Model units & connections

The table below and schematic diagrams describe the overall discretisation approach used in each tool. 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, 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 here. Although described on separate pages the discretisation and process algorithms 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.

Overarching approaches by tool

The schematic diagrams and tables below describe the overall discretisation approaches used in each tool and for two generalised set-up approaches in MIKE-SHE (rationale and description of these found here).

Model structure overviews by tool

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)	Catchments 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 specifically input. Runoff modules and sets of linked modules are effectively subcats.	Catchments composed of subcatchments. Subcats are linked in spatially determined flow network (map input).	Catchments composed of subcatchments. Subcats are linked in a user-specified flow network.	Catchments composed of subcatchments. Subcats are linked in a spatially determined flow network (delineated from input DEM by tool or given as a map input)	Catchments composed of subcatchments. Subcats are linked in spatially determined flow network (map input).	Catchments composed of a 3D grid. The surface is uniformly sized grid cells. Each cell has a column beneath: layers of unsaturated & saturated material (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 ‘runoff module’ with no linked special area modules functions as a subcat. If special area modules are linked, then a set of connected modules functions as a subcat.   Special area modules that can be added are: ·       tree plantations, ·       alien vegetation, ·       irrigation ·       mines Special area modules appear as separate units, but function as sub-areas inside a specified runoff module. Runoff modules can also have a specified: ·       impervious area ·       riparian area (ET from GW). These are specified within the module.	Subcatchments all contain a river channel and may have specified special sub-areas. Subcats can optionally include either a wetland or a reservoir/lake at the outlet. Special sub-areas are portions of a subcat’s area for which a different algorithm is applied. A 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, forestry plantation, IAP) ·       area with irrigation from dams ·       area with irrigation from river ·       small dams internal to subcat (lumped) The impervious, high ET cover, and irrigation areas do not have a defined spatial location within the subcat in relation to other areas (e.g. riparian areas, internal dams).	Subcatchments are composed of one or more HRUs and non-headwater subcats must contain a river channel. Subcats can optionally include additional channel units and reservoir units, linked in a user-specified network. There are special HRUs with different algorithms for impervious areas, irrigated areas, riparian areas & wetlands. For most HRUs, all runoff (‘quckflow’ & ‘baseflow’) is routed directly to a channel, reservoir, or subcat outlet node (in parallel). HRUs are not linked to one another. 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 specified as adjacent.	Subcatchments are composed of: ·       HRUs ·       shallow aquifer unit, ·       deep aquifer unit, ·       tributary channel unit, and ·       main river channel unit. Subcats can optionally include ponds and wetlands, which can be internal to the subcat, and reservoirs/lakes, at the subcat outlet. HRU surface & ‘lateral’ runoff output is routed directly to the subcat channel network (in parallel). This runoff passes via the tributary channel, where delays & infiltration can be imposed, before reaching 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.	Subcatchments are composed of: ·       grid cells ·       ‘overland flow zones’, ·       ‘interflow reservoirs’ ·       ‘baseflow reservoirs’ ·       river channels Zones and reservoirs have user-specified spatial extents (map input). Interception, ET infiltration, & percolation are calculated by grid cell. Surface runoff is lumped for cells in 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 in a downslope series to the channel (potential for vertical loss on route). Baselfow reservoirs are recharged by overlying interflow reservoirs and outflow to the channel in parallel.	No subcat boundaries are input; water surface elevations on the surface and in the 3D grid drive flow directions.   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, etc); they do not need to be aligned with one another.
River channels	Channels are modules in the network with multiple inflow and outflow links possible. Channel modules can have an associated wetland storage/area.	Channels are units linked to subcats (one per subcat). They receive the local  (incremental) subcat runoff & upstream subcat channel outflows.	Channels are necessary units in non-headwater subcats. Subcats can have multiple channel units. Channels receive runoff from linked HRUs, reservoirs, & upstream subcat outflows.	Channels are units linked to subcats (one main channel per subcat). They receive the local (incremental) subcat runoff & upstream subcat channel outflows.	Channels are composed of spatially explicit reaches between node points. Reaches with nodes mapped inside a subcat exchange water with the subcat.	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 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, but 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 surface runoff reaching the stream, i.e. the combined impact of the cliff and permeable toeslope on surface runoff reaching the channel. If modelling the cliff and the toeslope as separate units, surface runoff on the cliff unit could be calculated and then surface flow and infiltration on the toeslope unit could be calculated as a separate set of processes.