Height Above Nearest Drainage goes mainstream in QGIS and ArcGIS

Between 2008 and 2016 Camilo Rennó and Antonio Nobre and co-authors published a series of papers describing "Height Above Nearest Drainage" (HAND), an approach for measuring the height above a mapped stream channel. Their approach basically involves identifying watersheds for each cell in a stream and relating that the to the upslope area through a join.  HAND is defined, more of less, as the difference between the elevation of a grid cell and the downslope elevation where it flows into the stream.

In 2011 I created an ArcGIS Toolbox called the "Riparian Topography Toolbox" and added a tool to calculate HAND. Height Above Nearest Drainage differs "Height Above River", which is calculated as the difference in elevation between a grid cell and a weighted average of grid cells within a stream. The two methods are conceptually similar although HAR uses multiple grid cells within a river.  Hence, it is more appropriate for deriving vertical height for LIDAR DEMs rather than typ

ical 10 or 30 meter DEMs.  HAND, on the other hand, is expected to be more computationally-efficient and more appropriate on coarser-scale DEMs.

As of ArcGIS version 10.7.1, I noticed that ArcGIS is now including a tool called "Flow Distance" in the Hydrology Toolset in ArcToolbox. This tool has an option for calculating distance in either horizontal or vertical distances. The vertical option appears to be equivalent to HAND so I decided to check it out.  I ran the "Flow Distance" tool on some of my study watersheds to see what the output looks like. The image on the left shows five sample watersheds with streams in blue, pour points as red dots, and watershed boundaries in black. The output from the Flow Distance tool is shown as a yellow to green to blue color ramp.

The image on the right shows the output from Flow Distance after thresholding into two classes: above and below 15 meters vertical height above stream channels. Purples are the areas below 15 meters and tan are the areas above 15 meters. You can see the some watersheds tend to have narrower riparian areas than others.

QGIS has an equivalent version of this tool.  It is called Terrain Channels and it also has a vertical component.

ArcGIS Flow Distance - https://desktop.arcgis.com/
en/arcmap/latest/tools/spatial-analyst-toolbox/flow-distance.html

QGIS Terrain channels - https://docs.qgis.org/2.18/ en/docs/user_manual/processing_algs /saga/terrain_analysis_channels.html

Rennó, C. D., Nobre, A. D., Cuartas, L. A., Soares, J. V., Hodnett, M. G., & Tomasella, J. (2008). HAND, a new terrain descriptor using SRTM-DEM: Mapping terra-firme rainforest environments in Amazonia. Remote Sensing of Environment, 112(9), 3469-3481.

Nobre, A. D., Cuartas, L. A., Hodnett, M., Rennó, C. D., Rodrigues, G., Silveira, A., & Saleska, S. (2011). Height Above the Nearest Drainage – a hydrologically relevant new terrain model. Journal of Hydrology, 404(1-2), 13-29.

Nobre, A. D., Cuartas, L. A., Momo, M. R., Severo, D. L., Pinheiro, A., & Nobre, C. A. (2016). HAND contour: a new proxy predictor of inundation extent. Hydrological Processes, 30(2), 320-333.

Dilts, T. E., Yang, J., & Weisberg, P. J. (2010) Mapping Riparian Vegetation with Lidar Data Predicting plant community distribution using height above river and flood height. ArcUser Magazine - https://www.esri.com/news/arcuser/0110/mapping-with-lidar.html