The river monitoring stations included in the assessment vary yearly due to availability of time series for the whole period starting from 1992. In the 2013 assessment, data for a significant number of stations was not reported. Conversely, some new stations were added, if the QA/QC procedure showed that stations reported under different names or codes could be treated as identical. This optimisation needs further quality checking. In the end, 702 stations were assessed in 2013 (compared to 849 stations in 2012) for BOD5 and 921 stations were assessed in 2013 (compared to 952 stations in 2012) for total ammonium.

]]>The data requested on rivers includes the physical characteristics of the river monitoring stations, proxy pressures on the upstream catchment areas, as well as chemical quality data on nutrients and organic matter, and hazardous substances in rivers. It also includes the biological data (primarily calculated as national Ecological Quality Ratios), as well as information on the national classification systems for each Biological Quality Element and waterbody type. This reporting obligation is an EIONET Priority Data flow.

**Station selection**: No criteria are used for station selection (except for time series and trend analysis; see below)

**Determinants**: The determinants selected for the indicator and extracted from Waterbase are BOD5, BOD7, total ammonium and ammonium.

Most countries monitor BOD5. Finland monitors BOD7. Lithuania monitored BOD5 up to 1995 and started monitoring BOD7 in 1996. Latvia monitored BOD7 from 1996 to 2001. Estonia monitored BOD5 in 2010, while it monitored BOD7 up to 2009. BOD is commonly used for BOD5. For countries reporting BOD7, these values have been converted to BOD5 (BOD7 = 1.16 BOD5) for reasons of comparability.

All countries reported total ammonium until 2006. In 2007, Greece and Liechtenstein started reporting ammonium instead of total ammonium. Instead of total ammonium, Cyprus, Lichtenstein and Slovenia began reporting ammonium in 2008, Austria and Netherlands in 2009, Bulgaria and Latvia in 2010, and Estonia, Norway and Poland in 2011. Besides total ammonium, Slovakia also started to report ammonium for some stations in 2008. Belgium, Germany, Italy, Luxembourg, Slovakia and the United Kingdom report either ammonium or total ammonium for an individual station in a selected year from 2008 on. Data of either of the two determinants was included in the assessment. For those stations in Slovakia where both were reported, total ammonium data was included in the assessment.

All values are labeled as BOD5/total ammonium in the graphs, but it is indicated in the graph notes for which countries BOD7/ammonium data are used.

An automatic QA/QC procedure excludes data (stations*year) from further analysis. This is based on flagging in Waterbase, deriving from QA/QC tests. In addition a semi-manual QA procedure is applied, to identify outliers that are not identified in the QA/QC tests. This comprises e.g. values deviating strongly from the whole time series, values not so different from values in other parts of the time series, but deviating strongly from the values closest in time, consecutive values deviating strongly from the rest of the time series or whole data series deviating strongly in level compared to other data series in the country. If not explicitly confirmed valid by reporting countries, such values are flagged in Waterbase, but only excluded from the following year’s assessment due to timing issues. More details on the QA/QC procedure can be found here:

- groundwater QA/QC description
- rivers QA/QC description
- lakes QA/QC description

**Quality checked data: **In the table on nutrients ("Waterbase_rivers_v12_Nutrients"), QA-fields are treated as follows:

- Field "QA_MVissues": all flagged values are excluded from the indicator calculation, except for zero values (flag 103).
- Field "QA_LRviolation": all flagged values are allowed, except for flagged values that break the rule “Mean >= Minimum” (flag 201) and “Mean <= Maximum” (flag 202).
- Field "QA_outlier": all flagged values are excluded from the indicator calculation, except for outliers confirmed by country (flags 491, 493).
- Field "QA_station_issues: all flagged values are allowed (including wrong coordinates or missing coordinates), except for "Water Category value is incompatible with this particular dataset” (flag 511) and “station is not defined in the station table" (flag 599).
- Field "QA_CR violation": all flagged values are allowed.

**Inter/extrapolation and consistent time series**

For time series (Fig. 1-5) and trend analyses, only series that are complete after inter/extrapolation (i.e. no missing values in the station data series) are used. This is to ensure that the aggregated data series are consistent, i.e. including the same stations throughout the time series. In this way assessments are based on actual changes in concentration, and not changes in the number of stations.

*Changes in methodology: Station selection and inter/extrapolation. *

Until 2006, only complete time series (values for all years from 1992 to 2004) were included in the assessment. However, a large proportion of the stations was excluded by this criterion. To allow the use of a considerably larger part of the available data, in 2007 (i.e. when analysing data up until 2005), it was decided to include all time series with at least seven years of data. This was a trade-off between the need for statistical rigidity and the need to include as much data as possible in the assessment. However, the shorter series included might represent different parts of the whole time interval, and the overall picture may therefore not be reliable. In 2009, it was decided rather to inter/extrapolate all gaps of missing values of 1-2 year for each station. At the beginning or end of the data series one missing value was replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values were replaced by the values next to them for gaps of two years and by the average of the two neighbouring values for gaps of one year.

In 2010 this approach was modified, allowing for gaps of up to three years, both at the ends and in the middle of the data series. At the beginning or end of the data series up to three years of missing values are replaced by the first or last value of the original data series, respectively. In the middle of the data series, missing values are replaced by the values next to them, except for gaps of one year and for the middle year in gaps of three years, where missing values are replaced by the average of the two neighbouring values. Only time series with no missing years for the whole period 1992-2011 after such inter/extrapolation are included in the assessment. The number of gaps is unlimited, only gap length (size) of three years is defined. This procedure increases the number of stations that can be included in the time series/trend analysis. Still, the number of stations is markedly reduced compared to the analysis of the present situation, where all available data can be used. In Figure 1, the two time series are used: 1992–2012 and 2000–2012.

**Aggregation of time series**

The selected time series (see above) must be aggregated in to a smaller number of groups and averaged, before the aggregated series can be displayed in a time series plot. Determinants are grouped into five geographical regions of Europe, which contain the following countries:

Eastern: CZ, EE, HU, LT, LV, PL, SI, SK.

Northern: FI, IS, NO, SE.

Southern: CY, ES, GR, IT, MT, PT.

South-Eastern: AL, BA, BG, HR, ME, MK, RO, RS, TR, XK.

Western: AT, BE, CH, DE, DK, FR, IE, LI, LU, NL, UK.

*(List of country codes can be found here )*

Not all countries listed per region are included in the figures due to no data being reported or no stations with complete time series after inter/extrapolation. Due to changes in the monitoring network (adapting to monitoring networks under Water Directives) the time series are broken and limited number of time series is available for some countries.

Determinants are in addition grouped into six sea region catchments, which are defined not by countries but by river basin districts or river basin district subunits if consistent with catchment areas of seas. The data thus represents rivers or river basins draining into that particular sea. The sea regions are defined as Arctic Ocean, Greater North Sea, Celtic Seas, Bay of Biscay and the Iberian Coast, Baltic Sea, Black Sea and Mediterranean Sea. The sea region delineation is according to the Marine Strategy Framework Directive (MSFD) Article 4, with the Arctic Ocean added as a separate region. As the catchment area draining into what is defined as the North-east Atlantic Ocean region of the MSFD is very big, it was decided rather to use the sub-region level here, but merging the Celtic Seas and the Bay of Biscay and the Iberian Coast.

Determinants are also aggregated for the whole of Europe.

**Trend analyses**

Trends are analysed by the Mann-Kendall method (McLeod 2005) in the free software R (R Development Core Team 2006). The test was suggested by Mann (1945) and has been extensively used with environmental time series (Hipel and McLeod, 2005). Mann-Kendall is a test for monotonic trend in a time series y(x), which in this analysis is nutrient concentration (y) as a function of year (x). The test is based on Kendall's rank correlation, which measures the strength of monotonic association between the vectors x and y. In the case of no ties in the x and y variables, Kendall's rank correlation coefficient, tau, may be expressed as tau=S/D where S = sum_{i<j} (sign(x[j]-x[i])*sign(y[j]-y[i])) and D = n(n-1)/2. S is called the score and D, the denominator, is the maximum possible value of S. The p-value of tau under the null hypothesis of no association is computed by in the case of no ties using an exact algorithm given by Best and Gipps (1974). The tests reported here are two-sided (testing for both increasing and decreasing trends). Data series with p-value < 0.05 are reported as significantly increasing or decreasing ("strong trends"), while data series with p-value >= 0.05 and <0.10 are reported as marginally significant ("weak trends"). Data series with p-value >0.10 have no significant trend. The test is non-parametric which means that the amount of change from year to year is not considered, only the direction of the change.

The size of the change is estimated by calculating the Sen slope (or the Theil or Theil-Sen slope) (Theil 1950; Sen 1968) using the R software. The Sen slope is a non-parametric method where the slope m is determined as the median of all slopes (yj − yi)/(xj − xi) when joining all pairs of observations (xi,yi). Here the slope is calculated as the change per year for each unit (groundwater body/river station/lake station). This is summarised by calculating the average slope (regardless of the significance of the trend) for all units in Europe or a selected region. Multiplying this by the number of years of the time series gives an estimate of the absolute change over time. This can be related to the mean value of the aggregated time series to give a measure of relative change. The Sen slope was introduced for this indicator in 2013.

The Mann-Kendall method or the Sen slope will only reveal monotonic trends, and will not identify changes in the direction of the time series over time. Hence a combination of approaches is used to describe the time series: A visual inspection of the time series, describing whether the general impression is a monotonic trend, no apparent trend, clear shifts in direction of the trend or high variability with no clear direction; an evaluation of significant versus non-significant and decreasing versus increasing monotonic trends using the Mann-Kendall results; an evaluation of the average size of the monotonic trends using the Sen slope results.

**Present concentration distributions**

The latest year for which there are concentration data for the selected river stations are extracted from Waterbase. The number of stations with annual mean concentrations occurring in the selected concentration bands or classes are then calculated and presented. The allocation of a station to a particular class is based only on the face value concentration and not on the likely statistical distribution around the mean values.

- The new/revised class defining values for BOD5 concentrations (mg O
_{2}/l): <1.4, 1.4 to 1.99, 2 to 2.99, 3 to 3.99, 4 to 4.99, >5. The two highest classes are merged to >4. - The new/revised class defining values for total ammonium concentrations (mg N/l): <0.04, 0.04 to 0.09, 0.1 to 0.19, 0.2 to 0.39, 0.4 to 0.99, >1. The two highest classes are merged to >0.4.

More information is given in the WISE maps on Water quality in rivers and lakes under section "Help": http://www.eea.europa.eu/themes/water/interactive/soe-rl (BOD in rivers, Total ammonium in rivers).

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