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Time series analysis of environmental data maps irregularities

The reliability of environment monitoring can be adversely affected by changes in sampling methods or laboratory staff.

Karl Wahlin, PhD student in statistics, has made a time series analysis of surface water and ground water data collected over lengthy periods. The data show percentages of nitrogen, phosphorus, organic materials and resistance to acidification.

He encountered several instances of sudden or irregular variations in the trend. The most notable deviations coincided with a point in time when a laboratory or instrument methodology was replaced or other documented problems occurred.

One such variation occurred in 1996 when the phosphorus content dropped sharply. Remarkably, in 1996 the reported content dropped at exactly the same dates and in equal amounts in several northern rivers as well as Lake Vänern. Neither weather conditions nor human activity can explain these sudden changes in such geographically distant areas.

As early as 2007, LiUs environmental statisticians pointed out that several series of environmental data indicates changes that are not substantiated by reality. Anders Grimwall, professor of statistics and Karl Wahlin's academic supervisor points out that no specific laboratory is being blamed.

"The cause is a systems error in the environment monitoring," he explains. "The task of tracing small variations from year to year is simply more complicated than was previously thought."

Karl Wahlin has created a road map for analyzing large sets of environmental data. It is essential to be able to follow the trends in concurrent series. And just as in the present case, climate researchers need to adjust the old measurements to eliminate known errors.

"It is better at this time to not expand data collection but instead apply resources to analyzing and adjusting the historic data bases," he suggests. The Swedish Environmental Protection Agency has announced it will further explore the findings presented in Karl Wahlin's doctoral dissertation, which was recently defended at LiU.


Page manager: therese.winder@liu.se
Last updated: 2009-06-03