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).

TEMPLATE TABLE

General term	WRSM-Pitman (Sami GW)	SPATSIM-Pitman (Hughes GW)	ACRU4	SWAT2012	MIKE-SHE, semi-distributed, more conceptual	MIKE-SHE, fully-distributed, more physical

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.