Flow modelling tools

There are a number of tools available in Whitebox GAT that can be used to extract information about the distribution of water and other flow-related phenomena (e.g. sediment and nutrients) across a landscape. These tools utilize digital elevation model (DEM) based flow routing algorithms in order to trace the flow path from each grid cell in a raster DEM to an outlet point, usually at the edge of the DEM. Common applications include mapping the spatial distribution of upslope contributing areas, watershed boundaries, subbasins, and stream networks from DEM data. These data are also useful for calculating a number of secondary terrain attributes such as the topographic wetness index (used in the TOPMODEL hydrological framework) and the relative stream power index (sometimes used for modelling erosion and deposition patterns). Most of these applications require a DEM that has been hydrologically 'corrected' to remove topographic depressions and flat areas, which can artificially truncate flowpaths. Available tools for hydrological pre-processing DEM data include:

Many tools involving tracing flowpaths through a DEM grid require an input of a flow-pointer grid. A flow-pointer grid contains information about where the flow entering each grid cell should be directed, i.e. flow-direction data. In fact, there are several different ways of estimating the flow-pointer grid depending on the flow algorithm that is chosen. Although there are many different flow algorithms that have been developed, each can be classified as either being a single-flow-direction (SFD) algorithm or a multiple-flow-direction (MFD) algorithm depending on whether or not flow dispersion is permitted. MFD algorithms will route the flow enter a grid cell to one or more downslope neighbours. Available tools for creating a flow-pointer grid include:

The decision to use one algorithm over another depends largely on the application. For example, many applications that involve stream network analysis require the D8 SFD method because divergence of flow is relatively rare once water is channelized, greatly simplifying the analysis. When modelling the pattern of upslope contributing area or soil wetness on hillslopes, however, flow dispersion may well occur, in which case an MFD flow algorithm such as Dinf will likely provide more suitable results.

Many applications of flow modelling require the estimation of upslope contributing area. This is a flow accumulation operation. Most flow algorithm actually measure the flow accumulation to an area, i.e. a grid cell, rather than a point (interestingly points have no contributing areas because the are infinitely small). Because the pattern of contributing area is dependent on the grid resolution, it is common to normalize this pattern by the resolution, effectively yielding the upslope contributing area per unit contour length, called the Specific Catchment Area (SCA). Each of the above flow-pointer algorithms have corresponding flow-accumulation tools, including D8/Rho8, FD8, and Dinf.

In addition to flow accumulation, common operations that include flow modelling include watershed delineation and many stream network analysis operations.

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