2.3. Delimitation of closed depressions and map elaboration methodology
The analysis of closed depressions was made using a Digital Terrain Model (DTM) elaborated as a part of the ISOK project (Informatic System for Country Protection against extraordinary hazards), involving LiDAR scanning of Poland in the period 2011-2014 (Woźniak, 2015). The DTM has a vertical accuracy of 0.15 m and a high spatial resolution of 1x1 m (Kurczyński et al., 2015). Closed depressions were extracted from this DTM using the CDs function, included in SAGA-GIS (Conrad et al., 2015). The resulting grid contained all CDs, strongly differentiated in shape and size. In the next stage, depressions with an area of more than 200 m2 were extracted and grid data was converted to shapefile polygons. Then depressions of anthropogenic origin were manually selected and removed based on the analysis of shaded relief, coloured DTM and field observations. Next, polygons were automatically smoothed and small errors were manually removed from some polygons. The spatial distribution of SOC was analysed using 10 ha hexagons. First, a simplified geomorphological map of the Nałęczów Plateau, containing the valley bottoms, slopes, plateaus and closed depressions, was drawn using GIS software. Then data from laboratory analyses of SOC content in the soils inside the CDs were assigned to their map polygons, while SOC content in soils outside CDs in the study area was obtained from published data and assigned to the polygons as well. Then the contribution (%) of the main relief elements (CDs, slopes and hilltops) inside each hexagon was calculated. Based on the SOC content of these relief elements, total SOC content within the hexagons were computed. Only hexagons where CDs, slopes and tops cover together entirely hexagons’s area were used in calculations. Separate maps of SOC content within the closed depressions and of closed depressions together with plateaus and slopes were elaborated. Hexagons of 10 ha area were used for the spatial analysis of the CDs in the study area (consisting of 4960 hexagons). The number of CDs per unit area (density of CDs) in the study area was calculated by summing all CDs per hexagon and then converting these values into the number of CDs per ha.
For the calculation of the spatial distribution of SOC in the eroded loess landscape, measured SOC content in CDs and on slopes were used. SOC values for each hexagon are calculated as sum of SOC content in all the corresponding CDs and the SOC content of the remaining sloping areas. These calculations cover 100% of the Nałęczów Plateau. SOC present in valley bottoms was not taken into account in these calculations.
For calculating SOC from all CDs in the study area only the areal fraction of the CDs in each hexagon was taken into account. This calculation covers 23 010 ha (46.3% of the Nałęczów Plateau). Contribution of SOC stored in CDs (SOC enrichment in Mg·ha-1) to overall SOC distribution in the studied landscape was calculated as the difference between SOC content in CDs and the SOC content on slopes.
SOC enrichment for each hexagon (% SOC) is the fraction of SOC content in CDs (Mg·ha-1) to total SOC content in CDs and on slopes. This calculation is only made for hexagons, where CDs and slopes cover together 100% of the hexagon area (in total 19 820 ha, 39.95 % of the Nałęczów Plateau).
Next Pearsons’ correlation coefficients were calculated between the density of CDs and SOC enrichment (Mg·ha-1 and %) per hexagon.