Modelling tool capability overview

A set of commonly used modelling tools in South Africa was reviewed for the WRC “Critical catchment model inter-comparison and model use guidance development” project.

This set included major tools developed in South Africa: ACRU and the Pitman model-based tools, WRSM-Pitman and SPATSIM-Pitman. It also included two tools that were developed in the northern hemisphere, but have been used in South Africa and globally: SWAT and MIKE-SHE. Locally developed tools can have certain advantages from being designed with the South African context in mind, both in terms of local data availability and in terms of local climate characteristics, ecosystems, soils, geologic types, and land and water management practices (e.g. landscapes with many small farm dams). SWAT and MIKE-SHE have resourced development teams behind them that continually update the tools and adapt them to make use of developing globally available data sources, such as remotely sensed data, and generally improved access to greater computing power.

This suite of tools covers a diversity of model structure and algorithm type options, but users should be aware there are many many tools available (see more examples)

The tables below summarise basic information about the tools in this set. These tables provide a broad overview, which may be all you need in many instances.

More detailed information about the structural options and capabilities of the tools is given across the other tool inter-comparison pages: Model units & connections, Process representation, Water balance outputs across tools, User interfaces, Documentation & support, and Specific use cases (e.g. irrigation, farm dams, groundwater pumping, riparian zones, wetlands).

Tool background & intended uses

The modelling software tools and the specific versions of these tools covered here are given in the table below.

Modelling software tool background & version covered

Characteristic	WRSM-Pitman	SPATSIM-Pitman	ACRU	SWAT	MIKE-SHE
Developed in South Africa	yes	yes	yes	no	no
Current curator / developer	Bailey & Pitman Water Resources Ltd	Rhodes University, Institute of Water Resources (IWR)	University of KwaZulu Natal, Centre for Water Resources Research (UKZN-CWRR)	Texas A&M University & US Department of Agriculture (USDA)	Danish Hydrologic Institute (DHI)
Free to access	yes	yes	yes	yes	no* free student licenses & free/reduced research licenses by arrangement
Version reviewed in wiki	WRSM-Pitman version 2.9	SPATSIM GWv3 Global Options Threaded model	ACRU 4	SWAT2012; ArcSWAT2012	MIKE-SHE & MIKE Hydro River, version 2019 & 2020
Reference documents consulted (see documentation links page)	Theory manual: Bailey, A.K. (2015). WRSM2000/Pitman: Water Resources Simulation Model for Windows - Theory Manual (Water Research Commission). User manual: Bailey, A.K., and Pitman, W.V. (2016). WRSM/Pitman User’s Manual: WR2012 Volume 7 (Water Research Commission).	Theory documentation: Hughes, D.A. (2004). Incorporating groundwater recharge and discharge functions into an existing monthly rainfall–runoff model. Hydrological Sciences Journal 49. Hughes, D.A. (2013). A review of 40 years of hydrological science and practice in southern Africa using the Pitman rainfall-runoff model. Journal of Hydrology 501, 111–124. Kapangaziwiri, E. (2007). Revised parameter estimation methods for the Pitman monthly rainfall-runoff model. MSc. Rhodes University. User manual: Hughes, D.A. (2019). SPATSIM v3 & IWR version of the Pitman model (IWR Rhodes University).	Theory manual: Schulze, R.E. (1995). Hydrology and Agrohydrology: A Text to Accompany the ACRU 3.00 Agrohydrological Modelling System (Water Research Commission). User manuals: Clark, D.J., Smithers, J.C., Thornton-Dibb, S.L.C., and Lutchminarian, A. (2012). ACRU 4 User Manual: User Interface & Tutorials (Volume 3 of Deployment, Maintenance, & Further Development of SPATSIM-HDSF) Schulze, R.E., and Davis, N.S. (2018). Practitioners’ Handbook for Undertaking Current and Projected Future Climate Related Risk and Vulnerability Modelling Assessments (an update of the ACRU user manual) (Schulze and Associates).	Theory manual: Neitsch, S.L., Arnold, J.G., Kiniry, J.R., and Williams, J.R. (2011). Soil and Water Assessment Tool (SWAT) Theoretical Documentation, Version 2009 (Texas Water Resources Institute, Texas A&M University). User manuals: Winchell, M., Srinivasan, R., Di Luzio, J., and Arnold, J. (2013). ArcSWAT Interface for SWAT2012: User’s Guide (Texas Water Resources Institute, Texas A&M University). Arnold, J.G., Kiniry, J.R., Srinivasan, R., Williams, J.R., Haney, E.B., and Neitsch, S.L. (2012). Soil & Water Assessment Tool (SWAT) - Input/Output Documentation, Version 2012 (Texas Water Resources Institute, Texas A&M University).	Theory manuals: DHI (2019). MIKE SHE Manual, Volume 2: Reference Guide, MIKE 2019 (Danish Hydrologic Institute). DHI (2019). MIKE 1D: DHI Simulaton Engine for 1D river and urban modelling - Reference Manual, MIKE 2019 (Danish Hydrologic Institute). User manuals: DHI (2019). MIKE SHE Manual, Volume 1: User Guide, MIKE 2019 (Danish Hydrologic Institute). DHI (2019). MIKE Hydro River: User Guide, MIKE 2019 (Danish Hydrologic Institute).

These software tools have different development histories and somewhat different focuses, however all have adapted over time, adding features that allow them to be used in more contexts and have generally overlapping intended applications (see table below).

The structural options and design of each tool reflect the intended applications as well as the balance struck by the developers between potentially competing concerns and goals, such as achieving parsimony, including detailed representation of land cover differences to estimate the impacts of change, maintaining ease of use of the tool, facilitating quantification of model output uncertainty, ensuring applicability in data limited contexts, etc.

For these reasons, despite the overlap in general intended uses, the tools offer some fairly different modelling strategies (see structure overview below). This impacts how they can be applied in specific settings (documented in more detail here). For example, all the tools are capable of modelling the impacts of land use change to some degree, however WRSM-Pitman and SPATSIM-Pitman place greater limitations on the number and types of land cover that can be explicitly modelled compared to the other tools.

A summary table of some key modelling capabilities across the tools is presented below.

Intended uses & development focuses summary

Characteristic	WRSM-Pitman	SPATSIM-Pitman	ACRU4	SWAT2012	MIKE-SHE
Specific tool development focuses	Flexible network for managed systems: many for options transfers, withdrawal points, reservoir locations, etc. Irrigated area representation IAP & plantation forestry water use Groundwater-surface water interaction (vegetation & GW pumping impacts)	Parsimony, Uncertainty assessment & stochastic modelling, Groundwater-surface water interaction (vegetation & GW pumping impacts)	Detailed land cover type representation, In-built local parameter database for SA vegetation & soil types, supports the level of representation Crop & irrigation detail, IAP & plantation forestry water use, including deep rooted vegetation in riparian areas	Detailed land cover type representation, In-built local parameter database for vegetation & soil types - North American focus, some more general, supports the level of representation Crop & irrigation detail with source flexibility, Coupling to GIS tools	Flexible spatial discretisation & distribution (climate, land cover, soil, geology, etc), Potential for fine scale process representation Crop & irrigation detail with source flexibility, Groundwater-surface water interaction, Coupled hydraulic channel model & flooding processes
Intended applications:
Water balance estimation	yes	yes	yes	yes	yes
Design hydrology & flood analyses	no	no	yes	yes	yes
Supply planning (general)	yes	yes	yes	yes	yes
Reservoir yield	yes	yes	yes	yes	yes
Irrigation planning	yes	(limited)	yes	yes	yes
Groundwater recharge	yes	yes	yes	yes	yes
GW-SW interactions & GW pumping impacts	yes	yes	no	yes	yes
Land cover change impacts	yes	yes	yes	yes	yes
Climate change impacts	yes	yes	yes	yes	yes
Application limitations	Monthly model: not applicable for peak flow, flood assessment, design hydrology No irrigation direct from groundwater	Monthly model: not applicable for peak flow, flood assessment, design hydrology No irrigation direct from groundwater	Simplified groundwater modelling; Does not model groundwater pumping	Highly simplified deep groundwater modelling	(None documented for the modelling system as whole, only for certain process options within it. The learning curve for its use and computing power required can pose practical limitations to its application )

Model structure & capabilities overview across tools

The table below gives a very basic overview of the type of model structure that can be built in each tool. More details about the model structure options across tools are presented here.

Model spatial & temporal scale overview by tool

Characteristic	WRSM-Pitman	SPATSIM-Pitman	ACRU4	SWAT2012	MIKE-SHE
Intended scale of catchment or modelled area	Local to regional: no suggested min-max model size	Local to regional: 10-10,000’s of km2  more typical: 100-1,000’s km2	Field to regional: no suggested min-max model size	Field to regional: no suggested min-max model size	Field to regional: no suggested min-max model size
Timestep	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* Calculation timesteps are dynamic and vary by process. All outputs saved for a user-selected step.
Spatial discretisation (model spatial units)	Modules connected by routes (“runoff” modules + special area modules + channel modules create subcatchments)	Subcatchments + limited internal sub-area types	HRUs within subcatchments	HRUs within subcatchments	Grid cells (3D), with optional calculation simplifications: surface flow modelled for zones, interflow & groundwater modelled for storage reservoirs within subcatchments.
Suggested model unit scales	Runoff module:  < 1,000 km2	(none listed)	Subcatchments: 5-50 km2 HRUs:  < 30km2	(none listed)	(none listed)

Despite having largely overlapping intended uses in general, there is notable diversity in model structure (table above, and more detailed coverage here) and in more specific modelling capabilities in this set of tools (table below).

Some key points arising from the capabilities comparison are:

• No one tool had all the capabilities listed.  All the tools have differing sets of advantages over the others.

• Although both based on the same predecessor model structure and sharing many basic process algorithms, WRSM-Pitman and SPATSIM Pitman have diverged in capabilities across several aspects. These differences are linked to WRSM’s modular network structure compared to the SPATSIM version’s focus on the subcatchment as the primary unit for process representation. For example, a WRSM model can include user-defined artificial water transfers between channel units in a model network, which SPATSIM does not include. SPATSIM includes an uncertainty or stochastic modelling routine, made more straightforward by the more uniform set-up of subcatchments, which WRSM does not include.

• WRSM-Pitman, SPATSIM-Pitman, and MIKE-SHE model dynamic, two-way exchange between surface water channels and groundwater aquifers (GW-SW exchange): groundwater can flow into the channel or channel water can recharge an aquifer in the model and the direction and magnitude of the exchange is calculated based on the water levels in each in every modelled timestep.

• Only MIKE-SHE has a fully coupled hydraulic model, allowing it to model channel-floodplain interactions in more detail, including the impact of changes in channel size, shape, depth of incision, etc. Other tools can represent some aspects of overbank flooding and the fate of the flood water, but this is limited to their wetland and riparian zone modules.

It's important to note that this capabilities summary table indicates what is technically possible within the modelling tools, but it does not cover the practicality and ease-of-use of the different features across each tool. Some things that are possible in a tool become a veritable labour of love to actually achieve, depending on the nature of the model set-up (i.e. how many subcatchments, how many HRUs, etc).
For example, while different climate inputs can be specified by HRU in ACRU4, to allow for spatial distribution of rainfall within a subcatchment, operationalising this when there are many HRUs becomes quite unwieldly compared to simply specifying one set of climate inputs for all units in the subcatchment scale. There is no batch-input of climate for other subgroups beyond the subcatchment, and so varying climate within a subcatchment requires manually clicking through three layers of menus each for four different climate inputs for every HRU. Various aspects of the user interfaces of the different tools and their implications are covered on a separate page here.

Modelling tool capabilities summary

Capability	WRSM-Pitman (+ Sami GW)	SPATSIM-Pitman (+ Hughes GW)	ACRU4	SWAT2012	MIKE-SHE
Climate (rain & PET)
Spatially variable across model domain	yes	yes	yes	yes	yes
Spatially variable within subcatchment	(limited)	no	yes	no	yes
Inter-annual variability in PET	no	yes	yes	yes	yes
Land cover & change
Processes explicitly linked to land cover	(limited)	(limited)	yes	yes	yes
Multiple land cover types included	(limited)	(limited)	yes	yes	yes
Cover has explicit location in subcatchment	(limited)	no	(limited)	(limited)	yes
Cover can vary over model run timespan	yes	no	(limited)	yes	(limited)
Irrigation, with dynamic demand & supply	yes	yes	yes	yes	yes
Potential for ET from GW (deep roots)	yes	yes	(limited)	(limited)	yes
Peak flows & flooding
Max daily or subdaily peak flow estimation	no	no	yes	yes	yes
Explicit impacts of channel capacity & shape on flow	(limited)	(limited)	(limited)	(limited)	yes
Calculation of flooded area extent	(wetland)	no	(wetland)	(wetland)	yes
Flood water subject to infiltration, ET, etc	(limited)	no	yes	no	yes
Reservoirs, dams & channel flow modification
Reservoirs explicitly modelled	yes	yes	yes	yes	yes
Lumped representation for many small dams	yes	yes	(limited)	(limited)	no
Abstractions & external inputs	yes	yes	yes	yes	yes
Internal transfers between model units	yes	no	yes	yes	(limited)
GW representation & GW-SW interactions
Dynamic, 2-way, GW-SW exchange	yes	yes	no	(limited)	yes
GW table elevation predicted	(limited)	(limited)	no	(limited)	yes
GW pumping included	yes	yes	no	yes	yes
Wetlands & riparian zones
Wetland processes included	yes	yes	yes	yes	yes
On-channel wetlands (valley bottom)	yes	yes	yes	yes	yes
Off-channel wetlands (fed by channel overbank spill)	yes	yes	yes	no	yes
Groundwater-fed wetlands (direct vs via channel's baseflow)	(limited)	(limited)	(limited)	(limited)	yes
Modelling other related catchment processes
Sediment movement	no	no	yes	yes	yes
Water quality	no	no	yes	yes	yes
Crop yield	no	no	yes	yes	no
Uncertainty & parameter calibration
Functions/routines for batch runs, uncertainty, parameter sensitivity, & calibration	no	yes	no	yes	yes