Geomorphometric analysis is the measurement of the three-dimensional geometry of landforms and has traditionally been applied to watersheds, drainages, hillslopes, beaches, and other groupings of terrain features. In particular basin morphometric parameters have received a lot of attention from hydrologists and geomorphologists since watersheds (catchments) have been used for analysis of various physical ecosystem processes, including soil erosion, deposition, runoff, stream discharge, sediment yield, sedimentation of streams, irradiation by sunlight, evaporation, evapotranspiration, and nutrient distribution. Currently the USGS (Harvey and Eash, 1996) is in the process of developing, in association with private enterprise, GIS software to precisely calculate a variety of catchment morphometric parameters which include variables such as average basin slope, the basin elongation ratio, compactness ratio, basin relief, stream density ( http://www.basinsoft.com ). Some parameters, such as slope length or stream sinuosity, can be used to represent both basin and hillslope, or basin and stream channel properties.

Basin morphometry represents one set of variables recommended for use in watershed hydrologic condition analysis methods developed by the Department of Agriculture and Department of the Interior (McGammon, Rector, and Gebhardt, 1998). Because by contrast with the traditional topographic map based methods, the GIS methods are relatively easy to apply in a consistent way on large areas of landscape, and because they permit summation of terrain characteristics for any region, they can be utilized to provide geomorphometric data and therefore insight into the processes affected by terrain morphology for all types of mapping units.

Resource Notes:

• The Spatial Terrain Analysis Resource Toolset (START)
• Tools to Models: Some Applications of Automated Resource Analysis Tools for Ecosystem Management.

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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 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. This is because the retained intermediate maps have default names, and all programs 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 results.

FOCALCURVATURE.AML, C program, and Documentation

The FOCALCURVATURE program can be used to evaluate, using a negative average slope technique, whether a particular point in the landscape is in a concave, flat, or convex location (Blaszczynski, 1997). negative slope technique involves analyzing averages of slope gradient values within a specified neighborhood (focus) while treating elevation differences as signed values (slope gradient generation methods usually use the absolute value of the elevation difference or rise).  The easiest and most accurate way to use the program is using a 3x3 default roving window which provides most detail. An interesting feature of this tool is that unlike ARC GRID curvature commands it permits generalized view of the landscape. The output is fractional and can be multiplied by 100 to obtain units of average slope percent.  Unfortunately the program is currently available only in uncompiled format, although a compiled version can be provided for the Unix AIX machines upon request. In the near future we intend to provide compiled versions for the NT workstations.  The program can be used in conjunction with ARC/INFO terrain hydrologic analysis techniques to perform fourth-order-of-relief analysis (Blaszczynski, 1997) and provides several additional diagnostic maps.

&r SHEDSTATS input watersheds input DEM

The program SHEDSTATS.AML is misnamed somewhat because it can produce elevation derivative data about any class in a discrete grid. However, all of this information makes most sense in the catchment context and so we kept the name. For a watershed, or any zone (a discrete class) in a zone grid SHEDSTATS calculates the maximum, minimum, range, mean, and standard deviation for the following topographic parameters:

• elevation (ELEV_);
• slope gradient (SLOPE_);
• slope length (LENGTHA_) and flow accumulation (FLOWA_) for an unfilled (original) DEM;
• slope length (LENGTHZ_) and flow accumulation (FLOWZ_) for a DEM that had its depressions filled;
• aspect (ASPECT);
• terrain ruggedness index (RUGGED_); and
• the Revised Universal Soil Loss Equation (RUSLE) terrain factor calculated using two different methods (LSFACT1_ and LSFACT2_).

The data obtained using the SHEDSTATS program are attached to each watershed, class, or zone as their attributes. These topographic attributes can then be used to characterize the terrain morphology contained in the zone. Since it’s possible to do this for any type of a zone, it can be done for other types of class maps such as vegetation, soils, and geology providing summaries of terrain characteristics for these regions. Through simple copying, pasting, and few corrections it is possible to add capabilties to this AML so that it summarizes variables for other continuous maps such as surface temperature or precipitation.

&r RUGGEDNESS <input DEM>

The terrain ruggedness index (TRI) is a measurement developed by Riley, et al. (1999) to express the amount of elevation difference between adjacent cells of a digital elevation grid. The process essentially calculates the difference in elevation values from a center cell and the eight cells immediately surrounding it. Then it squares each of the eight elevation difference values to make them all positive and averages the squares. The terrain ruggedness index is then derived by taking the square root of this average, and corresponds to average elevation change between any point on a grid and it’s surrounding area. The authors of the TRI propose the following breakdown for the values obtained for the index where:

• 0-80 m is considered to represent a level terrain surface;
• 81-116 m represents nearly level surface;
• 117-161 m a slightly rugged surface;
• 162-239 m an intermediately rugged surface;
• 240-497 m a moderately rugged;
• 498-958 m a highly rugged; and
• 959-4367 m an extremely rugged surface.

From running the AML on several regions in the Rocky Mountains this author found that the procedure rarely produced values over 400 meters. The main value of this measurement when compared to it's cousin, slope gradient measurement, is that it gives a relatively accurate view of the vertical change taking place in the terrain model from cell to cell. While the slope gradient map provides data on the steepness of a hillslope (rise / run), terrain ruggedness index provides data on the relative change in height of the hillslope (rise), such as side of a canyon.

&r TOPOAREA <input DEM > <class grid> <cell size>

This routine estimates the topographically correct area values for particular classes or geographic zones within your project area utilizing slope gradient map generated from a Digital Elevation Model (DEM) for the project area. The theory behind it is that the slope gradient for each particular cell can be understood as directly proportional to the elongation of the cell in one direction. This elongation can be calculated using trigonometric equation:

H (elongated side) = B (current cell size) / cosine S  (slope gradient per cell)

and the new area of the cell which accounts for it's slope can be calculated as:

topographically corrected area estimate per cell (At) = B x H

while the existing area estimate per cell (no topo correction) = B x B. For any particular zone we can use the ARC GRID ZONALSUM function to add the areas for each cell of a particular class or zone. This AML produces a number of default maps which can then be inspected to better understand the process. Since the very high slope values for near vertical slope gradient angles can produce unrealistic estimates of surface area, the most accurate interpretation of the difference between map view value and topographically corrected values comes from area summaries for grid classes, such as watersheds.  When summarized for some microbasins in the Rocky Mountains, the highest ratio of corrected to the uncorrected (flat) area values appeared to be 1.4.

The value of this tools should be immediately apparent especially to hydrologists, who often use flat map measurements to calculate areas of hydrologic units used for modeling. Terrain corrected estimates should help any hydrologic model results that rely on area calculations as one of the variables.