Surface flow evaluators#

Assorted evaluators used for surface flow, including potential surfaces, Manning’s conductivity, and their frozen equivalents.

Like the subsurface flow evaluators, many of these evaluators show up in nearly all ATS simulations. For real examples, see ats-demos

Ponded Depth or Water Height#

Computes ponded depth from surface water pressure.

\[h = \frac{H(p - p_{atm})}{\rho g}\]

where \(H\) is the Heaviside function.

“evaluator type” = “ponded depth”

height-evaluator-spec

KEYS:

  • “mass density”

  • “pressure”

Ponded Depth, Frozen#

Effective, or Smoothed Height#

Computes ponded depth from surface water pressure using a smoothed term to make derivative smooth near 0. This is pretty much never used anymore.

“evaluator type” = “effective height”

effective-height-evaluator-spec

  • “smoothing width [m]” [double] 0.01 the width over which smoothing is applied.

KEYS:

  • “height” The unsmoothed ponded depth

Unfrozen fraction#

An empirical equation for freezing ponded water – this is simply a smooth sinusoidal curve from 0 to 1 over a given transition in temperature.

“evaluator type” = “unfrozen fraction”

unfrozen-fraction-evaluator-spec

  • “unfrozen fraction model” [unfrozen-fraction-model-spec]

KEYS:

  • “temperature”

unfrozen-fraction-model-spec

  • “transition width [K]” [double] 0.2 Degrees over which to transition from no ice to all ice.

  • “freezing point [K]” [double] 273.15 Center of the transition, at this point unfrozen fraction is 0.5.

  • “minimum unfrozen fraction [-]” [double] 0 Sets a minimum value.

Unfrozen Flowing Depth#

Evaluates the unfrozen mobile depth.

In freezing conditions, water is only mobile if it is unfrozen. This evaluator determines how much water is allowed to flow given that it is partially frozen.

Given a ponded depth, an unfrozen fraction, and an optional power-law exponent, which we call the ice retardation exponent.

unfrozen-effective-depth-evaluator-spec

  • “ice retardation exponent [-]” [double] 1.0 exponent alpha controlling how quickly ice turns off flow.

DEPENDENCIES: - “depth” DOMAIN-depth - “unfrozen fraction” DOMAIN-unfrozen_fraction

SurfacePotential#

Evaluates the potential surface upon which overland flow acts.

\[h + z\]

“evaluator type” =

pres-elev-evaluator-spec

KEYS:

  • “height” DOMAIN-ponded_depth Names the height variable. [m]

  • “elevation” DOMAIN-elevation Names the elevation variable. [m]

NOTE: This is a legacy evaluator, and is not in the factory, so need not be in the input spec. However, we include it here because this could easily be abstracted for new potential surfaces, kinematic wave, etc, at which point it would need to be added to the factory and the input spec.

NOTE: This could easily be replaced by a generic Additive_ Evaluator.

Overland Conductivity, sheet flow#

This implements the conductivity of overland flow, which is the nonlinear coefficient in the diffusion wave equation. The term is given by:

Optionally, this may include a density factor, typically a molar density, which converts the flow law to water flux rather than volumetric flux.

Also, this evaluator can be used in snow redistribution, and in that case needs some extra factors (timestep size) to ensure the correct flow law in that case.

“evaluator type” = “overland conductivity”

overland-conductivity-evaluator-spec

  • “include density” [bool] true Include the density prefactor, converting the flux from volumetric flux to water flux.

  • “dt factor [s]” [double] -1 The artificial timestep size used in calculating

    snow redistribution, only used in that case.

  • “swe density factor [-]” [double] 10 Ratio of water to snow density.

DEPENDENCIES:

  • “mobile depth” DOMAIN-mobile_depth Depth of the mobile water; delta in the above equation.

  • “slope” DOMAIN-slope_magnitude Magnitude of the bed surface driving flow; | nabla z | above.

  • “coefficient” DOMAIN-manning_coefficient Surface roughness/shape coefficient; n_{mann} above.

  • “molar density liquid” DOMAIN-molar_density_liquid If “include density” is true, the density.

Overland Conductivity, litter resistance#

(missing documentation!)