Scope: what’s covered (and not!) in this wiki

This wiki focuses on catchment-scale hydrological modelling, as opposed to plot-scale, hillslope-scale, or global-scale modelling of water movement.

At present, this wiki does not cover specifically groundwater-focused modelling tools (e.g. MODFLOW) or hydraulics-focused tools (e.g. HEC-RAS). However, modelling tools intended for catchment hydrological modelling necessarily include some level of representation of groundwater storage and fluxes as well as some representation of river channel flows. This representation is generally far more simplified than groundwater-focused and hydraulic modelling tools. Recognition of the need for more detailed representation of groundwater and channel/floodplain hydraulics in some catchment modelling contexts has led to the integration of algorithms used by these more specific tools into catchment modelling tools. An example of this is the MIKE-SHE modelling platform.

At present, this wiki does not cover modelling of sediment transport or other water quality parameters, although some of the modelling tools described here do have these capabilities. It is the intention to add more coverage of these eventually.

A focus of this wiki is helping modellers to make informed choices about their model structures and modelling tool selection. The intention is to help users compare across modelling tools more easily by describing different aspects of them side-by-side. Each modelling project will have it's own needs and constraints. There will almost always be multiple modelling tools that are similarly suitable for a given use-case, although each one may have a different type of limitation or drawback. For this reason this wiki will describe issues that need to be considered when applying a given tool in a particular case and how the tools compare across various relevant factors, and then let the user decide how to weight the various factors involved for their specific situation.

This wiki is aimed at readers who a have at least a basic background in hydrology and exposure to catchment hydrological modelling. The material is not intended to replace courses or foundational instructional material on hydrology and modelling in general or on the use of specific modelling tools. One needs a solid grounding in hydrology before getting into catchment modelling. This wiki describes and compares various aspects of different modelling tools, but it is not an exhaustive coverage of any of them and so it cannot replace reading the tool's supporting material!

Modelling software tools & versions covered

Currently, the modelling tools included in the detailed comparison in this wiki are:

Modelling tool	Version	References
ACRU, Agricultural Catchment Research Unit model	ACRU4	Schulze, 1986, 1995; Schulze and Davis, 2018
WRSM-Pitman, Water Resources System Model	WRSM2000	Pitman, 1973; Bailey and Pitman, 2015
SPATSIM-Pitman, Spatial and Time Series Information Modelling	IWR (GWv3)	Pitman, 1973; Hughes, 2005
SWAT, Soil and Water Assessment Tool	SWAT2012; ArcSWAT2012	Arnold et al., 1998; Neitsch et al., 2011
MIKE-SHE (& MIKE-Hydro), Système Hydrologique Européen	MIKE-SHE 2020	Abbott et al., 1986; DHI, 2014; Refsgaard and Storm, 1995

These were chosen as commonly used, accessible, and/or promoted versions of the modelling tools when this wiki was developed (2021).

ACRU, SPATSIM-Pitman, and SWAT are curated by university research groups and there are many versions of these platforms that were developed by researchers for specific studies. These may appear in published research articles, but may not be easily accessible for a general user.

The SWAT2012 version of SWAT describes the model units and process algorithm options. Models can be optionally built and operationalised in a GIS environment, using ArcSWAT2012 with ArcGIS ESRI software and Q-SWAT with Q-GIS software. ArcSWAT2012 was primarily used in this project and is the version referred to in the inter-comparison pages.

A new version of SWAT, called SWAT+ was launched during the inception of this project. Most of it's core algorithms are the same as SWAT2012, however it's user interface has changed and SWAT+ includes runoff routing from specified hillslope areas to floodplain areas. This is a significant change to the landscape process representation. The intention is to update wiki material over time as SWAT+ may become the dominant version in use.

Coverage of structural options within modelling tools

All of the tools reviewed have some flexibility in how a catchment model can be set-up. This adds a layer of complexity when trying to compare approaches across tools. In this review an effort has been made to document and consider the main structural options available and approaches considered ‘typical’ for each tool; however, it should be noted that not all potential options have been covered.

A description of how multiple structure options were handled for the modelling tools reviewed in the wiki is given here. This is perhaps most important for MIKE-SHE which has the largest number of structural options for the user to select from when setting up a model.