Model units & connections

This page describes how modelling tools allow users to discretise catchments into modelled units in order to represent and calculate hydrological processes. Differentiating the catchment into separate components or units, such as subcatchments, patches of similar land cover, or soil layers with distinct properties, allows hydrological processes to be modelled by algorithms that have been developed for that scale and type of unit.

Each modelling tool has a unique way of describing a catchment in terms of:

• The types surface and subsurface model units that can be included
• The connections that can be defined between these different units
• The process algorithms that are used to calculate inflows, storage, and outflows for each unit

This page summarises the unit types and connection options available across the tools, while their process algorithms are described on a processes page here. Although described on separate pages, the approach to discretisation and the process algorithms used are inextricably linked. This page also summarises some implications of the structural differences across the tools, which are dealt with in more detail for some specific model application contexts here.

The material on this page may go into more depth than you might be looking for. If all you need is a very brief overview of the structures and capabilities of the different modelling tools, that summary info can be found on the capabilities page, here.

Basic approaches for representing catchments in the different modelling tools

The schematic diagrams and tables below describe the main discretisation approaches used in each tool in terms of breaking up catchments into units and specifying linkages. For MIKE-SHE, because the tool offers many options, two generalised set-up approaches have been described (more background on these is given here).

Subcatchment delineation: Demonstration of: (a) a catchment area delineated into subcatchments and (b) the flow network connections of these subcatchments when used as model units. This approach is used to some degree in all the modelling tools covered (except MIKE-SHE when fully distributed). Subcatchments 4,5,6,8 are not headwaters and their river channels receive flow from upstream subcatchments. Figures from: Schulze, R.E. (1995). Hydrology and Agrohydrology: A Text to Accompany the ACRU 3.00 Agrohydrological Modelling System (Water Research Commission).	Subcatchments as units for process modelling: Diagram illustrating the approach taken in SPATSIM-Pitman of using the subcatchment as the primary unit for representing processes. Flow through the subcat can be conceptualised as flow through a generalised hillslope, which provides a means of modelling groundwater flow and exchange with the river channel. Special subareas (e.g. irrigated crops) impact the subcat's infiltration, surface runoff, ET, and recharge. Interflow and aquifer exchange with the channel are calculated based on the resulting subcatchment-scale storage. WRSM-Pitman & SWAT follow a related approach for groundwater in that aquifers are modelled at the subcatchment scale. Figures from: Hughes, D.A. (2004). Incorporating groundwater recharge and discharge functions into an existing monthly rainfall–runoff model. Hydrological Sciences Journal, 49.	Modular network of modelling units: Example WRSM-Pitman model network diagram (catchment of the Churchill Dam, quaternaries K90A&B), demonstrating a catchment composed of modules linked by routes. Subcatchment boundaries are not set-up in the tool; however, sets of linked modules are subcats in effect. (Also, though shown as separate units, each 'irrigation module' is modelled as part of a specified 'runoff module' for groundwater modelling.) ACRU4 uses a similar modular network approach, although ACRU units are grouped into subcatchments. Figure from: Bailey, A.K., and Pitman, W.V. (2015). Water Resources of South Africa 2012 Study (WR2012) (Water Research Commission).
Networks of modelling units within subcatchments: Demonstration ACRU4 model network diagram showing HRUs (as coloured squares), river channel, and dam units grouped within subcatchments. Climate can be input at the subcatchment scale and certain relationships may only be allowed between units in the same subcatchment (e.g. routing HRU subsurface flow to a special riparian HRU). The figure also shows that withdrawals and external inputs can be included for rivers and dams. Figure from: Clark, D.J., Smithers, J.C., Thornton-Dibb, S.L.C., and Lutchminarian, A. (2012). ACRU 4 User Manual: User Interface & Tutorials in Volume 3 of Deployment, Maintenance, & Further Development of SPATSIM-HDSF. (Water Research Commission).		HRUs as units for process modelling: Schematic diagram of process representation for an HRU (hydrological response unit) in ACRU4. Each HRU includes a groundwater/baseflow store. SWAT2012 also uses HRUs to model surface and shallow subsurface processes, but models groundwater at the subcatchment scale. Figure modified from: Schulze, R.E. (1995). Hydrology and Agrohydrology: A Text to Accompany the ACRU 3.00 Agrohydrological Modelling System (Water Research Commission).
Routing runoff from units in parallel or in series: Demonstration of subcatchment delineation into topographic units, which could serve as HRUs or process modelling zones, and related routing options. Runoff from each unit could be routed directly to the stream in parallel, or routed from one unit to the next in a downslope series. Series routing allows processes (infiltration, recharge, saturation seepage, etc) to occur along the path depending on states of downslope units (dry/saturated, flat/steep, rough/smooth, etc). It adds computational complexity. Different approaches can be more appropriate, or more important, in different settings, spatial scales, and for different runoff components (i.e. surface flow, interflow, groundwater flow). SPATSIM subareas in subcats modify subcat-scale runoff; no separate routing needed. WRSM's afforestation & alien veg modules are similar (act inside 'runoff modules'), but irrigation module surface runoff and interflow are routed in parallel to the linked 'runoff module'. ACRU4 routes all HRU 'quickflow' runoff in parallel and can optionally route 'baseflow' runoff into the soil of a specified 'riparian HRU'. SWAT2012 routes all HRU surface and 'lateral' (interflow) runoff in parallel, while groundwater is modelled at the subcat scale. MIKE-SHE grid cells are connected in series. If 'overland flow zones' are used, surface runoff can be routed in series or in parallel. If interflow and baseflow linear reservoirs are used, interflow is routed in series and groundwater in parallel (if multiple aquifers used).
3D grid cells as units for process modelling: Illustration of the approach used in MIKE-SHE for modelling processes using a 3D grid, i.e. grid cells on the surface (2D) with columns of material below that are broken into layers. (For those familiar with HRUs, each cell can be seen as similar to an HRU connected in series to its neighbors.) In the diagram part of the unsaturated zone is shown as transparent so that the gridded water table surface beneath can be more easily visualised. Figure source: DHI (2019). MIKE SHE Manual, Volume 1: User Guide, MIKE 2017 (Danish Hydrologic Institute).		Hybrid approach - different unit types for modelling different processes: Illustration of the more lumped, conceptual approach for representing interflow and groundwater flow with 'linear reservoir' units in MIKE-SHE. Other processes can still be modelled by grid cell. Interflow reservoirs have spatial extents and each receives percolating water from the grid cells that overly it. Similarly if multiple different mapped baseflow reservoirs (distinct aquifers) are included, these are recharged by overlying interflow units. SWAT2012 also uses a hybrid approach in that surface flow and 'lateral' flow (interflow) is modelled at the HRU scale, while the aquifer spans the subcatchment an is recharged by all the overlying HRUs. Figure source: DHI (2019). MIKE SHE Manual, Volume 2: Reference Guide, MIKE 2017 (Danish Hydrologic Institute).

Model structure overviews by tool

Level / Element	WRSM-Pitman (Sami GW)	SPATSIM-Pitman (Hughes GW)	ACRU4	SWAT2012	MIKE-SHE, semi-distributed, more conceptual	MIKE-SHE, fully-distributed, more physical
Catchments & subcatchments (subcats)	WRSM catchments are composed of ‘modules’ linked in a user-specified network. Module types: runoff modules, special area modules (see below) channel modules reservoir modules Subcatchments are not explicitly input, but runoff modules and sets of linked modules act as subcats.	SPATSIM catchments are composed of subcatchments. Subcats are linked in spatially determined flow network (map input).	ACRU4 catchments are composed of subcatchments Subcats are linked in a user-specified flow network.	SWAT catchments are composed of subcatchments Subcats are linked in a spatially determined flow network (delineated by SWAT from DEM input, or given as a map input)	MIKE catchments can be composed of subcatchments Subcats are linked in spatially determined flow network (map input).	MIKE catchments can be composed of 3D grid units. The surface is uniformly sized grid cells. Each cell has a column beneath: input layers of soil, sediment, rock. No subcat boundaries are input; grid cells have elevation (DEM input) and water elevation on the surface and in the 3D grid drive flow.
Units within subcatchments	A WRSM ‘runoff module’ with no linked special areas functions as a subcat. If special area modules are linked, then a set of modules functions as a subcat.   Special area modules that can be added are: afforestation (tree plantation), alien vegetation, irrigated crops mines These appear as separate units, but function as sub-areas inside a specified linked runoff module. Runoff modules can also have internally specified subareas: impervious area riparian area (ET from GW).	SPATSIM subcatchments all contain a river channel and may have specified special sub-areas. Subcats can optionally include either a wetland or a reservoir/lake on the channel at the subcat outlet. A special sub-area can be defined for any, or all, of the following in a subcat: impervious area riparian area (ET from GW) area with a higher ET land cover (forest, tree plantation, alien veg) crop irrigated from dams crop irrigated from river small dams internal to subcat (lumped) Impervious, high ET cover, and irrigation areas do not have defined spatial locations within the subcat.	ACRU4 subcatchments are composed of one or more HRUs. Non-headwater subcats must contain a river channel. Subcats can optionally include additional channels and reservoir units. Units are linked in a user-specified network. There are special HRUs for: impervious areas, irrigated areas, riparian areas wetlands. For most HRUs, all runoff is routed directly to a channel, reservoir, or subcat outlet node (in parallel). Optional exceptions: For ‘riparian HRUs’, ‘baseflow’ output from other HRUs can be routed to the riparian HRU soil. For ‘adjunct-impervious HRUs’, runoff is routed to the surface of another HRU.	SWAT subcatchments are composed of: HRUs (many) shallow aquifer unit, deep aquifer unit, tributary channel unit main river channel unit. Subcats can include an internal pond or wetland and a reservoir/lake on the channel at the subcat outlet. HRU surface & ‘lateral’ runoff output is routed directly to the subcat channel (in parallel). This passes via the tributary channel, where delays & infiltration can occur, to the main channel. Groundwater is modelled at the subcat scale. Aquifers are recharged by all subcat HRUs. Shallow aquifer outflow goes to the subcat main channel.	MIKE-SHE subcatchments are composed of: grid cells (veg & soil) ‘overland flow zones’, ‘interflow reservoirs’ ‘baseflow reservoirs’ river channel links Surface runoff is lumped for grid cells within an overland flow zone & routed across zones in a downslope series to the channel (potential for infiltration on route). Interflow reservoirs receive percolation from overlying grid cells. Interflow is laterally routed through a downslope series of reservoirs to the channel (potential for loss to recharge on route). Baselfow reservoirs are recharged by overlying interflow reservoirs and outflow to the channel in parallel.	Using fully distributed MIKE-SHE, no subcat boundaries are input. Relative water surface or head elevations on the surface and in the 3D grid units drives flow exchange between units.   Surface and subsurface properties for the grid cells & layers are input for different zones (map input). Different sets of zone polygons can be used for the different types of properties (e.g. vegetation types, soil types, aquifer extents); they do not need to be aligned with one another.
River channels	WRSM channels are modules in the network with multiple inflow and outflow links possible. Channel modules can have an associated wetland storage/area.	SPATSIM channels are units linked to subcats (one per subcat). They receive the local subcat runoff & upstream subcat channel outflows.	ACRU4 channels are units in a subcat network of units. (necessary in non-headwater subcats). They receive runoff from linked HRUs, reservoirs, & upstream subcat outflows.	SWAT channels are units linked to subcats (one main channel per subcat). They receive the local subcat runoff & upstream subcat channel outflows.	MIKE channels are composed of spatially explicit reaches between node points. Reaches with nodes mapped inside a subcat exchange water with the subcat.	MIKE channels are composed of spatially explicit reaches between node points. Reaches can exchange water with grid cells that border them.

Links between discretisation and process representation

The scale of discretisation of the landscape influences which hydrological processes are individually represented and the algorithms and the timesteps that are appropriate (discussed in more detail here). In general, for larger spatial/vertical units, longer timesteps, more lumped process representation, and different property parameters are applicable compared to modelling with smaller units. This has to do with how long it’s likely to take for water to move through a modelled unit. For example, it may take a week for interflow to move through the 10 km long hillslope of subcatchment, but a day to move through the 1 km slope length of a patch of grassland within that subcatchment. Speeds vary by process. This also has to do with the fact that what we may model as one process at a larger spatial scale can be the result of several different processes occurring at smaller scales. For example, rain falling on a steep, rocky cliff may form surface runoff. However, if this surface runoff then flows across an area of permeable and unsaturated soil before reaching a stream channel, some may infiltrate and not reach the stream as surface flow. If one models a subcatchment where this is happening as a single unit, the equation and parameters for estimating ‘surface runoff generation’ would account for the net outcome of the surface runoff that actually reaches the stream, i.e. the combined impact of the cliff and the permeable toe-slope on surface runoff reaching the channel. If modelling the cliff and the toe-slope as separate units, surface runoff on the cliff unit could be calculated and then surface flow and infiltration on the toe-slope unit could be calculated as a separate set of processes. 

Timesteps & spatial units for different processes

Units	WRSM-Pitman (Sami GW)	SPATSIM-Pitman (Hughes GW)	ACRU4	SWAT2012	MIKE-SHE, semi-distributed, more conceptual	MIKE-SHE, fully-distributed, more physical
Timesteps	Monthly* A daily version has been developed. Limited use to-date	Monthly* A daily version has been developed. Limited use to-date	Daily	Daily, subdaily	Daily, subdaily* Timesteps are dynamic, vary by process & unit size. Outputs saved for the same selected step.	Daily, subdaily* Timesteps are dynamic, vary by process & unit size. Outputs saved for the same selected step.
Spatial units for:
Climate input	Modules (all)	Subcatchments	Subcatchments or HRUs* *HRU-scale input is labor intensive	Subcatchments	Grid cells or zones	Grid cells or zones
Surface & shallow subsurface processes	Runoff modules + special area modules	Subcatchments (+ internal special sub-areas)	HRUs	HRUs	Grid cells + Overland flow zones + Interflow reservoir zones	Grid cells
Groundwater processes	Runoff modules	Subcatchments	HRUs	Subcatchments	Baseflow reservoir zones	Grid cells

Specific units, layers, & connections

Basic layers: land cover, soil & unsaturated zone, aquifers

The tables below describe in more detail how the modelling tools can represent the distribution of land cover types, soil types, and aquifer types in a catchment and vertical distribution of soil, sediment, and rock types using the modelling units described above and layers within these units. The linkages or routing of water between units and layers is highlighted here, and covered in more detail in sections on process algorithms.

Land cover: discretisation & spatial distribution approach

Component	WRSM-Pitman (Sami GW)	SPATSIM-Pitman (Hughes GW)	ACRU4	SWAT2012	MIKE-SHE, semi-distributed, more conceptual	MIKE-SHE, fully-distributed, more physical
Spatial units for land cover type/property distribution	Modules & sub-areas within them	Subcatchments & sub-areas within them	HRUs	HRUs	Cover type polygons applied to grid cells	Cover type polygons applied to grid cells
	Extents of cover types are either defined in the: ‘runoff module’ set-up special area modules (which are tied to, and act as sub-areas of, a ‘runoff module’) Each runoff module can have different general cover specifications & linked special areas.	Extents of cover types are defined as special sub-areas within a subcat. Each subcat can have different general cover & sub-areas specifications.	Each HRU has its own cover & vegetation parameters.	Each HRU has cover & vegetation parameters. HRUs are assigned cover types using an input cover type map. Properties are input by type.	Each grid cell is assigned a cover type using an input cover type map. Properties are input by type.  Surface flow parameters (roughness & detention storage) can be assigned for separately mapped zones. (Does not need to align with cover or soil type boundaries)	Each grid cell is assigned a cover type using an input cover type map. Properties are input by type.  Surface flow parameters (roughness & detention storage) can be assigned for separately mapped zones. (Does not need to align with cover or soil type boundaries)
Limitations to types & number of types	Yes	Yes	No	No	No	No
	Types that can be represented: General vegetation (runoff module parameters) Impervious area (runoff module parameters) Riparian zone vegetation (runoff module parameters) Afforestation (special module, 1 per runoff module) Invasive alien vegetation (special module, 1 per runoff module) Irrigated crops (special module, multiple per runoff module) Mines (special area module)	Types that can be represented: General subcat vegetation Impervious area Higher ET vegetation (forest, tree plantation, alien veg) Irrigated crops Only one area (one type) of each of these broad classes can be represented within a given subcat.	No limit on number of HRUs (hence cover types) per subcat.	No limit on number of HRUs or cover types per subcat.	No limit to number of cover types.	No limit to number of cover types.

Soils & unsaturated zone (UZ) material: spatial distribution, vertical layers, connections

Component	WRSM-Pitman (Sami GW)	SPATSIM-Pitman (Hughes GW)	ACRU4	SWAT2012	MIKE-SHE, semi-distributed, more conceptual	MIKE-SHE, fully-distributed, more physical
Spatial units for distribution of types / properties	Runoff modules & irrigation modules	Subcatchments	HRUs	HRUs	Soil type polygons applied to grid cells & ‘Interflow reservoir’ extent polygons	Soil type polygons applied to grid cells
	Each ‘runoff module’ can have its own soil moisture store & ‘percolation storage’ properties. Afforestation & alien veg modules use the soil of the runoff module they are linked to / part of. Irrigation modules have their own soil properties.	Each subcat can have its own soil moisture store properties.	Each HRU can have its own soil properties	Each HRU can have its own soil properties. HRUs are assigned soil types using an input soil type map. Properties are input by soil type. Individual HRU soil properties can by modified.	Each grid cell is assigned a soil type using an input soil type map.   Interflow reservoir ‘type’ extents are input as a map (boundaries do not need to align with soil types or subcatchments)	Each grid cell is assigned a soil type using an input soil type map.
Vertical layers	2 Layers	1 Layer	2 Layers	11 Layers (max)	2(3) Layers*	Unlimited Layers
	A runoff modules has 2 ‘UZ’ layers: Soil moisture storage Percolation lag storage	A subcat has a single soil moisture storage unit.	An HRU has 2 soil layers: A horizon   B horizon Both in root zone.	A soil type can have up to 10 layers (user input layers). Profile can extend below root zone. Each HRU also has a vadose zone (lag storage) ‘layer’ below the soil profile	A soil type has vertically uniform parameters (no layers), but each model grid cell has 2 computational UZ layers: Upper layer (root zone) Lower layer (below roots) Interflow reservoirs are storage units fed by all overlying grid cells	A soil type can have an unlimited number of layers (user input layers). Profile can extend below root zone.
Surface runoff routing w.r.t. units	Parallel*	(n/a)	Parallel	Parallel	Series or parallel	Series
	Parallel for runoff modules & irrigation modules. Other special area modules modify runoff module flow	Subcatchment scale	Parallel from HRUs in subcat	Parallel from HRUs in subcat	Across a series of mapped flow zones within a subcat OR each zone in parallel	Across grid cells based on elevation.
Interflow routing w.r.t. units	Parallel*	(n/a)	Parallel	Parallel	Series	Series
	Parallel for runoff modules & irrigation modules Other special area modules modify runoff module flow	Subcatchment scale	Parallel from HRUs in subcat	Parallel from HRUs in subcat	Through a series of interflow reservoirs in a subcat	Through grid cells based on head.