Terminology

Modelling terminology

Various terms related to hydrological modelling are used in different ways across different contexts. The table below provides definitions for basic modelling terminology that is used in this wiki.

Even the word "model" is used to refer to a range of different things. "Model" is often used to refer to a modelling software tool, e.g., it is normal to read "the ACRU model" referring to ACRU modelling software. It is also common to read “an ACRU model” referring to a model of a given catchment built using ACRU. ACRU modelling software does enforce certain ways of representing/calculating the hydrological processes in a catchment. In this way the ACRU modelling software does constitute a “model" of how catchments work. However, there are many things that can vary across different models that have all been built using ACRU, even when these are built to represent the same catchment. Different "ACRU models" of ‘catchment A’ could have different numbers of subcatchments, river elements, and separately represented land cover types; different linkages between parts of the landscape; and different parameter values. The same can be said of most modelling tools. Calling a particular model set-up for a catchment “an ACRU model" does say some things about the model structure, but it doesn't clarify many critical elements. A model built in SWAT and a model built in ACRU of the same catchment, could actually have very similar structures to one another, or they could be vastly different. Their differences may be more due to the set-up choices of the users than due to the differences across the software tools!

For this reason it can help to differentiate "a model" and "a modelling tool". Here an effort is made to use "model" to refer to a specific model set-up for a catchment, including its structure and parameter values, and use "modelling software tool" ("modelling software", "modelling tool", or "tool" for short) for software programmes that can be used to design and run catchment models. Each software tool comes with its own set of structural and algorithm options and choices within this set would have been made to build a “model" using that tool.


Term	Applied defintion
Model	Broadly: A physical object, a diagram, or a set of equations that provides a simplified representation of a more complex or larger object or system. Used here as ‘short form’ for ‘hydrological model’ - see definition below
Hydrological model	A model that describes the flow of water through an area of land to output a prediction of its water balance. It is a structured set of equations and logic statements (collectively referred to as algorithms) along with parameter and input variable values. Given precipitation, other climate variables, and parameters describing physical processes and properties, the algorithms produce estimates of how much of the precipitation will be stored in the modelled area, leave as evapotranspiration (ET), or leave as surface or subsurface outflow. A ‘hydrological’ model may or may not include a ‘hydraulic model’ (defined below). The area represented is typically a catchment, therefore "catchment hydrological model" is implied. (If the modelled area is not a full catchment, additional surface and subsurface flows at its boundaries need to be specified.) The term ‘model’ will be used to refer to the complete package required to produce the output, i.e. BOTH the ‘model structure’ and the ‘parameter values’. NB: Elsewhere "a model" often refers to "a model structure" or "a modelling software tool".
Hydraulic model	A model that describes surface flow of water across a specified area. This most often a channel network and adjacent floodplain. Given the flow entering the area, various system properties (channel size, roughness, slope), and algorithms representing an understanding of physics (laws of energy, mass, momentum), a hydraulic models outputs the water surface elevation, velocity, and flow rate for specified calculation points. Hydraulic models do not calculate the quantity of water entering the channel network. Input flows at boundaries must be measured, calculated by a hydrological model, or otherwise estimated/assumed.
Conceptual model (Perceptual model)	A representation of how a person or group understands the flow of water through a catchment, typically in the form of diagrams, flow charts, and text. This consists of how people decide to divide the catchment into different spatial and vertical units to be considered separately, and a description of the perceived processes, flows, and connections within and between these units. Also referred to as a 'perceptual model'. NB: The term "conceptual model" also commonly refers to a numerical model (defined below) with algorithms that are considered more 'conceptual' vs. 'physical', in that their parameter values are not individual physically measurable properties. It will generally not be used this way here unless specifically clarified.
Numerical model	Used here as ‘short form’ for ‘numerical catchment hydrological model.’ A set of mathematical equations and logic statements used to quantitatively describe the processes and connections in a conceptual model of catchment. When applied to the required numerical inputs, it produces quantitative predictions of flows.
Algorithm	A step-by-step set of operations used to obtain an output from certain inputs. This can be an ordered set of equations and/or logic statements and can diverge into branches. Numerical models are examples of complex algorithms. They are generally combinations of many internal, individually-described algorithms that predict the occurrence and output of different particular hydrologic processes (e.g. infiltration of water into soil, percolation of soil water downward to the groundwater).
Model structure	The form of a numerical model: the specific way in which the land surface and subsurface is divided into different units and connected and the specific set of process algorithms that are applied within and between units. For example this includes whether a catchment being modelled is subdivided spatially into subcatchments, into hydrological response units, into grid cells, and how these different units are then linked together. It includes how the catchment is subdivided vertically into layers such as the vegetation canopy, the soil surface, layers of soil, sediment, and rock, and how these interact with one another in the model.
Parameter	Numeric values that form part of model algorithms and describe properties of a system, such as the porosity of soil, the gradient of a hillslope, the leaf area index (LAI) of vegetation. These properties are often assumed to be constant in the model, at least over a period of time or within a scenario. Some model structures allow some parameter values to change over time, such as a seasonal pattern of LAI values for a vegetation type. Despite potentially varying, parameters differ from “input variables” in that parameters are part of the definition of how an input and output variable relate, e.g. the LAI value is part of the equation that calculates how the rainfall input becomes the through-fall output, representing the process of canopy interception.
Input Variables	Numeric value inputs to model algorithms that are considered to be an inherently changing feature or condition of the system, such as daily precipitation, evaporative demand, irrigation application, water withdrawals.
Validation	Evaluation of the model to determine whether or not it is a sufficient representation of the system, the catchment’s hydrology, to be used for its desired purpose. This includes assessment of the inputs, structure, and outputs compared to our understanding of the system. Statistical tests can be applied to compare model outputs to field measurements for quantitative assessments of accuracy. Criteria and thresholds of model acceptance need to be defined by users. When the term "validation" is used in conjunction with "calibration" (defined below) it refers to model performance testing that is done for a different time period or set of inputs than those that were used in the calibration exercise.
Calibration	Adjustment of model parameter values to improve the accuracy of model outputs against user-defined measures of accuracy (e.g. goodness-of-fit statistics of model outputs to comparable field measurements or patterns). Parameter value options used in calibration are typically constrained to value ranges considered realistic given the physical meaning of the parameter and knowledge about physical properties of the system.
Modelling software tool	Computer software programme designed to help users to build and run numeric models. Different programmes encode different sets of algorithms and require users to input parameter values and input variables. Different programmes allow for different levels of spatial discretization of the catchment area and subsurface layering. Some include several different options for discretisation and options for the algorithms used for hydrologic processes. This means that even within a single modelling software programme, different model structures can be built to represent the same catchment based on user decisions. For this reason ‘modelling software’ will be differentiated from ‘a model’. (Also referred to here as: ‘modelling software’, ‘modelling tool’, ‘modelling programme’, ‘modelling platform’)
Model building	Deciding upon the model structure with spatial discretisation, process algorithms, parameter values, and input variable data to use to represent a specific catchment for a specific time period and operationalising the implementation of this to produce outputs, using existing modelling tools and associated software and code. (This is differentiated from designing and testing a more generic modelling software tool that allows users to build models of a variety of catchments - see Modelling tool development)
Modelling tool development	Creating a software programme or set of code that can be used to build and run models of variety of catchments given structural specifications, parameter values, and input data that can be given by a user.

Hydrological process and parameter terms across tools

Modelling software tools each have their own set of terminology in their user interfaces and documentation. It is important to check the meanings in the tool being used, and to not assume that meanings are exactly the same across different tools or as defined in other references:

Different words may be used for the same or very similar concepts across tools (e.g. “interflow” in WRSM-Pitman is “lateral flow” in SWAT).
The same term, or a very similar term, may be used in different tools to refer to different objects or concepts, although they're likely related (e.g. In SPATSIM, ”groundwater outflow” refers to groundwater flowing from one subcatchment to a neighboring subcatchment, remaining as groundwater. In SWAT texts, “groundwater flow” refers to groundwater flowing out of an aquifer into a river channel within a subcatchment).

Some terms for basic hydrological processes and properties for the focus tools are covered in the tables below. The tables highlight when terms are essentially equivalent or just closely related. The list is not exhaustive.

Spatial units

Spatial unit terms across tools
General term	Concept	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
equivalent terms are bold; similar / related terms are not bold & are starred; notes on term usage in tool texts & interface are given in italics*
Catchment (Cat)	All land area that drains to a specific point in the landscape (catchment outlet), often a point on a river or a water body. Assumed to be a surface flow catchment: topographically delineated by the direction of potential surface flow. Boundaries are ridge lines/highest points. Groundwater flow may or may not have the same bounds.	Catchment, Network*	Catchment	Catchment	Basin, Watershed	Catchment, Model domain*
Catchment (Cat)		WRSM models are networks of connected modules. The ‘network’ refers to a model’s extent, which could include multiple catchments				Model domain: full extent of the area modelled, not forced to follow topographic catchment boundaries
Subcatchment (Subcat)	Smaller catchment (topographically defined) within a larger catchment. When a catchment is delineated into subcats, there will be: headwater subcats: no subcats upstream, non-headwater subcats: have other subcats upstream. For a non-headwater subcat: accumulated subcat: all land draining to the subcat outlet point, includes all upstream subcats as well; incremental subcat: only the additional area draining to the subcat outlet point that is not included in upstream subcats. (will have one or more channel inflow points from upstream.)	Subcatchment, Runoff module*	Subcatchment	Subcatchment	Subbasin, Subwatershed	Subcatchment
Subcatchment (Subcat)		WRSM "runoff modules" function as subcats, BUT don’t include channels and special area types that are represented with separate modules. A collection of linked modules (e.g. runoff module + irrigation module + channel module) would together represent what would be a 'subcat' in another tool. Output from channel modules linking subcats will represent the accumulated subcat.	Subcat runoff is output for the incremental subcat. Streamflow is output for the accumulated subcat.	Subcat runoff is output for the incremental subcat. Streamflow is output for the accumulated subcat.	Subcat runoff is output for the incremental subcat. Streamflow is output for the accumulated subcat.	Subcat runoff is output for the incremental subcat. Streamflow is output for the accumulated subcat.
Hydrologic response unit (HRU)	Area with relatively homogenous hydrological processes in comparison to the rest of the landscape. Often a specific combination of land cover, soil type, and topographic position. For many models using HRUs, the areas included in a single HRU are not necessarily contiguous in the landscape, but are located within the same subcat.	Module*	(not used)	HRU	HRU	(not used)
Hydrologic response unit (HRU)		WRSM runoff modules & special land area modules function similarly to HRUs, but the process algorithms used across the different module types are more diverse vs. across HRUs in other tools.				MIKE allows landscape property parameters (e.g. vegetation properties, surface roughness properties, soil properties) to input for user-defined zones. These different zones do not need line up with one another.

Runoff & streamflow

Runoff & streamflow terms across tools
General term	Concept	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
equivalent terms are bold; similar / related terms are not bold & are starred; notes on term usage in tool texts & interface are given in italics*
Runoff	All water leaving the catchment landscape to become streamflow. "Runoff" can also be water leaving an incremental subcat, HRU, or other land unit to enter a downslope unit or the channel network. Includes both surface and subsurface flow contributions.	Runoff	Runoff	Runoff	Water yield	Tool & texts only refer to individual components (overland flow, interflow, baseflow) and streamflow
Surface runoff (SRO)	Water flowing on the land surface. In modelling: water leaving a subcat or HRU as surface flow and reaching the modelled channel network. Includes surface flow created by both saturation excess and infiltration rate excess.	Surface runoff	Surface runoff	‘Non-delayed’ stormflow* ACRU calculates total ”stormflow” (also called "quickflow") generated in a rain event. Some of this is lagged in reaching the channel (“Delayed stormflow”) to represent interflow .The same-day portion can be considered surface runoff.	Surface runoff	Overland flow
Streamflow (Q)	Water flowing in the channel network at a point (generally at a subcat outlet). Includes contributions of the incremental subcat, and all upstream subcats. If there are diversions, transfers, dams, and/or if channel bed losses are handled separately, model streamflow output at a subcat outlet may not be equal to the modelled runoff from the contributing landscape area.	Streamflow, Route flow In WRSM, streamflow is output for a ‘route’ element leaving a runoff module or a channel module.	Streamflow, Total downstream flow	Streamflow, Channel flow	Streamflow, Channel flow	Streamflow, River Discharge
Channel transmission loss	River channel flow that infiltrates into the channel bed material. It could become bank storage, part of the unsaturated zone, and/or recharge groundwater.	Bedloss	Channel loss	(not used) ACRU doesn’t explicitly model channel transmission loss	Channel transmission loss	Saturated zone (SZ)-river exchange, River discharge to baseflow reservoir With finite difference groundwater modelling: dynamic 2-way exchange between ‘SZ’ and river channel With linear reservoir groundwater modelling: transmission loss is routed a “baseflow reservoir”

Evapotranspiration

Evapotranspiration terms across tools
General term	Concept	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
equivalent terms are bold; similar / related terms are not bold & are starred; notes on term usage in tool texts & interface are given in italics*
Potential evapo-transpiration (PET) & Reference PET	Max evapotranspiration (ET) from a surface given a set of climate conditions and no water availability restrictions. This is determined by the atmospheric 'demand' for ET (energy for evaporation & capacity to hold additional moisture: solar radiation, temperature, humidity, wind) and the properties of the surface (cover, stomatal conductivity of vegetation). Reference PET is PET for a standardised surface. It gives information about the atmospheric ET demand and can be used as a base to estimate PET and AET for other land covers. The frequently used FAO-56 method (Allen et al 1998) applies the Penman-Monteith equation to estimate PET for a reference grass.	Potential evaporation (PE) WRSM “PE” is calculated for veg being modelled using pan evaporation and a pan factor : PE = pan evap pan-factor for veg* Pan evaporation is the atmospheric ET demand input, not a veg reference PET. Symon’s pan (S-pan) evaporation is used in general, but some modules use A-pan.	Potential evaporation (PE), Pan evaporation, PEVAP, PET* “PET” and “pan evaporation” are used interchangeably in SPATSIM texts. Symon’s pan (S-pan) evaporation is generally used as the atmospheric ET demand input. The tool does not apply a pan factor to estimate PET for the specific vegetation being modelled. (This could be done externally by the user rather than inputting raw pan evap)	Reference potential evaporation (E_r)* & Maximum evaporation (E_m), PET ACRU typically uses A-pan evaporation as its “Reference potential evaporation (E_r)” input. Other options are provided. “Maximum evaporation (E_m)”, also referred to as PET in ACRU texts, is PET for the veg being modelled, estimated from the reference: E_m = A-pan evap A-pan crop coefficient for veg type*	PET SWAT can calculate PET or reference PET using different algorithm options. The full Penman-Montieth option calculates both PET and AET for modelled veg types using climate, LAI, stomatal conductance Without full climate data, SWAT estimates grass reference PET and adjusts this to get PET for the modelled veg based on LAI.	Reference evapotranspiration (ET_ref) & Crop Reference ET rate (ET_rate), PET MIKE-SHE uses FAO-56 grass reference PET as its “Reference evapotranspiration (ET_ref) ” “Crop reference ET rate (ET_rate)”, also referred to as PET in texts, is PET for the veg being modelled, estimated from the reference: ET_rate = ET_ref crop coefficient for veg type* (NB: There is some use of "ET_o" and "ET_p" in texts without clarification.
Crop coefficient (K_c)	Scaling factor to adjust a reference PET, or other measure of atmospheric ET demand, to estimate PET for the specific vegetation type being modelled: ET from veg type if soil moisture were not limiting = reference PET * K_c	Pan factor Pan factor used to adjust S-pan evaporation (not grass reference PET)	(not used)	Crop coefficient (K_c) Crop coefficient used to adjust A-pan evaporation (not grass reference PET)	(not used) When using SWAT methods that calculate reference PET first, a crop coefficient is calculated by SWAT from LAI	Crop coefficient (K_c) Crop coefficient used to modify grass reference PET
Actual evapo-transpiration (AET)	Total ET from an area, potentially including: evaporation from canopy interception storage, evaporation from open water surfaces, evaporation from soil moisture, transpiration by vegetation from soil moisture and from groundwater. AET will be less than the PET when water availability is limiting. NB: Sources differ in which of these components get included in “AET” for a landscape	Catchment evaporation (E)* WRSM “E” refers to: soil moisture evap + transpiration Does not include: canopy interception evap, evap from water bodies	AET* SPATSIM “AET” refers to: canopy interception evap + soil moisture evap + transpiration from soil Does not include: transpiration from groundwater, evap from water bodies	AET* ACRU “AET” refers to: soil moisture evap + transpiration Does not include: canopy interception evap, evap from water bodies	AET, ET* SWAT “AET” & “ET” refer to: soil moisture evap + transpiration Does not include: evap from water bodies (NB: canopy interception not explicitly modelled in standard daily timestep application)	AET MIKE “AET” refers to: canopy interception evap + soil moisture evap + transpiration + ponded surface water evap

Soils & unsaturated zones

Soil & unsaturated zone terms across tools
General term	Concept	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
equivalent terms are bold; similar / related terms are not bold & are starred; notes on term usage in tool texts & interface are given in italics*
Unsaturated zone (UZ)	Soil, sediment, regolith, and rock layers above the groundwater water table. May become temporarily saturated from storm events, but generally does not remain saturated for months at a time. May not all be strictly considered ‘soil’* under typical soil definitions, i.e. material having both organic & mineral content.*	Soil + Percolation storage zone (or Unsaturated storage) WRSM runoff modules have 2 UZ components: “Soil” (root zone) & “Percolation storage zone” (below root zone) The “percolation storage zone” is also referred to as the “Unsaturated storage” in the model interface.	Soil, Moisture store, Upper zone SPATSIM subcats have 1 UZ unit. Several terms for this unit are used in tool & texts	Soil ACRU HRUs' UZ is a 2 layer soil profile (all root zone) ACRU3 and some research versions include an optional ‘intermediate zone’ between root zone soil and aquifer	Soil + Vadose zone SWAT HRUs have 2 UZ components: “Soil” (above & below roots) & “Vadose zone” ‘Soil’ profile has separately parameterised layers and more complex handling vs the “vadose zone” below that lags recharge to aquifer.	*Unsaturated zone (UZ) (+ interflow reservoir)** MIKE’s UZ is a layered profile that can extend below the root zone. With finite difference groundwater: UZ thickness changes as the water table fluctuates. With linear reservoir groundwater: an ‘interflow reservoir’ is included below the ‘UZ’ profile.
Root zone	Soil, sediment, fractured rock layers that contain roots, allowing direct withdrawal of stored water for transpiration. Deeper layers can feed ET indirectly via capillary rise into the root zone. In some cases, roots may reach the groundwater, which makes the whole UZ profile part of the ‘root zone’.	Soil Soil unit functions as the root zone	Soil, Moisture store, Upper zone 1 UZ unit, functions as the root zone. Several terms for this unit are used in tool & texts	Soil 2 soil layers (‘horizons’) are included and both contain roots (can set the lower layer to contain very little of the roots)	Root zone Soil profile can include layers below the root zone.	Root zone, UZ upper layer* MIKE UZ algorithm options.. In the "2-layer": the “upper layer” is the root zone + the potential capillary fringe depth. If/when the water table rises to this level, there is no ‘lower layer’. In others: the UZ profile can include layers below root depths.
Saturation soil moisture (Sat SM)	Maximum water content of a soil layer (or sediment), determined by its porosity	Saturation moisture (ST)* Pitman ‘ST’ is a max water storage for a MONTH. It’s a threshold for surface flow generation, max interflow, and max recharge rates. Because of the monthly timestep, its not directly equivalent to porosity.	Saturation moisture (ST)* Pitman ‘ST’ is a max water storage for a MONTH. It’s a threshold for surface flow generation, max interflow, and max recharge rates. Because of the monthly timestep, its not directly equivalent to porosity.	Saturation, Total porosity	Saturation (SAT)	Saturation
Field capacity (FC)	Moisture content of porous media at which there is no vertical drainage due to gravity: all the pore water present held by capillary forces stronger than gravity	Drainage limit (SL)* Pitman ‘SL’ is a MONTHLY soil moisture storage threshold below which interflow and percolation stop. Because of the monthly timestep, it's not directly equivalent to FC.	Drainage limit (SL)* Pitman ‘SL’ is a MONTHLY soil moisture storage threshold below which interflow and percolation stop. Because of the monthly timestep, it's not directly equivalent to FC.	Drained upper limit (DUL)	Field capacity (FC)	Field capacity (FC)
Wilting point (WP)	Moisture content of porous media below which plants cannot withdraw water for ET because capillary forces are too strong	(not used) The monthly soil moisture limit for ET withdrawal is a function of: PE, ST, & shape parameter (R)	(not used) The monthly soil moisture limit for ET withdrawal is a function of: PE, ST, & shape parameter (R)	Wilting point (WP)	Wilting point (WP)	Wilting point (WP)
Infiltration	Water on the ground surface (from throughfall of rain or irrigation, from detained surface flow) entering into soil or sediment.	Catchment absorption, Infiltration	Catchment absorption, Infiltration	Infiltration	Infiltration	Infiltration
Interflow	Lateral flow in the porous material of the unsaturated zone, occurring above and separately from groundwater flow in an aquifer. In models, it is water in the UZ leaving an HRU, subcat, or unit to enter another unit or the model channel network. The ‘unsaturated zone’ material may be temporarily saturated or near saturated, i.e. following a storm, when interflow is occurring	Interflow Flow from the soil moisture store to the channel that occurs when moisture exceeds SL	Soil moisture runoff Flow from the soil moisture store to the channel that occurs when moisture exceeds SL	Delayed stormflow, Baseflow* ACRU calculates total ”stormflow” generated in a rain event. Some of this is then lagged in reaching the channel (“Delayed stormflow”) to represent interflow. Theory manual also suggests that some of the modelled ‘baseflow’ may also represent interflow.	Lateral flow SWAT ‘lateral flow’ from the soil profile can occur when moisture in a soil layer exceeds field capacity. There is no lateral flow from the SWAT ‘vadose zone’	Upper layer saturated zone flow to river, Interflow With finite difference groundwater: MIKE calculates lateral flow for saturated layers only. ‘UZ’ and ‘saturated zone’ (SZ) profiles overlap. Temporarily saturated layers are handled in the SZ. Lateral flow in a perched upper SZ layer is equivalent, but not referred to as 'interflow' in MIKE. With linear reservoir groundwater: an ‘interflow reservoir’ is included below the ‘UZ profile’. Interflow to the channel does not require saturation of this reservoir.
Percolation	Downward movement of water in the unsaturated zone. It can be movement between different layers or components of the UZ and so does not necessarily result in recharge of an aquifer.	Percolation,* Recharge* In WRSM “recharge” refers to water leaving the soil moisture store and entering the “percolation zone storage” (a.k.a “unsaturated storage”) This water will eventually reach the aquifer, but is lagged. WRSM uses “percolation” for water leaving the percolation storage and entering the aquifer.	Recharge* SPATSIM has 1 UZ unit and all water percolating out of this enters the aquifer below so is called recharge.	Drainage	Percolation	Vertical flow, Percolation* In MIKE “percolation” is only used in the context of the linear reservoir groundwater option, for flow from the “interflow reservoir” downward into the “baseflow reservoir” (i.e. recharge)

Aquifers & groundwater flows

Aquifer & groundwater flow terms across tools
General term	Concept	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
equivalent terms are bold; similar / related terms are not bold & are starred; notes on term usage in tool texts & interface are given in italics*
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