Process representation across tools

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building process representation overview tables + links to more specific pages about individual processes

Canopy interception and evaporation (vs throughfall)

Algorithm
description
WRSM-Pitman SPATSIM-Pitman ACRU4 SWAT2012 MIKE-SHE,
semi-distributed &
fully distributed

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


  • Rain
  • Cover properties:
    • max interception



  • Rain
  • Cover properties:
    • max interception



  • Rain
  • PET
  • Canopy storage (state)
  • Cover properties:
    • max interception (OR LAI)



Canopy interception is not explicitly modelled when using daily timestep modelling.
It is implicitly considered in the ‘initial abstraction’, see infiltration & surface runoff below


  • Rain
  • PET
  • Canopy storage (state)
  • Cover properties:
    • LAI
    • canopy interception coefficient


Function
type
exponential & threshold exponential & threshold threshold threshold
Thresholds

yes:
timestep interception capacity (calculated)

yes:
timestep interception capacity (calculated)

yes:
timestep interception capacity (calculated)

yes:
timestep interception capacity (calculated)


Infiltration into soil moisture (vs surface runoff or surface ponding)

Note: This excludes the case of a modelling unit (HRU, grid cell, area within a subcatchment, etc.) is designated as impervious. In this case, rain inputs would become surface runoff, potentially with a portion staying behind as surface storage/ponding if the area has attenuation specified (to represent roughness and flatness).

Algorithm
description
WRSM-Pitman SPATSIM-Pitman ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


  • Rain
  • Rainfall distribution factor
  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Infiltration rate distribution (min, max)



  • Rain
  • Rainfall distribution factor
  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Infiltration rate distribution (min, max)



  • Throughfall (rain-interception)
  • Soil moisture within "infiltration depth"(state)
  • Soil properties:
    • Saturation soil moisture
    • "Infiltration depth" parameter
    • Macropore by-pass (clay cracking)



  • Throughfall (rain-interception)
  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Curve number (SCS-CN)
    • Macropore by-pass



  • Throughfall (rain-interception)
  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Saturated hydraulic conductivity (Ksat),
    • Macropore by-pass



  • Throughfall (rain-interception)
  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Soil moisture retention curve
    • Saturated hydraulic conductivity (Ksat),
    • Macropore by-pass


Function
type
non-linear & threshold non-linear & threshold power & threshold power & threshold linear (rate) & threshold non-linear & threshold
Thresholds

yes:
Max infiltration rate,
Saturation soil moisture*
*Saturation excess becomes interflow, not surface runoff

yes:
Max infiltration rate,
Saturation soil moisture*
*Saturation excess becomes interflow, not surface runoff

yes:
Saturation soil moisture*
*Fraction of saturation excess becomes "delayed flow"~interflow, not "quickflow"~surface runoff

yes:
Max infiltration rate (calculated),
Saturation soil moisture

yes:
Max infiltration rate (uses Ksat),
Saturation soil moisture

yes:
Max infiltration rate (calculated),
Saturation soil moisture


Surface runoff to channel network

Note on surface runoff vs ponding and surface storage: In the Pitman models, all water not calculated to infiltrate in a timestep, which is a month, is assumed to be runoff. For models with shorter timesteps, water reaching the land surface which does not infiltrate into soil in a timestep can become surface runoff or remain as surface ponding/surface storage. In subsequent model timesteps, water still on the land surface can evaporate, infiltrate, and/or become surface runoff in the next timestep. Some models have relatively short timesteps (subdaily, daily) compared to the rate at which water would move all the way across a modelled land unit (which could be large, rough, and/or flat, slowing the flow rate). This is why some water will be 'surface storage' in one timestep and then 'surface runoff' in the next timestep. MIKE-SHE can also consider that land surfaces can be very rough, or have dips that trap water, and so some amount surface water will not be able to run off at all (detention storage). This water will both evaporate and infiltrate over time.
ACRU4 differs notably, assuming the water that does not infiltrate into soil accounts for both surface runoff and interflow runoff.

Algorithm
description
WRSM-Pitman SPATSIM-Pitman ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


  • Surface water present* (state)


*Month timestep: all water not infiltrating becomes runoff


  • Surface water present* (state)


*Month timestep: all water not infiltrating becomes runoff


  • Surface water present* (state)
  • Lag coefficient*


*Lag coefficient separates "quickflow"~surface runoff from "delayed-flow"~interflow. Quickflow portion reaches the channel on the same day generated (rain day). The rest is lagged over subsequent days


  • Surface water present* (state)
  • Surface path properties:
    • Path length
    • Path slope
    • Path roughness (Manning's n)



  • Surface water present* (state)
  • Surface path properties:
    • Detention storage
    • Path length
    • Path slope
    • Path roughness (Manning's n)



  • Surface water present* (state)
  • Surface path properties:
    • Detention storage
    • Path length (from gridded topography)
    • Path slope(from gridded topography)
    • Path roughness (Manning's n)


Function
type
(no transformation) (no transformation) (no transformation) non-linear non-linear & threshold non-linear & threshold
Thresholds

no*
*none after infiltration has been calculated

no*
*none after infiltration has been calculated

no*
*none after infiltration has been calculated

no*
*none after infiltration has been calculated

yes:
Detention storage

yes:
Detention storage


Evapotranspiration (ET) from soil moisture (SM)

Note: More coverage of evapotranspiration related terminology and inputs across different tools can be found here

Algorithm
description
WRSM-Pitman SPATSIM-Pitman ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


  • S-Pan evaporation
  • Soil moisture (state)
  • Vegetation properties:
    • "Crop" coefficient (vs pan)
    • Pitman ET coefficient*
  • Soil properties:
    • Saturation soil moisture

*determines linear decline of ET with soil moisture decline


  • S-Pan evaporation
  • Soil moisture (state)
  • Vegetation properties:
    • "Crop" coefficient (vs pan)
    • Pitman ET coefficient*
  • Soil properties:
    • Saturation soil moisture

*determines linear decline of ET with soil moisture decline


  • A-Pan evaporation
  • Soil moisture (state)
  • Vegetation properties:
    • "Crop" coefficient (vs pan)
    • Root depth distribution
  • Soil properties:
    • Saturation soil moisture*
    • Field capacity SM
    • Wilting point SM

*ET assumed to also decline if soil gets close to saturation, waterlogging, unless wetland plants


  • Remaining ET demand (PET - Canopy evap.)
  • Soil moisture (state)
  • Vegetation properties:
    • LAI
    • Root depth (max)
    • Root distribution
    • Demand redistribution coefficient
  • Soil properties:
    • Field capacity SM
    • Wilting point SM



  • Remaining ET demand (PET - Canopy evap. - Ponded evap.)
  • Soil moisture (state)
  • Vegetation properties:
    • "Crop" coefficient
    • ET curtailment SM
    • Root depth (max)
    • Root distribution
  • Soil properties:
    • Field capacity SM
    • Wilting point SM



  • Remaining ET demand (PET - Canopy evap. - Ponded evap.)
  • Soil moisture (state)
  • Vegetation properties:
    • "Crop" coefficient
    • ET vs SM curve parameter
    • Root depth (max)
    • Root distribution
    • ET demand depth distribution
  • Soil properties:
    • Soil moisture retention curve


Function
type
linear & threshold linear & threshold multi-part linear & threshold non-linear & threshold multi-part linear & threshold non-linear & threshold
Thresholds

yes:
ET demand,
Wilting point SM

yes:
ET demand,
Wilting point SM

yes:
ET demand,
Wilting point SM

yes:
ET demand,
Wilting point SM

yes:
ET demand,
Wilting point SM

yes:
ET demand,
Wilting point SM


Evapotranspiration (ET) from groundwater (GW)

Note: More coverage of evapotranspiration related terminology and inputs across different tools can be found here

Algorithm
description
WRSM-Pitman
(Sami GW)
SPATSIM-Pitman
(Hughes GW)
ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


riparian area only

  • A-Pan evaporation
  • Riparian area
  • Aquifer storage (state)
  • Vegetation properties:
    • "Crop" coefficient (vs pan)
  • Aquifer properties:
    • Aquifer storage limit for ET



riparian area only

  • Remaining ET demand (PET - ET from soil)
  • Riparian area
  • Aquifer storage (state)
  • Vegetation properties:
    • "Crop" coefficient (vs pan)
    • Pitman ET coefficient*
  • Aquifer properties:
    • Aquifer storage limit for ET



riparian area only

Indirect representation:
Specified riparian zone HRU can receive a user-specified fraction of the aquifer outflow from connected upslope HRUs. This water is added to riparian HRU soil in the B-horizon, where it can be accessed for ET and can recharge GW.

No ET (or capillary rise) is simulated from the GW store of the riparian HRU.


represented through capillary rise into the soil driven by ET demand

  • PET
  • Aquifer storage (state)
  • Soil properties:
    • Capillary rise rate coefficient
  • Aquifer properties:
    • Aquifer storage limit for capillary rise

+ all inputs for ET from soil


riparian area only
represented through capillary rise into the soil driven by ET demand

  • Remaining ET demand (PET - Canopy evap. - Ponded evap. - ET from soil)
  • Aquifer storage (state)
  • Aquifer properties:
    • Aquifer storage limit for capillary rise

+ all inputs for ET from soil


capillary rise is calculated, but roots can also extend below the water table and withdraw directly

  • Remaining ET demand (PET - Canopy evap. - Ponded evap. - ET from soil)
  • Water table depth (state)
  • Aquifer properties:
    • Saturated hydraulic conductivity (Ksat)

+ vegetation property inputs for ET from soil

Function
type
non-linear & threshold non-linear & threshold non-linear & threshold non-linear & threshold non-linear & threshold
Thresholds

yes:
ET demand,
Aquifer storage limit for ET

yes:
ET demand,
Aquifer storage limit for ET

yes:
ET demand,
Aquifer storage limit for capillary rise

yes:
ET demand,
Aquifer storage limit for capillary rise

yes:
ET demand,
Water table depth vs root depth


Interflow generation & routing to channel network

Algorithm
description
WRSM-Pitman SPATSIM-Pitman ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


Interflow from soil layer & from "percolation zone"

  • Soil moisture (state)
  • Percolation zone storage (state)
  • Aquifer storage (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Interflow rate power coefficient
    • Max interflow rate
    • Interflow lag coefficient
  • Aquifer properties
    • Percolation zone max capacity
    • Aquifer max capacity



Interflow from soil layer

  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Interflow rate power coefficient
    • Max interflow rate
    • Interflow lag coefficient



Interflow amount calculated in the same step as surface runoff, not as a separate process:
total surface runoff + interflow runoff is calculated in the infiltration vs runoff step, then these are separated through lagging

  • Surface water present* (state)
  • Lag coefficient*

*Lag coefficient separates "quickflow"~surface runoff from "delayed-flow"~interflow. The lagged flow that doesn't reach the channel on the rain day, but instead reaches the channel over subsequent days can be considered interflow


Interflow from soil layer

  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Drainage slope
    • Saturated hydraulic conductivity (Ksat)



Interflow from "interflow zone (IZ)"
to calc input into IZ:

  • Macropore flow (Throughfall*by-pass coefficient)
  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Drainage slope
    • Saturated hydraulic conductivity (Ksat)

to calc outflow from IZ:

  • IZ storage (state)
  • IZ properties:
    • IZ storage threshold for outflow
    • lateral outflow rate constant



Interflow not modelled as a separate process to "saturated zone" flow.
Lateral flow through a "saturated zone" material layer in the model which is not frequently saturated and/or is perched above more permanently saturated layers can be considered as interflow. Flows through different layers can be exported as separate model outputs.

See groundwater outflow to channel

Function
type
non-linear & threshold non-linear & threshold non-linear & threshold two step: non-linear & threshold, linear reservoir & threshold
Thresholds

yes:
Field capacity SM,
Percolation zone max storage + Aquifer max storage

yes:
Field capacity SM

yes:
Saturation SM

yes:
Field capacity SM
IZ storage threshold for outflow


Aquifer recharge

Algorithm
description
WRSM-Pitman
(Sami GW)
SPATSIM-Pitman
(Hughes GW)
ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


Recharge via "percolation zone" (vadose zone)

  • Soil moisture (state)
  • Vadose zone storage (state)
  • Aquifer storage (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Percolation power coefficient
    • Max percolation rate
  • Vadose zone properties:
    • Recharge lag coefficient
    • Vadose zone max storage
  • Aquifer properties:
    • Aquifer max storage



Recharge from soil column

  • Soil moisture (state)
  • Aquifer storage (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Percolation power coefficient
    • Max percolation rate
  • Aquifer properties:
    • Aquifer max storage



Recharge from soil column*

  • Soil moisture (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Percolation rate (~Ksat)


*Aquifer storage is unlimited


Recharge via vadose zone

  • Soil moisture (state)
  • Vadose zone storage (state)
  • Aquifer storage (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Saturated hydraulic conductivity (Ksat)
  • Vadose zone (VZ) properties:
    • VZ field capacity
    • VZ saturated hydraulic conductivity (Ksat)
  • Aquifer properties:
    • Aquifer max storage



Recharge via "interflow zone"

  • Soil moisture (state)
  • Interflow zone storage (state)
  • Aquifer storage (state)
  • Soil properties:
    • Saturation soil moisture
    • Field capacity SM
    • Saturated hydraulic conductivity (Ksat)
  • Interflow zone (IZ) properties:
    • IZ storage threshold for outflow
    • vertical outflow rate constant
  • Aquifer properties:
    • Aquifer max storage



Recharge from soil column

  • Soil moisture (state)
  • Aquifer storage (state)
  • Soil properties:
    • Saturation soil moisture
    • Soil moisture retention curve
    • Saturated hydraulic conductivity (Ksat)
  • Aquifer properties:
    • Aquifer max storage
    • Aquifer conductivity (Ksat)


Function
type
two step: non-linear & threshold, non-linear non-linear & threshold non-linear & threshold two step: non-linear & threshold, non-linear & threshold two step: non-linear & threshold, linear reservoir & threshold non-linear & threshold
Thresholds

yes:
Field capacity SM,
Vadose zone max storage
Aquifer max storage

yes:
Field capacity SM,
Aquifer max storage

yes:
Field capacity SM

*Aquifer storage is unlimited

yes:
Field capacity SM,
Vadose zone field capacity
Aquifer max storage

yes:
Field capacity SM,
Aquifer max storage

yes:
Field capacity SM,
Aquifer max storage


Groundwater (GW) flow: lateral flow within the saturated zone

Note: This refers to groundwater flow between modelled land units, such as grid cells, HRUs, or subcatchments, depending on the scale that groundwater stores and flows are modelled at (see more on scales of process representation for GW here)
For coverage of groundwater outflow into a river channel, see the table below

Algorithm
description
WRSM-Pitman
(Sami GW)
SPATSIM-Pitman
(Hughes GW)
ACRU4 SWAT2012 MIKE-SHE,
semi-distributed,
more conceptual
MIKE-SHE,
fully-distributed,
more physical

Algorithm
inputs

(input data,
parameters,
model-calculated
states/storages)


GW flow between subcatchments

  • Aquifer storage (state)
  • Aquifer properties:
    • Aquifer transmissivity
    • Aquifer storage threshold for flow
    • Regional groundwater slope



GW flow between subcatchments

  • Aquifer storage (state)
  • Aquifer properties:
    • Aquifer transmissivity
    • Aquifer storage threshold for flow
    • Regional groundwater slope
    • Aquifer max storage



GW flow between HRU aquifers is not modelled

Special case, partial exception: upland HRU aquifer outflow routed to riparian HRU soil


GW flow between subcatchment aquifers is not modelled


GW flow between aquifer storage units is not modelled


GW flow between grid cells (Darcy's Law)

  • Aquifer level gradient (state - compare levels in neighboring cells)
  • Aquifer properties:
    • Aquifer specific yield
    • Aquifer saturated conductivity (Ksat)


Function
type
non-linear & threshold non-linear & threshold non-linear
Thresholds

yes:
Aquifer storage threshold for flow

yes:
Aquifer storage threshold for flow

n/a


Aquifer exchanges with channel: aquifer outflow to channel and channel transmission loss