|
Excel Spreadsheets for Statistically
Analyzing Pebble Count Data
Two
Excel 2000 spreadsheet-workbooks have been developed to assist with
the statistical analysis of pebble count data. One is designed
to assist with the proper implementation of the zig-zag pebble count
procedure, while the other can be used to perform similar analyses
on pebble count data tallied by size class. The thrust of each
analysis is to identify shifts in the fine gravel and smaller portions
of the distribution, rather than the median.
Zig-Zag
Pebble Count Analyzer
The
Zig-Zag Pebble Count Analyzer was developed by Greg Bevenger, Forest
Hydrologist, Shoshone National
Forest,
and Rudy King, Station Statistician, Rocky Mountain Research Station,
to help users properly implement the zig-zag pebble count procedure
(Bevenger and King, 1995, A pebble count
procedure for assessing cumulative watershed effects. Rocky Mountain Forest and Range Experiment Station Research
Paper RM-RP-319, 17 pages). The zig-zag
method is a pebble count procedure using a zig-zag sampling pattern
along a longitudinal stream reach such that a stream is sampled
along a continuum instead of an individual site, reach, or cross-section.
By doing this, numerous meander bends and all associated habitat
features are sampled as an integrated unit rather than as individual
cross-sections.
Macro
enabled worksheets are provided to help users: (1) estimate sample
size, (2) enter field data, (3) produce tables and graphs, (4)
perform statistical analysis using contingency tables and the Pearson
chi-squared statistic, and (5) make notes. The spreadsheet-workbooks
also contain case studies to illustrate typical application of the
procedure and provides examples of typical analysis scenarios. The intent
is to assist users with the development of study plans and to help
them interpret results.
Figure
1 illustrates typical contingency table output. Additional information
about the statistical analysis is included in the spreadsheet to
help users properly interpret the results. Complete description
of the statistics is included in RM-RP-319 and users are cautioned
to become thoroughly familiar with the paper before proceeding.
Figure 2 shows example tabular and graphic output from one of the
case studies. The graphs show the departure of the study pebble
count from the reference pebble count, particularly for the smaller
size classes.
Figure
1. Example contingency tables produced
by the analyses spreadsheet. Summary data are presented as (1)
total number of pebbles counted that are less than and greater than
the designated particle size criterion and (2) percentage of pebbles
counted that are less than the designated particle size criterion.
Also presented is a p-value for each particle size criterion. A
small p-value (for instance, less than 0.05) indicates that the
proportion of particles less than the criterion is probably different
between your reference and study reaches.
Figure
2. An example of particle size tabulations and particle size distribution
and histogram plots automatically generated by the analysis spreadsheets.
Size-Class
Pebble Count Analyzer
The Size-Class Pebble
Count Analyzer was developed to allow analysis of pebble count data
tallied by size class. This spreadsheet was developed by John Potyondy,
Hydrologist, Stream Systems Technology Center, and Kristin Bunte, Fluvial Geomorphologist, Colorado State University,
to be similar to that developed by Bevenger and King. The size-class
pebble count analyzer assumes that sampling is geomorphically stratified
based on the natural sorting of grain sizes into distinct channel
features to sample homogeneous populations. Depending on study
objectives, this may involve sampling in riffles, pools, or combinations
of riffles and pools. The intermediate axis is measured with a
ruler or a gravel template (gravel-o-meter) and tallied into standard
Wentworth size classes. Potyondy and Bunte strongly recommend the
use of templates because they avoid incorrectly identifying the
intermediate axis and have been shown to reduce error among observers.
Additional information
about pebble counts is available in Bunte, K. and S.R. Abt, 2001,
Sampling surface and subsurface particle-size distributions in wadable
gravel– and cobble-bed streams for analyses in sediment transport,
hydraulics, and streambed monitoring. Rocky Mountain Research Station
RMRS-GTR-74, 428 pages.
Download
the Spreadsheets
Zig-Zag
Pebble Count Analyzer (v1)
Size-Class
Pebble Count Analyzer (v1)
If you have questions
about or need assistance using the size-class spreadsheets, contact
John Potyondy, (970) 295-5986, jpotyondy@fs.fed.us. If you have
questions about the zig-zag spreadsheets, contact Greg Bevenger,
(307)-527-6241, gbevenger@fs.fed.us.
Rocky
Mountain Research Station publications RM-RP-319 and RMRS-GTR-74
are available from RMRS Publications Distribution: (970) 498-1392,
or e-mail rschneider@fs.fed.us.
| 
