## Material and Methods
### Analysis of Changes in the Depth to GroundwaterTable  
The Vistula River Valley is a typical shallow groundwater area. The groundwater levels are under regular monitoring within a stationary monitoring network. Groundwater level measurements have been conducted since 1999 on a fortnight basis in 56 piezometers in KPN. An analysis of changes of the groundwater table level has provided data on the quantity of groundwater resources. They include the determination of: changes over a long period [4; 25], seasonal changes of resources [30], changes in the amount of resources during the vegetation period and the periods and sites of possible moisture deficit in the GDE areas [22], and duration of and depth to groundwater table during hydrogeological drought [17]. Statistical analysis of groundwater levels in Vistula valley included basic statistics for the 1999-2013 period and particular years of period were analyzed and the scope of groundwater levels was defined. The statistics allow for determining the range of long-term changes in the depth to groundwater table and seasonal changes, as well as defining the highest and lowest levels in a long period. The average depth to groundwater table in the piezometers of the monitoring network during the period of 1999-2013 was 1.87 m b.g.l., and the range of changes varied from 1.11 to 2.46 m. The individual piezometers record typical seasonal level fluctuations. The years 2003 and 2011 stand out for a distinctly different trend of seasonal variations in the groundwater table depth. This phenomenon was observed in all zones of the KNP--marsh, dune, and floodplain. In 2003, the average depth was 2.11 m b.g.l., and the amplitude was 0.87 m. The highest groundwater levels were recorded in 2011, when the average depth to groundwater table was 1.35 m b.g.l., and the amplitude was 0.56 m (Table 1). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Groundwater Depth**} & \multicolumn{3}{c}{**All Monitoring Points**} & \multicolumn{3}{c}{**Selected Piezometers in**} \\  & \multicolumn{3}{c}{**(_n = 56_) in:**} & \multicolumn{3}{c}{**1999–2013**} \\ \cline{2-7}  & **1999–2013** & **2003** & **2011** & **P11** & **P12** & **P14** \\ \hline Average & 1.87 & 2.11 & 1.35 & 1.89 & 0.47 & 0.74 \\ Median & 1.92 & 2.09 & 1.35 & 1.94 & 0.43 & 0.75 \\ Maximum & 2.46 & 2.46 & 1.67 & 2.43 & 1.15 & 1.42 \\ Minimum & 1.11 & 1.59 & 1.11 & 1.15 & \(-\)0.45 & \(-\)0.10 \\ Standard deviation & 0.28 & 0.21 & 0.16 & 0.28 & 0.31 & 0.32 \\ Maximum amplitude & 1.35 & 0.87 & 0.56 & 1.28 & 1.60 & 1.52 \\ Annual amplitude & 0.68 & 0.87 & 0.56 & 0.73 & 0.88 & 0.78 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main statistics of groundwater depth based on monitoring observations in all the piezometers and in selected piezometers of dune (P11), marsh (P12), and Vistula flood plain (P14). Figure 1: 3D view of study area location (vertical exaggeration 5.0; view from west; gridlines every 2 km). In the period of 1999-2013, in marsh zones, the groundwater table occurred at average depths ranging 0-1 m in the central parts (25% of the total area) to 1-3 m in the peripheries (46%) (Figure 2). The zones where the groundwater table was above the ground surface comprised ca. 6.5% of the study area. The greatest depth to groundwater table (>5 m) was found in dune zones and at the boundary with the upland area, attaining a maximum value of 13.5 m.  
Figure 2: Groundwater depth (kriging): (**a**) lowest (2003); (**b**) highest (2011); (**c**) average (1999–2013). There are distinct differences between the groundwater level for the long period and the lowest (2003) and highest (2011) levels in the period of 1999-2013. In 2003, the groundwater table in the marsh zones was at depths of 0-1 m (12%) or 1-2 m (49% of the total area), which indicates drying of organic soils whose maximum thickness does not exceed 1.5 m [27].