Stream Drop Analysis

Title  Stream Drop Analysis

Summary

Applies a series of thresholds (determined from the input parameters) to the input accumulated stream source grid (*ssa) grid and outputs the results in the *drp.txt file the stream drop statistics table. This function is designed to aid in the determination of a geomorphologically objective threshold to be used to delineate streams. Drop Analysis attempts to select the right threshold automatically by evaluating a stream network for a range of thresholds and examining the constant drop property of the resulting Strahler streams. Basically it asks the question: Is the mean stream drop for first order streams statistically different from the mean stream drop for higher order streams, using a T-Test. Stream drop is the difference in elevation from the beginning to the end of a stream defined as the sequence of links of the same stream order. If the T test shows a significant difference then the stream network does not obey this "law" so a larger threshold needs to be chosen. The smallest threshold for which the T test does not show a significant difference gives the highest resolution stream network that obeys the constant stream drop "law" from geomorphology, and is the threshold chosen for the "objective" or automatic mapping of streams from the DEM. This function can be used in the development of stream network rasters, where the exact watershed characteristic(s) that were accumulated in the accumulated stream source grid vary based on the method being used to determine the stream network raster.


Usage

Command Prompt Syntax:

mpiexec -n <number of processes> DropAnalysis -fel <felfile> -p <pfile> -ad8 <ad8file> -ssa <ssafile> -drp <drpfile> -o <outletfile> [ -lyrname <layer name>] [ -lyrno <layer number>] [ -par 5 500 10 0]

ad8file: File name for D8 contributing area grid 'ad8'

pfile: File name for D8 flow direction grid 'p'

felfile: File name for hydrologically correct elevation grid 'fel'

ssafile: File name for an accumulated stream source grid suitable for drop analysis. This needs to have

the property that it is monotonically increasing downslope along D8 flow directions.

outletfile: input outlets file (OGR readable dataset)

layer name: OGR layer name if outlets are not the first layer in outletfile (optional)

layer number: OGR layer number if outlets are not the first layer in outletfile (optional)

Layer name and layer number should not both be specified.

dropfile: File name where stream drop analysis output is to be written as txt

par: Array of parameters consisting of:

- threshmin: Minimum value of threshold to be used in drop analysis

- threshmax: Maximum value of threshold to be used in drop analysis

- nthresh: Number of drop thresholds to be used in drop analysis

- steptype: Type of threshold step to be used in drop analysis (0 = log, 1=arithmetic)

The default parameter values of min=5, max=500, nthresh=10, steptype=0 are assumed unless -par is

specified.


Syntax

StreamDropAnalysis (Input_Pit_Filled_Elevation_Grid, Input_D8_Flow_Direction_Grid, Input_D8_Contributing_Area_Grid, Input_Accumulated_Stream_Source_Grid, Input_Outlets, Minimum_Threshold_Value, Maximum_Threshold_Value, Number_of_Threshold_Values, Use_logarithmic_spacing_for_threshold_values, Input_Number_of_Processes, Output_Drop_Analysis_Text_File)

Parameter Explanation Data Type
Input_Pit_Filled_Elevation_Grid Dialog Reference

A grid of elevation values. This is usually the output of the "Pit Remove" tool, in which case it is elevations with pits removed. Pits are low elevation areas in digital elevation models (DEMs) that are completely surrounded by higher terrain. They are generally taken to be artifacts of the digitation process that interfere with the processing of flow across DEMs. So they are removed by raising their elevation to the point where they just drain off the domain. This step is not essential if you have reason to believe that the pits in your DEM are real, but with drop analysis ensures that the elevation drop downstream along a stream is positive.

 Raster Layer
Input_D8_Flow_Direction_Grid Dialog Reference

A grid of D8 flow directions which are defined, for each cell, as the direction of the one of its eight adjacent or diagonal neighbors with the steepest downward slope.

 Raster Layer
Input_D8_Contributing_Area_Grid Dialog Reference

A grid of contributing area values for each cell that were calculated using the D8 algorithm. The contributing area for a cell is the sum of its own contribution plus the contribution from all upslope neighbors that drain to it, measured as a number of cells or the sum of weight loadings. This grid can be obtained as the output of the "D8 Contributing Area" tool. This grid is used in the evaluation of drainage density reported in the stream drop table.

 Raster Layer
Input_Accumulated_Stream_Source_Grid Dialog Reference

This grid must be monotonically increasing along the downslope D8 flow directions. It it compared to a series of thresholds to determine the beginning of the streams. It is often generated by accumulating some characteristic or combination of characteristics of the watershed with the "D8 Contributing Area" tool, or using the maximum option of the "D8 Flow Path Extreme" tool. The exact method varies depending on the algorithm being used.

 Raster Layer
Input_Outlets Dialog Reference

A point feature defining the outlets upstream of which drop analysis is performed.

 Feature Layer
Minimum_Threshold_Value Dialog Reference

This parameter is the lowest end of the range searched for possible threshold values using drop analysis. This technique looks for the smallest threshold in the range where the absolute value of the t-statistic is less than 2. For the science behind the drop analysis see Tarboton et al. (1991, 1992), Tarboton and Ames (2001).

 Double
Maximum_Threshold_Value Dialog Reference

This parameter is the highest end of the range searched for possible threshold values using drop analysis. This technique looks for the smallest threshold in the range where the absolute value of the t-statistic is less than 2. For the science behind the drop analysis see Tarboton et al. (1991, 1992), Tarboton and Ames (2001).

 Double
Number_of_Threshold_Values Dialog Reference

The parameter is the number of steps to divide the search range into when looking for possible threshold values using drop analysis. This technique looks for the smallest threshold in the range where the absolute value of the t-statistic is less than 2. For the science behind the drop analysis see Tarboton et al. (1991, 1992), Tarboton and Ames (2001).

 Double
Use_logarithmic_spacing_for_threshold_values Dialog Reference

This checkbox indicates whether logarithmic or linear spacing should be used when looking for possible threshold values using drop ananlysis.

 Boolean
Input_Number_of_Processes Dialog Reference

The number of stripes that the domain will be divided into and the number of MPI parallel processes that will be spawned to evaluate each of the stripes.

 Long
Output_Drop_Analysis_Text_File Dialog Reference

This is a comma delimited text file with the following header line:

Threshold, DrainDen, NoFirstOrd, NoHighOrd, MeanDFirstOrd, MeanDHighOrd, StdDevFirstOrd, StdDevHighOrd, T

The file then contains one line of data for each threshold value examined, and then a summary line that indicates the optimum threshold value. This technique looks for the smallest threshold in the range where the absolute value of the t-statistic is less than 2. For the science behind the drop analysis, see Tarboton et al. (1991, 1992), Tarboton and Ames (2001).

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