## Description

This tool calculates the cartographic depth-to-water (DTW) index described by Murphy et al. (2009). The DTW index has been shown to be related to soil moisture, and is useful for identifying low-lying positions that are likely to experience surface saturated conditions. In this regard, it is similar to each of wetness_index, elevation_above_stream (HAND), and probability-of-depressions (i.e. stochastic_depression_analysis).

The index is the cumulative slope gradient along the least-slope path connecting each grid cell in an input DEM (`dem`) to a surface water cell. Tangent slope (i.e. rise / run) is calculated for each grid cell based on the neighbouring elevation values in the input DEM. The algorithm operates much like a cost-accumulation analysis (cost_distance), where the cost of moving through a cell is determined by the cell's tangent slope value and the distance travelled. Therefore, lower DTW values are associated with wetter soils and higher values indicate drier conditions, over longer time periods. Areas of surface water have DTW values of zero. The user must input surface water features, including vector stream lines (`streams`) and/or vector waterbody polygons (`lakes`, i.e. lakes, ponds, wetlands, etc.). At least one of these two optional water feature inputs must be specified. The tool internally rasterizes these vector features, setting the DTW value in the output raster to zero. DTW tends to increase with greater distances from surface water features, and increases more slowly in flatter topography and more rapidly in steeper settings. Murphy et al. (2009) state that DTW is a probablistic model that assumes uniform soil properties, climate, and vegetation.

Note that DTW values are highly dependent upon the accuracy and extent of the input streams/lakes layer(s).

## References

Murphy, PNC, Gilvie, JO, and Arp, PA (2009) Topographic modelling of soil moisture conditiTons: a comparison and verification of two models. European Journal of Soil Science, 60, 94–109, DOI: 10.1111/j.1365-2389.2008.01094.x.

## See Also

wetness_index, elevation_above_stream, stochastic_depression_analysis

## Function Signature

`def depth_to_water(self, dem: Raster, streams: Optional[Vector] = None, lakes: Optional[Vector] = None) -> Raster: ...`

## Project Links

WbW Homepage User Manual Support WbW