Model changes
While the previous implementation of tidal hydrology for a microtidal
marsh used a sinusoidal function to represent tidal variations, modeling
vegetation responses to tidal cycles across a broader range of coastal
systems requires the ability to drive the model with site-specific tidal
hydrology and salinity concentrations. We added the capability to read
in an external forcing file containing a time series of water level
heights and salinity concentrations (add github link?). The tide file
may be created with measured water levels and salinity concentrations,
water levels calculated from tide constituents, or hypothetical values
for scenario simulations, thereby facilitating flexibility in simulating
different site conditions. This approach builds the groundwork for
coupling ELM to water level and salinity boundary conditions determined
by ocean and river components of E3SM in future coupled simulations.
To represent the influence of salinity on vegetation growth, we used a
Gaussian function (Equation 2, Li et al., 2021)
\(f_{i}(s)=\ e\ \frac{{-(s-\mu)}^{2}}{{2y}^{2}}\) Equation 2
In this function, s represents salinity, µ represents the
salinity concentration at which maximum growth occurs (optimal salinity
for a PFT), and ʎ represents the salinity tolerance. The functionfi(s) yields a value between 0 and 1, andfi(s) is multiplied by
wiri for each layer to decreaseꞵt proportionately to the salinity concentration
(Fig 1a). ELM does not represent salinity variations with depth in this
study; the model is forced with a single salinity concentration applied
to the entire marsh column, assuming that salinity in the marsh is equal
to that in the tidal channel.
To represent the effects of flooding on vegetation growth, we used a
linear function that limits carbon uptake by the proportion of
vegetation that is submerged (Equation 3).
\(f_{i}\left(z\right)=\left\{\par
\begin{matrix}0,\ \ z\geq H\\
\frac{H-z}{H},0<z<H\\
1,z\leq 0\\
\end{matrix}\right.\ \) Equation 3
The inhibition of growth due to inundation,fi(z) , is calculated by the plant height Hand the surface water height z . Water levels above the plant
height set fi(z) to zero, while water levels
below the marsh surface set fi(z) to 1. When 0
< z < H , the function inhibits root
water uptake in proportion to the fraction of plant height that is
submerged (Fig 1b). As with the salinity response,fi(z) is multiplied by the root water uptake
resistance in each layer to represent the impact on root function. The
improved model thus represents the soil water stress via a transpiration
function that includes salinity and flood stress (Equation 4).
\(\beta_{t}=\ \sum_{i}{w_{i}\text{\ r}_{\text{i\ }}f_{i}\left(s\right)}f_{i}(z)\)(Equation 4)