4.2 Trend analysis
The trend detection and subsequent analyses have been performed only for the 254 rivers with at least 10 years with a minimum of 5 consecutive zero-flow days. First trend analysis has been carried out on annual, winter and summer mean dates of zero-flow occurrence, with the metric (eq. 4) computed on an annual or seasonal basis. The results in terms of significant trends are shown in Figure 3, indicating for most rivers in southern Europe a trend towards earlier occurrence in zero-flow days, mostly for the annual and summer timescales. For the river in the Baltic region, the trend toward later occurrence in zero-flow days for the summer and annual periods can be observed. Conversely, more contrasted trends patterns are detected for winter, with both positive and negative trends in the mean date for stations, but close to each other. A significant increasing trend is visible in southern France and central Spain for the winter period.
The second trend analysis is concerning annual and seasonal sums and maximum lengths of zero-flow days, the results are presented in Table 1 and figure 4. At the annual scale, 60 stations (24%) have positive trends in the number of zero flow days and more generally there are more positive trends detected by comparison to the negative trends for all indicators. Since on average the trends affect about 34% of stations, these trends (both positive and negative) are all field-significant according to the FDR procedure. These results indicate that at the annual and seasonal timescales the majority of trends are towards an increase in dryness. At the seasonal timescale, there is a marked trend towards an increase in summer zero-flows, but fewer trends detected for winter. Overall, fewer trends are detected for extreme durations compared to annual or seasonal totals, with the notable exception of the winter maximum lengths of zero-flow days that are increasing in the majority of stations.