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The DTM-ANALYSIS program is used to derive a distribution of ln(a/tanB) values from a regular raster grid of elevations for any catchment or subcatchment using the multiple direction flow algorithm of Quinn et al. (1995). Output from the program is a histogram of the distribution of the ln(a/tanB) values, a map file of ln(a/tanB) values that can be used for map output in the TOPMODEL programs There are three options in the program.

Topographic Index Distribution Calculation

This Topographic Index Distribution Calculation is the very simplest version of the algorithm in that while pits and sinks in the elevation matrix are identified, no attempt is made to create continuous flow pathways to the catchment outlet. Sinks should be removed by modifying the elevation data using the Automatic Sink Removal which uses successive averaging of surrounding elevations to resolve pits. This is simple but does handle the case of small river channels for which the elevation grid cannot resolve the continuous flow pathway.

This Topographic Index Distribution Calculation also requires that only elevations of points within the catchment are supplied, all other values in the matrix being set to greater than a value of 9999.0 (m). The Catchment Identification option can be used to cut out the catchment above a specified pixel using a hill climbing algorithm.

It is recommended that the elevation data should be 50 m resolution or better. It should be noted that the derived ln(a/tanB) distribution will be dependent on the resolution of the elevation data used and on the particular rules for distributing upslope areas and dealing with river channels that are smaller than the grid size. Different distributions may result in different effective parameter values for a given catchment.

Automatic Sink Removal

This option can be used to automatically correct for sinks and pits in an elevation array before running the Topographic Index Calculation option. The program will identify any local sinks (values surrounded on all nodes by equal or higher elevations) and will try to correct them by averaging the surrounding elevations. After each run through all the identified sinks, the program can check to see if there are any remaining sinks.

Note that this algorithm will not identify any large internal basins in the elevation map directly. However, such basins will always result in one local sink. Therefore by iterating through the program it should be possible to correct such features.

At the end of the option the new elevation data can be saved in a file. The output file is in exactly the same format as the input file. A separate file identifying all corrected sinks and changes in elevation values can also be saved.

Catchment Identification

The topographic index calculation requires that all elevation values outside the catchment be labelled as values of 9999. or greater. This option takes a raster elevation data file and identifies the catchment area draining to a specified point. The algorithm works by linking all points that are continuously upslope of the catchment outlet. At each iteration, the algorithm searches for points adjacent to those already identified as being in the catchment that are at an elevation greater than a specified threshold value. A value of zero may be specified for the threshold (i.e. adjacent points of equal elevation will be added to the catchment but this can cause problems is some elevation arrays that contain flat benches extending around catchment divides. If this is the case the original elevation interpolation should be improved, or the resolution of the elevation matrix should be degraded.