In order to evaluate the potential of each GIS package, we identified all the available tools and algorithms for the TWI computation. This step permitted to investigate the possibility offered by each GIS. As a consequence, we were able to assess the freedom given to the user in setting the index extraction.
Finally, to compare the results, a simple difference in absolute value between the TWI maps was computed.
Grass GIS
The Geographic Resource Analysis Support System (GRASS) manages raster, vector, point data and contains image processing modules. Developed by the U.S. Army Corps of Engineers, it was released to public in the 1989 [
25]. The GRASS package can be extended through custom modules, which can be written in common programming languages (C, C++, Python, etc.), or developed by people and institutes from all over the world. GRASS is an official project of the Open Source Geospatial Foundation and since 1997 it is maintained by “The GRASS Research Group” at Baylor University, Waco (Texas), U.S.A.
In this environment, it is possible to obtain the TWI with three different tools. The first one, which is also the specific tool for the extraction of the TWI, is the r.topidx tool.
The second possibility is given by the
r.watershed tool (Fig.
4a), which is able to generate many different dataset for the hydrological analysis. Among these maps it is possible to extract the TWI, which is called “topographic index” in this case. The default flow routing algorithm is the Multiple Flow Direction (MFD; [
11,
33]), but it is also possible to use the single flow direction algorithm Deterministic 8 (D8) as proposed in O’Callaghan and Mark [
27]; it has been however suggested in different works to avoid the calculation of the TWI using single flow direction algorithms [
1,
8,
20,
35]. For the MFD algorithm it is possible to set a convergence factor between 1 and 10, the default value being 5. This value makes the flow accumulation to converge more strongly with higher values.
The last option is the
r.terraflow tool (Fig.
4b); similarly to the
r.watershed tool, it generates many different spatial datasets and one of them is the TWI, here called “Topographic Convergence Index (TCI)”. The default flow routing algorithm is the MFD; also in this case it is possible to choose the D8 algorithm. This tool allows to set a threshold for the flow accumulation. Above the threshold, the flow dispersion will be switched from MFD to D8 in order to represent in a more realistic way the channelization of the water.
Whitebox GAT
Whitebox Geospatial Analysis Tools (GAT) is an open-source GIS designed to provide a platform for the rapid development and testing of experimental geospatial analysis methods, supported by its extensible design, integrated facilities for custom plug-in tool authoring, and its.
open-access design philosophy. One of the unique characteristics of Whitebox GAT is the ease with which users can inspect and modify the algorithms for individual geoprocessing tools. The open-access software model that Whitebox GAT adopts is designed to lessen the barriers that are often imposed on end-users when attempting to gain deeper understanding of how a specific function operates. It is a portable application running on the Java Virtual Machine; it is developed using a combination of programming languages targeting the Java Runtime Environment (JRE) including Java, Groovy, Jython (the Python implementation for Java), and Javascript. The Whitebox GAT project began in 2009 through the development efforts of researchers at the University of Guelph, Canada. The project was conceived as a replacement for the Terrain Analysis System, a freeware software package with an emphasis on analysis of digital elevation data [
22].
To calculate the TWI with Whitebox GAT the user has to follow a step-by-step approach. Firstly, the DEM has to be hydrologically corrected; the user can choose between the
Fill Depression (Wang & Liu) tool, the
Fill Depression (Planchon & Darboux) and the
Breach Depression tool (which is the recommended one by the developers). Then, the corrected DEM can be used to obtain the flow accumulation map. If the desired flow routing algorithm is the Dinf, D8 or Rho8, the user has to run respectively the
Dinf Flow Pointer,
D8 Flow Pointer or
Rho8 Flow Pointer tool first; the maps obtained can be used as input for the
D-infinity Flow Accumulation tool or for the
D8 and Rho8 Flow Accumulation tool, specifying the specific catchment area (SCA) as output type, to calculate the SCA [
10,
38]. If the user wants to use the TMFD (here called MDinf) or the MFD (here called FD8) flow routing algorithm, the SCA can be directly calculated using the corrected DEM as input for the
MDinf Flow Accumulation or the
FD8 Flow Accumulation tool, selecting as output type the specific catchment area [
34]. The latter tools give also the possibility to set a non-dispersive threshold. This threshold establishes the flow accumulation value above which flow dispersion is no longer allowed. Those grid cells with flow-accumulation values above this limit will have their flow routed with a single-flow-direction algorithm similar to the D8 algorithm. Therefore, the flow will be routed entirely to the steepest downslope neighbouring cell. This, under the assumption that flow dispersion is not realistic once flow becomes channelized, while it is opportune on hillslope areas. In addition, the user can set a different exponent parameter, the default value is 1, to modify the amount of flow dispersion.
The slope map can be computed using the
Slope tool, which uses the Horn’s method to estimate the slope, taking as input the DEM [
17]. If the DEM has different vertical and horizontal units, the user can specify a Z conversion factor.
Finally, the tool to extract the TWI map is called Wetness Index, which needs as input the SCA map and the slope map.
Saga GIS
The System for Automated Geoscientific Analyses (SAGA) is an open source GIS, that since its first release in 2004 has rapidly developed from a specialized tool for digital terrain analysis to a comprehensive and globally established GIS platform for scientific analysis and modeling. It is a portable application, so it does not require installation, coded in C++ in an object-oriented design. SAGA has been designed for an easy and effective implementation of spatial algorithms and hence serves as a framework for the development and implementation of geoscientific methods and models. This is possible thanks to an application programming interface (API). In 2005 the SAGA User Group Association was founded to support a sustainable long-term development covering the whole range of user interests. Since 2007, the core development group of SAGA has been situated at the University of Hamburg, coordinating and actively driving the development process [
6].
To extract the TWI with SAGA, it is possible to follow two different paths: the step-by-step path or the one step tool. The first step is the pre-processing of the DEM, which can be done with three different tools: the
Fill Sinks (Wang & Liu) tool, the
Fill Sinks (Planchon & Darboux) tool and the
Sink Removal tool [
28,
41].
All of them fill all the depression of the DEM and give as output a depressionless DEM, which can be used as input to compute a Flow Accumulation map. The user can also specify a minimum slope value, that is the minimum slope angle that has to be preserved from cell to cell, or a threshold for the maximum depth of a sink to be considered for removal.
The tools that can be used to extract the flow accumulation map are three: Flow Accumulation (Top-Down), Flow Accumulation (Recursive), Flow Accumulation (Flow-Tracing).
The
Flow Accumulation (Top-Down) tool (Fig.
4c) needs as input the filled DEM and gives as output the map of the flow accumulation, computed by processing the DEM from the highest cell to the lowest cell. This tool allows to choose the output unit between the cell area and the number of cells, and to set a threshold for the linear flow, i.e. the possibility to apply a linear flow routing algorithm (D8) to those cells having a flow accumulation greater than an user defined threshold. The flow routing algorithm implemented in the tool are: D8, Braunschweiger Reliefmodell, Rho 8, MFD, Deterministic Infinity (Dinf), Triangular Multiple Flow Direction (TMFD) and Multiple Flow Direction based on Maximum Downslope Gradient (MFD-md) [
2,
10,
32,
34,
38]. If the MFD algorithm or the TMFD algorithm are chosen, it is possible to define the convergence factor, which default value is 1.1.
The second possible tool is the
Flow Accumulation (Recursive) tool (Fig.
4d). The input required is the filled DEM and the output is the flow accumulation map. This tool processes recursively all upwards connected cells until each cell of the DEM has been processed. The flow routing algorithms that can be choose are: D8, Rho 8, MFD and Dinf. The user can choose to express the flow accumulation in cell area or in number of cells.
The third available tool is the
Flow Accumulation (Flow Tracing) (Fig.
4e): it traces the flow of each cell in a DEM separately until it leaves the DEM or ends in a sink. As for the other tools the user can choose the unit (between cell area and number of cells) and the flow routing method among the kinematic routing algorithm [
21], DEMON [
7] and Rho8. For the Rho8 method the tool adopts the original algorithm only for the flow routing and that will give different results. It is also possible to choose to apply a flow correction.
From the flow accumulation map, which is none but the total catchment area (TCA), it is possible to compute the SCA using the
Flow Width and Specific Catchment Area tool (Fig.
4f). This tool needs as input the filled DEM and the Flow Accumulation map. They are generated by one of the previously described tools. The outputs are the Flow Width and the SCA, needed for the extraction of the TWI. The available methods to calculate these outputs are the following: D8, MFD and Aspect.
To generate the TWI it is also necessary to compute the slope map: the dedicated tool is called
Slope, Aspect, Curvature (Fig.
4g). This tool allows the generation of the slope, aspect and curvature map, taking as input the DEM. The user can choose the method to compute the slope map among the following models: maximum slope - Travis, maximum triangle slope - Tarboton, least squares fitted plane - Horn\Costa-Cabral, 6 parameter 2nd order polynom - Evans, 6 parameter 2nd order polynom - Heerdegen, 6 parameter 2nd order polynom - Bauer, 9 parameter 2nd order polynom - Zevenbergen and 10 parameter 3rd order polynom - Haralick [
2,
7,
9,
14,
15,
17,
38,
40,
43]. The user can also define the unit of the output among radians, degree and percent; in this case, it is important to leave the unit of output in radians, otherwise it will lead to errors in the TWI calculation.
The last step corresponds to the generation of the TWI by mean of the Topographic Wetness Index (TWI) tool. This tool requires as inputs the slope and the SCA maps. It is also possible to give as input the slope and the TCA, but in that case the user has to set the area conversion to “1/cell size (pseudo specific catchment area)”; however, it is better to provide directly as input the SCA. The user can also choose between two options to generate the TWI: the Standard method and the TOPMODEL method.
Furthermore, SAGA GIS offers the possibility to compute the TWI in a single step using the Topographic Wetness Index (One Step) tool. This tool only needs as input the DEM and the user just has to set the preferred flow routing algorithm among the following: D8, Braunschweiger Reliefmodell, Rho 8, MFD, Dinf, TMFD and MFD-md. This tool directly gives as output the TWI map. The Topographic Wetness Index (One Step) tool is a tool chain, in which the DEM is preprocessed with the Sink Removal tool and the slope is computed using the Slope, Aspect, Curvature tool, then the TWI is computed using the following tools in sequence: Flow Accumulation (Top-Down), Flow Width and Specific Catchment Area, Topographic Wetness Index (TWI). The user has only to specify the algorithm used to route the flow in the flow accumulation computation, all the other settings are left as default: this means, for example, that the threshold for the linear flow is always set to 500.
In addition, it is worth mentioning the
SAGA Wetness Index tool, which computes the Topographic Wetness Index as before, but modifies the calculation of the catchment area. This method does not consider the flow as a very thin film. Consequently, the cells located in valley floors, with a small vertical distance to a channel, result in a more realistic soil moisture potential compared to the TWI calculated with the standard catchment area [
3].