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New approach to the statistical trend analysis of the watercourse sediment quality

Issue 03-06-2013At the request of MIRA, INBO introduced a new method to analyse trends in the evolution of the watercourse sediment quality in a statistically underpinned manner.

Since 2000, the Flemish Environment Agency has intensively monitored the watercourseFoto statistical trend analysis of the watercourse sediment quality sediment quality using the triad method. This method integrates the results of chemical, biological and ecotoxicological analyses and makes it possible to classify watercourse sediments into quality classes ranging from unpolluted to heavily polluted. The monitoring network, which originally consisted of 600 or so measuring locations that were sampled every four years, has recently been halved. The general evolution of the watercourse sediment quality has for some years been depicted by selecting the measuring locations that were sampled in each of the periods considered and then looking at the evolution of the distribution over the quality classes. This approach provides a general overview of the evolution of watercourse sediment quality in Flanders, but is not statistically underpinned.

The purpose of this study is to introduce a method that allows trends in the evolution of the watercourse sediment quality in Flanders to be analysed in a statistically underpinned manner. This presents a number of specific problems. Different observations at the same measuring location are not independent of each other, not all measuring locations are sampled with the same frequency and at the same interval, and for certain substances  concentrations below the detection limit are often encountered. Moreover, the detection limits change over time. The method presented here is that of the so-called mixed models regression technique.

The final report is an intuitive introduction to the mixed models regression technique. The underlying principles are explained through a simulation study and then illustrated by two examples from the watercourse sediment monitoring network. The aim is to use this method, which is new to MIRA, on a regular basis to monitor the evolution of the watercourse sediment quality.

Read the English summary of the Dutch report ‘An intuitive introduction to the mixed models regression technique. The principles of mixed models explained through simulation examples and an analysis of two concrete measurement series from the watercourse sediment monitoring network’

Study commissioned by MIRA, the Environment Reporting Unit
Research report MIRA/2013/03

researchers: Ivy Jansen, Paul Quataert, INBO

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