Water balance outputs across tools

Modelling tools differ in the hydrological processes, or water balance elements (fluxes and storages), that they calculate individually, save, and allow users to export for further analyses. The table below covers what each software tool outputs, meaning what can be saved as a timeseries of flow rates, fluxes, or storage volumes or depths, and for what spatial scale or modelled units these can be output (e.g. output for each grid cell, HRU, subcatchment, or catchment total). Output file formats are covered here.

The spatial scales at which the software can output values gives an indication of how much external calculation work a user may need to do in order to find out about a flux or store of particular interest. For example, we often wish to know the ET water use of a particular vegetation type, likely to compare with another type. ACRU and SWAT will output AET components for individual HRUs, however if there are multiple HRUs of the same land cover type within the model type, these tools will not compile an area-weighted average for the cover type across the whole modelled area. The user would need to do that manually outside the tool using the individual HRU outputs. MIKE-SHE can do this compilation within the software as the user can specify a polygon that covers the full area of a cover class and the tool will compile fluxes for that polygon.

Note: The table differentiates between the case in which a tool does not output its calculation of a certain flux or storage (Output = "no") and the case in which the tool does not explicitly model the listed process ("not modelled"), meaning it's not part of the model's calculations. In the case when a tool does model a process, but does not output the results of this calculation, the user could potentially consult the tools' user guide and attempt to calculate the process themselves using the algorithm that is applied in the software and other components that can be output. In cases where a calculation would be relatively simple, and based only on other outputs covered in the table, the calculation approach has been given in the table.
For cases where the tool does output a flux but uses a different name for it that has been used in the table row heading, this has been noted in brackets below. For more terminology comparison across tools, see further coverage here.
Some of the fluxes listed in the table are net totals of component fluxes which are also listed in the table (e.g. total AET or total evaporation & transpiration = canopy interception evaporation + AET from soil + AET from GW + evaporation off water bodies, where applicable). Some tools model and output each separately and may or may not output each individually. Some provide outputs of the total and the components, some just the components and the user would need to sum them if the total was of interest. Some calculate a total of some or all in a single step and output the total.

Flux or storage		Tool output and/or calculation approach
		WRSM-Pitman (Sami GW)	SPATSIM-Pitman (Hughes GW)	ACRU4	SWAT2012	MIKE-SHE, semi-distributed, more conceptual	MIKE-SHE, fully-distributed, more physical
AET total	Output	no * calculate: precip – runoff – Δ soil storage – Δ GW storage (works for long-term average, not monthly due to recharge lag)	no * calculate: ET output (includes canopy interception) + AET from GW (external calculation)	no * calculate: canopy evap. + AET from soil	no * calculate: AET from soil + AET from GW	YES	YES
	Scale	(runoff module)	(subcatchment)	(HRU)	(HRU)	grid cell, polygon, catchment	grid cell, polygon, catchment
Canopy interception evaporation	Output	no	YES	YES	not modelled	YES	YES
	Scale		subcatchment	HRU		grid cell, polygon, catchment	grid cell, polygon, catchment
AET from soil	Output	no	YES	YES	YES	YES	YES
	Scale		subcatchment	HRU	HRU, subcatchment, catchment	grid cell, polygon, catchment	grid cell, polygon, catchment
AET from GW	Output	no	no	not modelled (exception: riparian HRU approximation)	YES (‘Revap’)	YES	YES
	Scale				HRU	subcatchment aquifer, catchment	grid cell, polygon, catchment, (aquifer layer or all)
Evaporation from surface water body	Output	no	YES	YES	YES	YES	YES
	Scale		water body	water body	water body (includes river reaches)	water body (includes river reaches), surface ponding: grid cell, polygon, catchment	water body (includes river reaches), surface ponding: grid cell, polygon, catchment
Runoff (RO) total	Output	YES	YES	YES	YES	YES	YES
	Scale	runoff module, route (any scale)	subcatchment	HRU, subcatchment, catchment	HRU, subcatchment, catchment	river cross section	river cross section
Surface runoff	Output	no * calculate: RO total – interflow – aquifer RO	YES	no * calculate: “Quickflow” – “DelayedStormflow”	YES	YES	YES
	Scale	runoff module	subcatchment	HRU	HRU, subcatchment, catchment	grid cell, polygon, catchment	grid cell, polygon, catchment
Interflow	Output	YES	YES (‘soil moisture runoff’)	YES (‘DelayedStormflow’)	YES (‘Lateral Q’)	YES	YES (saturated zone, upper layer)
	Scale	runoff module	subcatchment	HRU	HRU, subcatchment, catchment	subcatchment interflow reservoir	grid cell, polygon, catchment
Aquifer to channel	Output	YES	YES	YES	YES	YES	YES
	Scale	runoff module	subcatchment	HRU, subcatchment	HRU, subcatchment, catchment	subcatchment aquifer, catchment	grid cell, polygon, catchment, (aquifer layer or all)
Channel to aquifer	Output	YES	YES	not modelled	no	YES	YES
	Scale	river module	subcatchment		channel to "bank store", not output, need to back-calculate for reach	river reach	river reach; grid cell, polygon, catchment (aquifer layer)
Aquifer GW flow, into or out of a model unit, remaining as GW	Output	no	YES	not modelled	no * GW can ‘leave’ catchment by recharging an inactive deep aquifer	no * GW can ‘leave’ catchment by recharging an inactive deep aquifer or ‘dead storage’	YES
	Scale		subcatchment		(subcatchment)	(subcatchment)	grid cell, polygon, catchment, (aquifer layer or all)
Water body storage	Output	YES	YES	YES	YES	YES	YES
	Scale		water body	water body	water body	water body, surface ponding: grid cell, polygon, catchment	water body, surface ponding: grid cell, polygon, catchment
Soil profile storage	Output	YES	YES	YES	YES	YES	YES
	Scale	runoff module	subcatchment	soil layer in HRU	HRU, subcatchment, catchment	grid cell, polygon, catchment	grid cell, polygon, catchment
Aquifer storage	Output	YES	no	YES	YES	YES	YES
	Scale	runoff module		HRU, subcatchment	HRU, subcatchment	subcatchment aquifer, catchment	grid cell, polygon, catchment, (aquifer layer or all)
Percolation out of soil storage	Output	YES (‘total recharge’ - lagged to aquifer, excess becomes interflow)	YES (same as aquifer recharge below)	YES (same as aquifer recharge below)	YES (lagged to aquifer)	YES (goes to 'interflow store')	YES (same as aquifer recharge below)
	Scale	runoff module	subcatchment	HRU	HRU, subcatchment	grid cell, polygon, catchment	grid cell, polygon, catchment
Aquifer recharge	Output	YES (‘aquifer recharge’)	YES	YES (B-horizon soil ‘SaturatedFlow’ + ‘UnsaturatedFlow’)	YES	YES	YES
	Scale	runoff module	subcatchment	HRU, subcatchment	HRU, subcatchment	subcatchment aquifer, catchment	grid cell, polygon, catchment, (aquifer layer or all)