This tool is used to generate a flow accumulation grid (i.e. contributing area) using the FD8 algorithm (Freeman, 1991), sometimes referred to as FMFD. This algorithm is an examples of a multiple-flow-direction (MFD) method because the flow entering each grid cell is routed to each downslope neighbour, i.e. flow divergence is permitted. The user must specify the name (`dem`) of the input digital elevation model (DEM). The DEM must have been hydrologically corrected to remove all spurious depressions and flat areas. DEM pre-processing is usually achieved using either the breach_depressions_least_cost (also breach_depressions_least_cost) or fill_depressions tool. A value must also be specified for the exponent parameter (`exponent`), a number that controls the degree of dispersion in the resulting flow-accumulation grid. A lower value yields greater apparent flow dispersion across divergent hillslopes. Some experimentation suggests that a value of 1.1 is appropriate (Freeman, 1991), although this is almost certainly landscape-dependent.

In addition to the input DEM, the user must specify the output type (`out_type`). The output flow-accumulation can be 1) `cells` (i.e. the number of inflowing grid cells), `catchment area` (i.e. the upslope area), or `specific contributing area` (i.e. the catchment area divided by the flow width. The default value is `cells`. The user must also specify whether the output flow-accumulation grid should be log-tranformed (`log`), i.e. the output, if this option is selected, will be the natural-logarithm of the accumulated flow value. This is a transformation that is often performed to better visualize the contributing area distribution. Because contributing areas tend to be very high along valley bottoms and relatively low on hillslopes, when a flow-accumulation image is displayed, the distribution of values on hillslopes tends to be 'washed out' because the palette is stretched out to represent the highest values. Log-transformation provides a means of compensating for this phenomenon. Importantly, however, log-transformed flow-accumulation grids must not be used to estimate other secondary terrain indices, such as the wetness index, or relative stream power index.

The non-dispersive threshold (`threshold`) is a flow-accumulation value (measured in upslope grid cells, which is directly proportional to area) above which flow dispersion is no longer permitted. Grid cells with flow-accumulation values above this threshold will have their flow routed in a manner that is similar to the D8 single-flow-direction algorithm, directing all flow towards the steepest downslope neighbour. This is usually done under the assumption that flow dispersion, whilst appropriate on hillslope areas, is not realistic once flow becomes channelized.

## Reference

Freeman, T. G. (1991). Calculating catchment area with divergent flow based on a regular grid. Computers and Geosciences, 17(3), 413-422.

## See Also

`D8FlowAccumulation`, quinn_flow_accumulation, qin_flow_accumulation, `DInfFlowAccumulation`, `MDInfFlowAccumulation`, rho8_pointer

## Function Signature

`def fd8_flow_accum(self, dem: Raster, out_type: str = "sca", exponent: float = 1.1, convergence_threshold: float = float('inf'), log_transform: bool = False, clip: bool = False) -> Raster: ...`

## Project Links

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