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.

For example, 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 to represent the same catchment. For example, different "ACRU models" of ‘catchment A’ could have very different numbers of subcatchments, river elements, separately represented land cover types, hydrological response units, or flow linkages between parts of the landscape and very different parameter values. The same can be said of many of the common 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 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.

Spatial units

General term (& abbreviation)	Concept	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
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.	Catchment, Network* WRSM models are networks of connected modules. The ‘network’ refers to a model’s extent, which could include multiple catchments	Catchment	Catchment	Basin, Watershed	Catchment, Model domain* Model domain: full extent of the area modelled, not forced to follow topographic (surface water) catchment boundaries