{\left[RDON\right]}{\left[RDOC\right]} \\ \\ F_{photo\langle computed\rangle}^{RDOP} =& F_{photo-tot}^{RDOC} \times \frac{\left[RDOP\right]}{\left[RDOC\right]} \end{aligned}\right\} \tag{N.23} \end{equation}\]

N.6.2 Production

The consumptive fluxes from Equation (N.23) result in the labile dissolved organic carbon, nitrogen and phosphorus productive fluxes in Equation (N.24). \[\begin{equation} \left.\begin{aligned} F_{photo\langle computed\rangle}^{DOC} =& F_{photo\langle computed\rangle}^{RDOC} \times f_{photo}^{RDOM} \\ \\ F_{photo\langle computed\rangle}^{DON} =& F_{photo\langle computed\rangle}^{RDON} \times f_{photo}^{RDOM} \\ \\ F_{photo\langle computed\rangle}^{DOP} =& F_{photo\langle computed\rangle}^{RDOP} \times f_{photo}^{RDOM} \end{aligned}\right\} \tag{N.24} \end{equation}\] \(f_{photo}^{RDOM}\) is the proportion of the total photolysed refractory dissolved organic matter that becomes the corresponding labile organics. The remainder of the photolysed refractory dissolved organic matter fluxes produce inorganic carbon and nutrients via Equation (N.25). \[\begin{equation} \left.\begin{aligned} F_{photo\langle computed\rangle}^{DIC} =& F_{photo\langle computed\rangle}^{RDOC} \times \left(1.0 - f_{photo}^{RDOM}\right) \\ \\ F_{photo\langle computed\rangle}^{NH_4} =& F_{photo\langle computed\rangle}^{RDON} \times \left(1.0 - f_{photo}^{RDOM}\right) \\ \\ F_{photo\langle computed\rangle}^{FRP} =& F_{photo\langle computed\rangle}^{RDOP} \times \left(1.0 - f_{photo}^{RDOM}\right) \end{aligned}\right\} \tag{N.25} \end{equation}\]

The WQ Module does not currently support simulation of dissolved inorganic carbon (DIC) so the corresponding carbonic productive flux in Equation (N.25) has no effect on inorganic water quality dynamics. Contact support@tuflow.com if the DIC model is required.