Computerized statistical and graphical procedures were developed for use in U.S. Geological Survey (USGS) investigations of trend in stream water-quality data. These procedures, identified as EStimate TREND (ESTREND), are described in this paper to assist USGS investigators involved in multiple-station studies of water-quality trends. Additional discussion focuses on certain statistical and operational decisions required in multiple-station analysis of trends. The statistical methods used in ESTREND overcome common statistical problems encountered by conventional statistical trend techniques in the analysis of water-quality data. The problems include data that are non-normal and seasonally varying and water-quality records with missing values, "less-than" (censored) values, and outliers, all of which adversely affect the performance of conventional statistical techniques. Parametric and nonparametric statistical trend tests are used in ESTREND. A nonparametric method, the Seasonal Kendall test, is used for data that have few less-than values or data that have been censored at only one reporting limit. A parametric test for trend involving a maximum likelihood estimation method is used for data that have been censored at multiple reporting limits. The Seasonal Kendall test for uncensored data allows for the removal of flow variability in water-quality data which improves the performance of the statistical trend tests. Menu-driven procedures in ESTREND allow the user to easily retrieve water-quality data, analyze data for trend, and view tabular and graphical results of analyses.
Abstract
Introduction
Background
Purpose and Scope
Station and data characteristics
Geographic locations of stations
Sample collection
Period of record
Overview of statistical procedures for detection of trends in water quality data
Scope of the methods
Summary of the methods
Seasonal Kendall test for uncensored data
Seasonal Kendall test for censored data
Tobit test for censored data
Testing criteria
Statistical procedures for uncensored water-quality data
Seasonal Kendall test
Adjustment for seasonal effects in water-quality data
Selection of seasonal water-quality values
Selection of seasonal periods
Comparison of seasonal trends
Trend slope estimator
Interpretation of trend slopes
Trends in flow-adjusted concentration
Regression flow adjustment
LOWESS flow adjustment
Interpretation of flow-adjusted trends
Criteria for trend analysis of uncensored data
Estimates of moments and percentiles
Statistical procedures for censored water-quality data
Seasonal Kendall test
Criteria for trend analysis of censored data with the Seasonal Kendall test
Tobit test
Tobit trend slope estimate
Estimates of reporting limits in Tobit
Criteria for trend analysis of uncensored data with the Tobit test
Estimates of moments and percentiles
Data selection and management
ESTREND system
Overview of programs
Capabilities
Processes
Data files
Support files
Organization
Retrieval of water-quality data
Preparation of support files
Pathnames.file
Header.file
Param info.file
Season info.file
Constants.ins
Execution
Selection 1--make datafile
Selection 2--define seasons
Selection 3--run. seasonal comparisons
Selection 4--select best seasonal definition
Selection 5--select flow models or seasons
Selection 6--run trends
Selection 7--table results for a constituent
Selection 8--map results for a constituent
Selection 9--plot results
Selection 10--table results for all stations
Selection 11--table flow model information
Selection 12--table seasonal trend results
References
Appendix I: Subdirectory contents in ESTREND
The text and graphics are presented here in pdf format (print quality):
The full report is 1.07 MB
If you have Adobe Acrobat Reader installed on your computer, you may view and/or print the PDF version of this report. If you do not have Acrobat Reader, you may download it here.
| AccessibilityFOIAPrivacyPolicies and Notices | |
| |
|