Biochemical oxygen demand and ammonium in European rivers
Chart (interactive)
The figure shows the development in Biochemical oxygen demand (BOD) levels and ammonium concentration in European rivers.
The data series are calculated as the weighted average of country time series, using river water body length per country as weights. The country time series are averages of gap-filled water body time series from each country.
Biological oxygen demand in rivers: Milligrams of O2 per litre (mgO2/l)
Ammonium in rivers: Milligrams of NH4-N per litre (mgNH4-N/l)
Methodology
The indicator analysis is based on annual mean concentrations per monitoring site. The annual mean concentrations are calculated from sample data, unless the country has reported annual data only. The data undergo automatic and manual quality checks to remove errors and outliers.
Monitoring site time series from the same water body are combined and gap filled before further analysis. Gap filling is necessary to ensure that the average European time series represents the same water bodies every year. Water body time series are only included in the analysis if they have data within the first and last five years of the time range and data from at least 40% of the years in the time range. Two different time ranges are analysed: From 1992 for long-term trends and from 2007 for better spatial coverage (more water bodies meet the selection criteria).
Data are aggregated from site to water body level and gap-filled using generalized additive models (GAM) in the free software R, using the mgcv package. For water bodies with more than one site, a GAM model with concentration as response variable, year as fixed effect and site as random effect is used to estimate annual concentrations per site. Annual values for the water body are subsequently estimated by averaging the annual site predictions. For water bodies with only one site, or where the random effect for site is non-significant, a GAM without a random site effect is used to estimate annual values and for gap filling. The gap-filled water body time series are averaged by country and a weighted average of the country time series (weighting by the total length of rivers in the country) is calculated to give the time series for Europe. A 95% confidence interval for the weighted average is calculated using of the formula for standard error of a weighted mean recommended in Gatz and Smith (1995).
