Resource Notes:

• Automated Drainage Network and Watershed Delineation from Digital Elevation Data

Software Downloads:

Instructions for use: Place the AML programs in your AML directory or in the directory you are running the analysis in. The programs share various intermediate products. Since these maps are important they are retained in the workspace saving time that would be needed to reproduce them each time. However, the user should be careful to apply all the programs to the entire project area, and keeps studies of portions of the project area in separate workspaces (directories). This is because the retained intermediate maps have default names, and most programs provided here test for their existence. If a map with a specific default name already exists in the workspace but is for another area, running the AML will give faulty, partial, or null results.

&r STREAMNETWORK <input DEM> <flow accumulation threshold>

This Arc Macro Language (AML) program models stream networks to create drainage pattern grids and coverages from digital elevation data. It first removes all the internal drainages from the terrain model. Then it routes the flow of water finding a flow direction for every cell of the elevation grid following the steepest paths. Flows from cell to cell on the same flowpath result in a flow accumulation map in which the value for each cell represents the accumulated flow along a particular path. We can use flow accumulation map to delineate stream networks by setting a lower limit to flow accumulations value, i.e. a flow accumulation threshold. The flow accumulation threshold represents the area (flow accumulation threshold value x cell area) which has to drain before the streams begin to be delineated. This AML creates a number of maps which then can be used by other terrain analysis AMLs without having to be remade. It is therefore necessary to allow for enough space (at least 60 times the size of the elevation grid) in the directory where terrain products are being generated, and to keep close check that the area covered by the default maps represents our most recent project area we have been working with. The AML also generates stream order information (Strahler and Shreve schemes) for each stream segment. This introduces the limitation that if a map containing more than 100,000 stream segments is being modeled portions of the program will not work. REACHSHEDMODEL.AML offers a simplified method of delineating streams which is not impacted by this problem.

&r INTERACTSHEDS <input DEM> <output shed> <POLY (outlet)|POINT(outlet)>

This ARC MACRO LANGUAGE program delineates watersheds based on graphic user input of either a point outlet or a polygonal outlet area. Since all values in cell processing are tied to a cell, which is always an area, the "point" represents a single cell outlet, while a "polygon" represents an area of any shape which makes a multi-cell outlet (e.g. a lake or reservoir, or a rectangular dam). Furthermore, for "point" outlets the program SNAPPOUR moves the outlet to fall on a drainage that is within a specified distance in map units from the user placed the outlet. In other words, since it can be difficult to interactively place the outlet so that it falls on the drainage modeled from a DEM, SNAPPOUR adjusts the location of the outlet to the nearest location with high flow accumulation level (drainage). If the user specifies "0" for the snap distance SNAPPOUR will not change the location of the "point" outlet.

&r REACHSHEDMODEL <input DEM> <flow accumulation threshold>

This AML program delineates watersheds for watershed outlets located at stream junctions for a stream network started at a user specified flow accumulation level (equivalent to drainage area). The stream network is delineated using a slightly simplified version of the same process as described for STREAMNETWORK.AML. The operation is performed on a filled DEM which provides for continuous distribution of water with no internal drainages. (The "z" suffix on product maps indicates that this operation was performed on a filled DEM.) Then the nodes, or intersection points of all the streams in the line drainage network coverage, are converted into a point coverage. The point coverage also includes all the starting points ("sources") of the delineated stream networks. This point coverage is then transformed into a grid of outlet locations for catchment delineation using the ARC GRID WATERSHED function.

&r POLYGONSHEDS <watersheds_to_be_filtered> <output_shed_grid> <area_threshold>

Converts a grid of modeled watersheds into a polygonal coverage by providing an area filter which eliminates unnecessary polygons created during the watershed delineation process. The problem with vectorizing a watershed grid modeled from digital terrain data and making a polygonal coverage out of it are what could be referred to as "slivers", which is something one wouldn't expect when source data is in grid or raster format. When GRID creates watersheds it does so basing their boundaries on the location of the watershed outlet, cells flowing into it, and the location of the next watershed outlet. The program "grows" watersheds following flow direction until all the cells that flow into any particular outlet are included. This can result in strings of usually two or three cells attached at the edge of the greater watershed area. When this grid is vectorized these individual cells, attached to the greater watershed or often to each other diagonally by their corners, become separate polygons which create many tiny new vector "watersheds". This AML provides a simple way to remove the diagonally adjacent cells by setting an area filter for individual regions (using ARC GRID REGIONGROUP function) and removing ones that fall below the user defined area threshold value (usually not more than 20000 square meters).

&r STREAMLINESHED <in_elev_grid> <in_stream_cover> <out_watersheds> <snap_distance>

This AML delineates watersheds for watershed outlets located at junctions and sources of a stream line (vector) ARC/INFO coverage. The program first creates basic hydrologic terrain analysis grid layers of a filled DEM, flowdirection map for the filled DEM, and a flowaccumulation map. Then it uses a line vector stream map to identify the endpoints and points of intersection, or nodes of stream arcs, and makes them into a point coverage. The point coverage is then rasterized to serve as outlets for watershed delineation. SNAPPOUR function is used to locate the streams in high flow accumulation areas (channels) modeled from the DEM within user specified distance in number of cells. Then the GRID WATERSHED function calculates the watershed areas based on the outlets and the flow direction map.