## 7 Conclusions  
Daily groundwater level data during a 15 hydrological years period in a semi-urban area of Wroclaw, Poland, have been examined with various statistical methods. First research results are presented in this article and show that the used methods for groundwater level fluctuation analysis are viable and allowed to identify patterns typical for a hydrological year. Both, the agglomerative clustering results and the correlation investigations clearly showed that groups of similar groundwater level-precipitation behaviour can be identified for some hydrological years. Typical hydrological year patterns can be influenced by non-identified factors resulting in groundwater level fluctuations that significantly differ from the preceding year. Yet, using data decomposition into winter and summer seasons reveals otherwise unnoticed similarities. In the case of the investigated semi-urban groundwater data, the winter seasons showed clear similarities, while the summer data shows groundwater level fluctuations based on so far unidentified factors. Thus, referring to the earlier research questions (i) and (iii), we can conclude that: (i) the groundwater level in a semi-urban area does not behave in a similar way every hydrological year and season, but we can indicate some groups of annual and seasonal groundwater level fluctuations and (iii) we defined a typical pattern of groundwater level fluctuations and identified abnormal years, but the question about the reasons for the anomalies is still open. From the results of this investigation, it can be concluded that a day to day analysis of precipitation and groundwater level fluctuation in a complex system is not possible, though three different correlation methods were applied. These results might be misleading in understanding the groundwater level--precipitation relationship, as there seems to be no direct relationship between these two factors. Though the general shapes of the groundwater level--precipitation matrix seem to be similar (Figure 18), the correlation is weak. Because of the site-specific conditions, the precipitation needs some time to fill the unsaturated soil column and reach the groundwater level. Therefore, the cumulative sum of the precipitation shows in most cases a better correlation with the groundwater level than the precipitation by itself, in some cases even a linear relationship. Another result is that positive correlations prevail in the winter season and negative ones in the summer season. We conclude that this negative correlation results from (i) a high insolation and elevated evapotranspiration resulting in a decrease in groundwater recharge and (ii) from effects of the water regulation of the Oda river during the summer seasons or one of each. Challenging, still, are cases where a nonlinear relationship exists. In conclusion, the results allow identifying relationships between precipitation and groundwater level fluctuations and a classification of hydrological years into a set of classes. The applied statistical and signal processing methods are universal and the assumption about the geological setting of the region is not required. Thus, we can extend the proposed approach to consider for instance the relationship between precipitation and groundwater level for regions of different geological settings and observe if it has an influence on the correlation coefficients. Conceptualization, R.Z., M.W.-K.; methodology, A.M., R.Z. and A.W. ; formal analysis, A.M.; investigation, M.K. ; data curation, A.M.; writing--original draft preparation, M.W.-K., R.Z., A.M.; writing--review and editing, C.W. ; visualization, A.M., C.W. and M.W.-K. All authors have read and agreed to the published version of the manuscript. The research work co-founded with the research subsidy of the Polish Ministry of Science and Higher Education granted for 2020.