The initial hypothesis assumed in our research was that an improvement in the water cycle in the river catchments chosen for analysis could be obtained using their natural basins without outlets to store rainwater from natural and urbanised areas. The chief goal of the research was to identify the types and conditions of retention in each of the catchments. Fieldwork was carried out in the catchments of the upper Parsęta and the Potok Oliwski, rivers situated in the Baltic drainage area in the coastal zone of northern Poland.
The upper Parsęta catchment includes an abundance of depressions without outlets, where a total of 358 landforms with no surface runoff have been distinguished, or 4.84 per 1 km². Of this number 164 are absorptive basins and 194 are closed evapotranspiration basins. The distinguished closed basins take up a substantial proportion of the upper Parsęta catchment at 8.33 km², or 11.26%. The Potok Oliwski catchment includes depressions without outlets, where a total of 35 landforms with no surface runoff have been distinguished, or 4.83 per 1 km². Of this number, 27 are absorptive basins. Closed basins occupy a total of 82,565 m², or 1.14% in the upper Potok Oliwski catchment.
The total capacity of all the closed basins in the upper Parsęta catchment amounted to 6,292,160.89 m³. In turn, in the Potok Oliwski catchment the total volume of closed evapotranspiration basins was 1,677.1 m³, and when filled with water, absorptive basins would have a total volume of 23,182.8 m³.
The result is considerable potential reservoirs in both catchments. Young glacial catchments, particularly areas with no outlet, can be a place of existing surplus drinking water. Water retention is a significant potential with more and more apparent water shortage in the world.