Impact of Pakoslaw Lateral Reservoir on Groundwater Levels in Adjacent Areas

The paper presents a preliminary study of the surface waters and groundwater levels in the basin of Pakoslaw lateral reservoir. The storage reservoir, built in the year 2007 represents a lateral reservoir placed outside of the water course, but in its direct neighbourhood. The reservoir is filled by the water of Orla river from the water intake localized before the weir, while the water discharge into the river bed takers place through a discharge construction localized below the weir. The results described in this paper contain an excerpt from a long-term study on the depth to a groundwater table in the area adjacent to Pakoslaw reservoir for 2010–2012. A direct influence on the groundwater table in the particular control wells has been exerted by the water levels in the reservoir. Analysis of nearly three years of observationshas indicated that the groundwater table in the zone of potential influence of Pakoslaw reservoir is highly varied. In all control wells, the shallow water table changed in a cyclical way. The observations of the decreasing groundwater tables have shown a direct connection to rainfalls and evapotranspiration. Excessive evapotranspiration during the summer periods with shortages of rainfalls led to soil drying and lowering the groundwater table. Contrary, during the autumn-spring periods, rainfalls exceeded evapotranspiration and resulted in refilling the soil retention storage and rising groundwater levels. The usefulness of the reservoir for anti-flood purposes and its utilization in the periods of water shortage has been confirmed. Water gained during spring thaw replaces water shortage in the spring period. Therefore, the main function of the reservoir and its purpose has fulfilled its expected task. One can state that the table of the groundwater in the zone of Pakoslaw reservoir has been realized in a differentiated way. The average groundwater oscillation amplitude is 1.88 m. The smallest change of groundwater level was observed in the well P-5 and it amounted to 1.70 m, while the highest level was 2.12 m in the P-2 well indicating a high variability of the groundwater level in the particular localities of the reservoir activity. All wells are built on permeable sand layers providing good water filtration conditions. The dynamics of changes in the groundwatrer levels is mainly determined by meteorological conditions and by ait temperature. A higher variability of groundwater has been observed on agriculturally utilized areas, but in the direct neighbourhood of the reservoir