## 5 Discussion  
Groundwater table fluctuations in Poland have been divided by [PERSON] [59] into four types depending on the geological setting of the observation point and the behavior of the groundwater table during the HY. If the observation point had been located in a undisturbed environment, it would be characterized as the type III (shallow, wide-spread aquifers within river or lake regime characterized by small amplitudes of the groundwater level with a typical increase in the winter season and decrease during the summer season) or as a type IV (shallow aquifers of rivers valleys and terraces with differentiated amplitudes and a few phases of increase and decrease of the groundwater levels during the HY). Which characteristics can be identified in the semi-urban area of Wroclaw? The results of year to year AHP and the Pearson correlation matrix clearly show that some groups of similar HYs and seasons exist. The best option to identify the character of the groundwater level fluctuations and the general shape of the groundwater level diagram during the HY is clustering (Figure 12). Generally, the two largest classes, class 2 (HYs: 2004, 2008, 2012, 2017) and 5 (HYs: 2005, 2006, 2009, 2010, 2013, 2016) can be classified as type III _sensu_ Pleczynski with a rising groundwater level during the winter season, a maximum in the springtime (typically in May) and a falling water table during the summer season. These two classes are mirrored by the green and blue groups in the dendrogram (Figure 11) as well as the Pearson correlation matrix (Figure 8). The HYs 2007, 2011, 2018 combined in class 3 represent the type IV _sensu_ Pleczynski. Groundwater levels in these years have been fluctuating irregularly with several high and lows of the groundwater level. This class is represented in the red group of the dendrogram. Finally, there are the HYs 2014 and 2015 that differ significantly from all the others and are grouped into classes 1 and 4, represented by black lines in the dendrogram. Figure 22: Scatterplots for the summer seasons. On the OY axis, the detrended groundwater levels and on the OX axis the cumulative sums of precipitation are presented. The bottom right plot presents the summer seasons with the highest correlation: 2004, 2005, 2008, 2012, 2018. Seasonal analysis of the HYs shows different results than the yearly ones. The winter seasons' character is similar to the whole years (Figures 14 and 15): the two largest classes (class 2 and 3) consist of the same years as classes 2 and 3 in the annual clustering. They can be classified as type III _sensu_ Pleczynski and be considered 'typical' seasons. Class 5 consists of the winter seasons in years 2014 and 2018 and might be Pleczynski's type IV. Winter seasons 2011 and 2015 are significantly different from all other winters seasons and fall into classes 1 and 4. Different results have been obtained for the summer seasons analysis (Figures 15 and 16). Two similar and dominant classes 1 and 4 were identified and can be classified as type III _sensu_ Pleczynski. Yet, the individual years do not fall in the same classes as the results of the annual or winter analysis. What are the implications of these results? Some groundwater fluctuations in the semi-urban area show characteristics that allow the definition of a typical HY. Yet, the results also show a substantial variability in the inter-annual groundwater levels, especially in the summer seasons, and that some HYs deviate significantly from all the other HYs. All these differences might be explained with a deeper investigation of the various factors influencing the groundwater levels (GWL):  
\[\text{GWL}=f(R,T,P,U,H,SWL,E,G,A), \tag{7}\]  
where \(R\): precipitation, \(T\): air temperature, \(P\): atmospheric pressure, \(U\): total insolation, \(H\): air and ground humidity, \(SWL\): surface water level, \(E\): evapotranspiration, \(G\): geological conditions and ground parameters, and \(A\): anthropogenic influence.