Using scaling factors for characterizing spatial and temporal variability of soil hydraulic properties of topsoils in areas heavily affected by soil erosion

Abstract

Extensive agricultural use of land coupled with intensive water erosion at geomorphologically diverse areas with uniform soil substrate and originally similar soils can lead to high diversity in soil types and, correspondingly, to a high spatial and temporal variability of soil properties (including soil hydraulic properties). Therefore, this study aimed to propose a procedure to characterize the variability of soil hydraulic properties of topsoils and the soil–water regime. Soil hydraulic properties were measured after sowing and harvesting at 5 sampling points of 1 elevation transect at 5 experimental areas with diverse soil types. The reference soil hydraulic properties and 3 scaling factors (related to the pressure head, αh, water content, αθ, and hydraulic conductivity, αK) were used to characterize the spatial and temporal variability of soil hydraulic properties. The best correspondence between scaled soil hydraulic curves was observed for the Chernozem area, followed by the two Haplic Cambisols areas, Leptosol area and Luvisol area. In general, matching of the scaled curves increased with a decreasing variability of n values (i.e., increasing similarity of a soil pore space geometry) and increasing average n values (e.g., for pronounced S-shape soil water retention curves). The sampling day significantly affected αh, αK, and αθ in the Chernozem area; αh and αK in the Cambisol (developed on shale) area; and αh and αθ in the Leptosol area. While values for the Chernozem area indicated an improvement in soil hydrological conditions during the vegetation period, the values for the Luvisol, Leptosol, and Cambisol (developed on shale) areas indicated aggravation. No considerable changes were observed for the Cambisol (developed on granodiorite and shale) areas. The sampling position significantly affected αθ in both the Cambisol areas. At some locations (e.g., Chernozem area), the spatial variability reflected different erosion-accumulation processes within the transect. To illustrate a practical application of the scaling factors at such locations, the scaling factors obtained for the Chernozem area were linearly interpolated within the surface layer of the theoretical vertical two-layer soil transect, and HYDRUS-2D program was used to simulate water flow in the surface 50 cm layer. The results showed a more gradual wetting front after sowing than after harvesting.