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: 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	total surface runoff + interflow runoff is calculated in the infiltration vs runoff step 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) Aquifer storage (state) IZ properties: IZ storage threshold for outflow lateral outflow rate constant vertical outflow rate constant (GW recharge) Aquifer properties: Aquifer storage capacity	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 flow and groundwater outflow to channel sections
Function type	non-linear & threshold	non-linear & threshold	fraction & 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 Max infiltration rate	yes: Field capacity SM IZ storage threshold for outflow Aquifer storage capacity