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  • 16. Extrapolation

16. Extrapolation

The Extrapolation button opens a window that is similar to the extrap program (Mueller, 2013), except the extrapolation window has been redesigned with buttons instead of menus. QRev defaults to the extrapolation settings determined automatically from the extrap algorithms. The user can use the Extrapolation window to switch to or explore other extrapolation settings, apply manually selected extrapolation settings, and evaluate the sensitivity of the discharge to the extrapolation settings. The user can explore different extrapolation settings without applying them to the measurement if “Cancel” is selected to close the window.
 
16. Extrapolation
1

Data Panel

1. Data Panel
 
The normalized distance from the streambed is divided into 5 percent segments (0.05; from 0 to 1). A median value of the normalized unit discharge is then computed for each segment, and the result is assigned to the average normalized depth of the data in that segment. The location of the mean normalized depth for each segment and number of points used in each segment are reported in the Data Panel. To help ensure that the median profile is representative of the data and not overly influenced by a few points in the top or bottom segments, only median values with sufficient points to exceed a specified threshold are used in the computation of the extrapolation. The default threshold is set to 20 percent of the median number of points for all segments. The threshold can be changed by the user using the Change Threshold button in the Data panel. The median points for an individual transect are colored blue if the number of points in the segment exceeds the threshold amd red if they do not..
 
Change Threshold button  allows the user to change the cutoff threshold from the default value of 20 percent of the median number of points in all the segments to a user defined value.
 
Subsection button allows the user to look only at a subsection of the data by entering a lower and upper discharge range. The range is applied from the beginning of the transect independent of the starting bank. This range is only applied to the profile evaluation and does not affect the discharge sensitivity analysis. For example, if the user wanted to look at the profile for the center 50 percent of the discharge, 25 should be entered for the lower limit and 75 for the upper limit.
 
The drop down menu at the bottom allows the user to select discharge or velocity data. Discharge is the default and is recommended for all moving-boat measurements.
2

Profile Panel

2. Profile Panel
The primary panel in the extrapolation dialog window is the Profile panel. The graph in the profile panel displays the following data depending on the selections made in the check boxes below the graph.
 
Data (Gray Dots)—The raw data represent the normalized discharge or velocity in each depth cell for all transects selected in the Fit panel.    
Surface Cells (Green circles)—Cells that are the top cell for each ensemble.
Median Points (Blue/Red/Black Squares)—If the number of points in a segment does not exceed the threshold value, the median point is colored red and is not used in the computation of the extrapolation. Solid black squares represent the median values of the composite of all transects in the measurement.
Whiskers (Horizontal Blue/Red/Black Lines)—The whiskers on each median value represent the 25th and 75th percentile of all the data in that 5 percent increment. Thus, 50 percent of the data for that increment fall within the limits of the whiskers. Colors are representative of the type of median value.
Extrapolation (Solid Blue/Magenta/Black Line)—The extrapolation fits are color coded such that magenta lines represent transects collected in the left to right direction, blue lines represent transects collected in the right to left direction, and the black line represents the composite of all transects in the measurement.
 
The controls at the bottom of the graph are described below:
 
Depth Cell
Data—Turns on and off the display of the cell data (gray dots).
Surface Cells—Highlights the cells that are the top cell for each ensemble.
 
Transects
Medians—Turns on and off the display of the median points for each transect.
Fit—Turns on and off the display of the extrapolation fit for each transect.
 
Measurement
Median—Turns on and off the display of the median points for the composite measurement.
Fit—Turns on and off the display of the extrapolation fit for the composite measurement.
 
If the transect data are shown the data for transects moving from left to right are shown in magenta and those moving from right to left in blue.
 
3

Fit Panel

3. Fit Panel
The Fit panel provides the user the ability to control what data are shown in the Profile and Data panels and change the fit type, extrapolation methods, and exponent. The transect selection list shows the filename of the transects loaded and the composite for the measurement (“Measurement”). The default is for the composite measurement to be shown. To view a specific transect, click on the transect filename, and the graph and fit characteristics for that transect selected in the Profile Panel  will be shown. The fit characteristics of an individual transect can be changed; however, because QRev uses only one set of fit characteristics for the measurement, only the settings for “Measurement” are used in the discharge computation.
 
Fit—The fit popup menu allows the user to change the fit type from the default automatic algorithms to Manual. If Manual is selected, the Top, Bottom, Exponent, and slider will become active.
Top—The Top popup menu displays the top extrapolation method. This popup menu cannot be changed unless Fit is changed to Manual. The options are Power and Constant.
Bottom—The Bottom popup menu displays the bottom extrapolation method. This popup menu cannot be changed unless Fit is changed to Manual. The options are Power and No Slip.
Exponent—The Exponent edit box displays the exponent for the power or no slip extrapolations. The exponent cannot be changed unless Fit is changed to Manual. If fit is changed to manual, the exponent can be changed by manually typing a value in the edit box or by using the slider below the box to incrementally change the value.
 
4

Previous Extrapolation Setting

4. Previous Extrapolation Setting
The Previous Extrapolation Settings are the extrapolation settings that will be used when exiting with Cancel.
5

Discharge Sensitivity

5. Discharge Sensitivity
The sensitivity of the extrapolation method for the top and bottom extrapolation is evaluated by computing the discharge for each combination of top and bottom extrapolation methods with a default and with least squares fit exponents and reporting the percent difference from the one-sixth power/power fit. If a manual fit is used for the composite measurement, then an additional line will be added to the table that represents the manual fit. This table can be used to help determine the effect of extrapolation choices on the final discharge.
6

Graphics Toolbar

6. Graphics Toolbar
Allows user to zoom, pan, and query the graphs. For more information see Toolbar.
7

Add Comment

7. Add Comment
Allows the user to add a comment. For more information see Comment Button.
8

User's Guide

8. User's Guide
Display this User's Guide.
9

Cancel and Close

9. Cancel and Close
Cancels any changes so the extrapolation remains as displayed in Previous Extrapolation Setting. Closes the window and returns control to the main window.
10

Apply Changes and Close

10. Apply Changes and Close
Applys the extrapolation settings from the Fit Panel to the data. Closes the window and returns control to the main window.