
class
BasicRtSa
¶
Model BasicRtSa¶
terrainbento BasicRt model program.
Erosion model program using depthdependent linear diffusion, soil production by exponential weathering, stream power with spatially varying erodibility based on two bedrock units, and discharge proportional to drainage area.
 Landlab components used:

class
BasicRtSa
(clock, grid, soil_production__maximum_rate=0.001, soil_production__decay_depth=0.5, soil_transport_decay_depth=0.5, **kwargs)[source]¶ Bases:
terrainbento.base_class.two_lithology_erosion_model.TwoLithologyErosionModel
BasicRtSa model program.
This model program combines the
BasicRt
andBasicSa
programs by allowing for two lithologies, an “upper” layer and a “lower” layer and explicitly resolving a soil layer. This soil layer is produced by weathering that decays exponentially with soil thickness and hillslope transport is soildepth dependent. Given a spatially varying contact zone elevation, \(\eta_C(x,y))\), a spatially varying soil thickness \(H\) and a spatially varying bedrock elevation \(\eta_b\), model BasicRtSa evolves a topographic surface described by \(\eta\) with the following governing equations:\[ \begin{align}\begin{aligned}\eta = \eta_b + H\\\frac{\partial H}{\partial t} = P_0 \exp (H/H_s)  \delta (H) K Q^{m} S^{n}  \nabla q_h\\\frac{\partial \eta_b}{\partial t} = P_0 \exp (H/H_s)  (1  \delta (H) ) K Q^{m} S^{n}\\q_h = D H^* \left[1\exp \left( \frac{H}{H_0} \right) \right] \nabla \eta\\K(\eta, \eta_C ) = w K_1 + (1  w) K_2\\w = \frac{1}{1+\exp \left( \frac{(\eta \eta_C )}{W_c}\right)}\end{aligned}\end{align} \]where \(Q\) is the local stream discharge, \(S\) is the local slope, \(m\) and \(n\) are the discharge and slope exponent parameters, \(W_c\) is the contactzone width, \(K_1\) and \(K_2\) are the erodabilities of the upper and lower lithologies, and \(D\) is the regolith transport parameter. \(w\) is a weight used to calculate the effective erodibility \(K(\eta, \eta_C)\) based on the depth to the contact zone and the width of the contact zone. \(H_s\) is the sediment production decay depth, \(H_0\) is the sediment transport decay depth, \(P_0\) is the maximum sediment production rate, and \(H_0\) is the sediment transport decay depth. \(q_h\) is the hillslope sediment flux per unit width.
The function \(\delta (H)\) is used to indicate that water erosion will act on soil where it exists, and on the underlying lithology where soil is absent. To achieve this, \(\delta (H)\) is defined to equal 1 when \(H > 0\) (meaning soil is present), and 0 if \(H = 0\) (meaning the underlying parent material is exposed).
The weight \(w\) promotes smoothness in the solution of erodibility at a given point. When the surface elevation is at the contact elevation, the erodibility is the average of \(K_1\) and \(K_2\); above and below the contact, the erodibility approaches the value of \(K_1\) and \(K_2\) at a rate related to the contact zone width. Thus, to make a very sharp transition, use a small value for the contact zone width.
Refer to Barnhart et al. (2019) Table 5 for full list of parameter symbols, names, and dimensions.
 The following atnode fields must be specified in the grid:
topographic__elevation
lithology_contact__elevation
soil__depth

__init__
(clock, grid, soil_production__maximum_rate=0.001, soil_production__decay_depth=0.5, soil_transport_decay_depth=0.5, **kwargs)[source]¶  Parameters
clock (terrainbento Clock instance) –
grid (landlab model grid instance) – The grid must have all required fields.
m_sp (float, optional) – Drainage area exponent (\(m\)). Default is 0.5.
n_sp (float, optional) – Slope exponent (\(n\)). Default is 1.0.
water_erodibility_upper (float, optional) – Water erodibility of the upper layer (\(K_{1}\)). Default is 0.001.
water_erodibility_lower (float, optional) – Water erodibility of the upper layer (\(K_{2}\)). Default is 0.0001.
contact_zone__width (float, optional) – Thickness of the contact zone (\(W_c\)). Default is 1.
regolith_transport_parameter (float, optional) – Regolith transport efficiency (\(D\)). Default is 0.1.
soil_production__maximum_rate (float, optional) – Maximum rate of soil production (\(P_{0}\)). Default is 0.001.
soil_production__decay_depth (float, optional) – Decay depth for soil production (\(H_{s}\)). Default is 0.5.
soil_transport_decay_depth (float, optional) – Decay depth for soil transport (\(H_{0}\)). Default is 0.5.
**kwargs – Keyword arguments to pass to
TwoLithologyErosionModel
. Importantly these arguments specify the precipitator and the runoff generator that control the generation of surface water discharge (\(Q\)).
 Returns
BasicRtSa
 Return type
model object
Examples
This is a minimal example to demonstrate how to construct an instance of model BasicRtSa. For more detailed examples, including steadystate test examples, see the terrainbento tutorials.
To begin, import the model class.
>>> from landlab import RasterModelGrid >>> from landlab.values import random, constant >>> from terrainbento import Clock, BasicRtSa >>> clock = Clock(start=0, stop=100, step=1) >>> grid = RasterModelGrid((5,5)) >>> _ = random(grid, "topographic__elevation") >>> _ = random(grid, "soil__depth") >>> _ = constant(grid, "lithology_contact__elevation", value=10.)
Construct the model.
>>> model = BasicRtSa(clock, grid)
Running the model with
model.run()
would create output, so here we will just run it one step.>>> model.run_one_step(1.) >>> model.model_time 1.0

run_one_step
(step)[source]¶ Advance model BasicRtSa for one timestep of duration step.
The run_one_step method does the following:
Creates rain and runoff, then directs and accumulates flow.
Assesses the location, if any, of flooded nodes where erosion should not occur.
Assesses if a
PrecipChanger
is an active boundary handler and if so, uses it to modify the erodibility by water.Updates the spatially variable erodibility value based on the relative distance between the topographic surface and the lithology contact.
Calculates detachmentlimited erosion by water.
Calculates topographic change by linear diffusion.
Finalizes the step using the
ErosionModel
base class function finalize__run_one_step. This function updates all boundary handlers handlers bystep
and increments model time bystep
.
 Parameters
step (float) – Increment of time for which the model is run.