Morphology, hydrology and land use/land cover of closed depressions in the Zbrzyca River sub-catchment (northern Poland)

Abstract

Closed depressions (CDs) are widespread geomorphological features of the young glacial landscapes of northern Poland; however, they remain insufficiently studied despite their important hydrological and ecological functions. This study aims to analyse the spatial distribution, morphometric characteristics, and land use/land cover (LULC) structure of closed depressions, distinguishing between infiltration (CD_I) and evapotranspiration (CD_E) forms, and to examine the relationships between their morphology and the surrounding landscape. The research was conducted in the Zbrzyca River sub-catchment, located in northern Poland. Spatial analyses were performed in a GIS environment using digital elevation model, cartographic materials, hydrographic datasets, and land use/land cover databases. Morphometric parameters, including area, perimeter, denivelation, slope, and catchment development index, were calculated and compared between the two depression types. A total of 2,154 closed depressions were identified within the study area. Infiltration closed depressions were the dominant type, covering 108 km² (46.5% of the analysed catchment). They were generally more circular, steeper, and predominantly covered by forests and semi-natural vegetation.  Evapotranspiration closed depressions occupied 31.5 km² (13.5% of the analysed catchment), were characterised by larger contributing areas and longer perimeters, and occurred mainly within agricultural landscapes. The spatial distribution of both depression types reflects the geological and geomorphological development of the catchment. These multifunctional geoecosystems play a key role in water retention, groundwater recharge, and biodiversity conservation. Their ecological significance and increasing anthropogenic pressure highlight the need for their effective protection and inclusion in sustainable land management and spatial planning strategies.