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@@ -985,7 +985,8 @@ The vegetation method is selected using ``method_vegetation``. The model current
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Vegetation Metrics
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^^^^^^^^^^^^^^^^^^^
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(**method** ``duran``) The basal vegetation density :math:`\rho_{\text{veg}}` (``rhoveg``) [:math:`\mathrm{-}`] can vary in space and time. It is determined by the ratio of the actual vegetation height :math:`h_{\text{veg}}` (``hveg``) [:math:`\mathrm{m}`] to the maximum attainable vegetation height :math:`H_{\text{veg}}` (``Hveg``) [:math:`\mathrm{m}`], varying between 0 and 1 :cite:`DuranHerrmann2006`:
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**Method** ``duran`
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The basal vegetation density :math:`\rho_{\text{veg}}` (``rhoveg``) [:math:`\mathrm{-}`] can vary in space and time. It is determined by the ratio of the actual vegetation height :math:`h_{\text{veg}}` (``hveg``) [:math:`\mathrm{m}`] to the maximum attainable vegetation height :math:`H_{\text{veg}}` (``Hveg``) [:math:`\mathrm{m}`], varying between 0 and 1 :cite:`DuranHerrmann2006`:
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.. math::
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:label: Vegetation_density_duran
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This assumption is based on the idea that burying vegetation reduces its height, which indicates a simultaneous decrease in actual cover. The change in vegetation density per grid cell is directly linked to the alteration in vegetation height within that specific cell.
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(**method** ``grass``) The new framework decouples plant structure into two independent state variables: tiller density :math:`N_t` (``N_t``) [:math:`\mathrm{tillers/m^2}`] and tiller height :math:`h_{\text{veg}}` (``hveg``) [:math:`\mathrm{m}`]. This separation enables the representation of distinct morphological states, such as sparse/tall canopies or dense/short canopies.
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**Method** ``grass``
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The new framework decouples plant structure into two independent state variables: tiller density :math:`N_t` (``N_t``) [:math:`\mathrm{tillers/m^2}`] and tiller height :math:`h_{\text{veg}}` (``hveg``) [:math:`\mathrm{m}`]. This separation enables the representation of distinct morphological states, such as sparse/tall canopies or dense/short canopies.
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To capture fine-scale spatial dynamics, such as clonal expansion, these vegetation metrics and their subsequent developmental processes are resolved on an automatically generated higher-resolution sub-grid (:math:`\Delta x \leq1` m). The resolution increase can be set using the ``veg_res_factor``. To prevent excessive computational demand, the timestepping for the vegetation module specifically can be adjusted using ``dt_veg``.
Vegetation growth and decay follow the model proposed by :cite:`DuranHerrmann2006`, modified to include an optimal burial rate :math:`\Delta z_{\text{b,opt}}` [:math:`\mathrm{m/yr}`] that shifts the peak of optimal growth:
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.. tip::
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Meaning of these variables: An intrinsic vertical growth rate of :math:`V_{\text{ver}} = 4` m/year does not mean the vegetation will be 4 meters high after 1 year, as growth follows a logistic curve that slows as it reaches :math:`H_{\text{veg}}`.
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**Method:** ``grass``
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**Method** ``grass``
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Vegetation development is simulated through two completely decoupled processes: vertical tiller growth and horizontal tiller establishment (:ref:`fig-vegetation-development`).
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.. _fig-vegetation-development:
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.. figure:: /images/vegetation_growth.mp4
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:width:900px
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:align:center
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.. video:: /images/vegetation_growth.mp4
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:autoplay:
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:loop:
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:muted:
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:width: 80%
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Simulated spatial and temporal evolution of tiller density and height demonstrating local growth, clonal expansion, seedling dispersal, and inter-species competition.
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**1. Vertical Tiller Growth:**
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Vertical growth utilizes a generalized logistic growth equation, driven by the intrinsic growth rate :math:`G_h` [:math:`\mathrm{m/yr}`] and an exponent :math:`\phi_h` [:math:`\mathrm{-}`] that provides greater control over the growth trajectory:
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Vertical growth utilizes a generalized logistic growth equation, driven by the intrinsic growth rate :math:`G_h` [:math:`\mathrm{m/yr}`] (``G_h``) and an exponent :math:`\phi_h` [:math:`\mathrm{-}`] (``phi_h``) that provides greater control over the growth trajectory:
The response to sediment burial and erosion :math:`B_h` relies on the sensitivity parameter :math:`\gamma_h` [:math:`\mathrm{-}`] and an optimal burial rate :math:`\Delta z_{\text{opt},h}` [:math:`\mathrm{m/yr}`] (e.g., 0.2–1.0 m/yr for marram grass), producing an asymmetric response to burial versus erosion.
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The response to sediment burial and erosion :math:`B_h` relies on the sensitivity parameter :math:`\gamma_h` [:math:`\mathrm{-}`] (``gamma_h``) and an optimal burial rate :math:`\Delta z_{\text{opt},h}` [:math:`\mathrm{m/yr}`] (e.g., 0.2–1.0 m/yr for marram grass) (``dzb_opt_h``), producing an asymmetric response to burial versus erosion.
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**2. Horizontal Tiller Establishment (Density):**
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Tiller density evolves through the recruitment of new tillers via seedling germination (:math:`s`) and clonal expansion (:math:`c`). Tiller production occurs in a source cell (:math:`j`) and is distributed to a target cell (:math:`i`) based on dispersal weights :math:`w_{ij}`:
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To account for inter-specific competition in multi-species simulations, the logistic saturation term utilizes a Lotka-Volterra approach, where :math:`\alpha_{AB}` [:math:`\mathrm{-}`] defines the competitive effect of species :math:`A` on target species :math:`B`. Production in the source cell is calculated via:
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To account for inter-specific competition in multi-species simulations, the logistic saturation term utilizes a Lotka-Volterra approach, where :math:`\alpha_{AB}` [:math:`\mathrm{-}`] (``alpha_comp``) defines the competitive effect of species :math:`A` on target species :math:`B`. Production in the source cell is calculated via:
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.. math::
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:label: tiller_production
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Vegetation-induced Shear Reduction
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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**Method:** ``duran``
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**Method** ``duran``
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Inspired by the Coastal Dune Model (CDM), AeoLiS incorporates vegetation-wind interaction using the simplified expression:
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The ratio of shear velocity in the presence of vegetation (:math:`u_{*,\text{veg}}`) [:math:`\mathrm{m/s}`] to the unobstructed shear velocity (:math:`u_*`) [:math:`\mathrm{m/s}`] is driven by the basal vegetation cover :math:`\rho_{\text{veg}}` and a fixed vegetation-related roughness parameter :math:`\Gamma` (``gamma_vegshear``, default = 16) [:math:`\mathrm{-}`].
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**Method:** ``grass``
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**Method** ``grass``
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Rather than relying on the basal cover assumption, the updated framework calculates local shear velocity reduction by explicitly using the frontal area index :math:`\lambda_{\text{veg}}`:
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In the standard advection scheme, the model implicitly assumes that local bed properties dictate the saturation concentration for the entire transport column. Therefore, the bed-interaction factor :math:`\zeta` [:math:`\mathrm{-}`] is essentially assumed to be 1, meaning any reduction in shear stress due to vegetation immediately forces the entire sediment flux to deposit.
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**Method:** ``grass``
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**Method** ``grass``
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To capture realistic "skimming" flows over dense grass canopies, the new framework divides the saturation concentration :math:`c_{\text{sat}}` into two distinct modes (:ref:`fig-vegetation-sediment-transport`):
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Vegetation Mortality
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^^^^^^^^^^^^^^^^^^^^^^^^
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**Method:** ``duran``
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**Method** ``duran``
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Vegetation is subject to destruction caused by hydrodynamic processes. In the event of cell inundation by high water levels, the vegetation density :math:`\rho_{\text{veg}}` in the affected grid cells is instantaneously or proportionally reduced to mimic storm-induced erosion of the canopy.
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**Method:** ``grass``
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**Method** ``grass``
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Because tiller height and tiller density are fundamentally coupled, changes in one necessitate updates in the other. Mortality and structural changes occur through three primary drivers:
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