Predicting the saturated hydraulic conductivity of compacted subsoils using the non-similar media concept

Abstract

It is widely recognized that saturated hydraulic conductivity is dominated by the micromorphology of soil pores rather than by the merely total porosity or dry bulk density. Nevertheless, some researchers are reporting that the decrease in saturated hydraulic conductivity of subsoil is simply associated with the decrease in soil porosity or increase in dry bulk density. Based on these understandings in published papers and on our preliminary field investigation, we assumed that micromorphology of soil pores in topsoils is subjected to be destroyed with continuous disturbance by frequent tillage while subsoils tend to be compacted without serious changes of micromorphology of soil pores. Thus, we focused on finding the dependence of saturated hydraulic conductivity on dry bulk density by separating the soils into tilled layer and compacted layer. The objective of this study was to describe the relationship between saturated hydraulic conductivity and dry bulk density using a theoretical model, the non-similar media concept (NSMC) model, capable of predicting saturated hydraulic conductivities of soils with different values of dry bulk densities. The study area was located near the Tone River in Saitama Prefecture, Japan, where the soils were classified into Haplic Brown Lowland Soils according to the Classification of Cultivated Soils in Japan (Eutric Fluvisol according to FAO/UNESCO). Two sites, where the topsoils were seasonally tilled while the subsoils were sustained as it is, and another site where the topsoil was seasonally tilled, too, but extra deep tillage (1 m tillage depth) had been done, were chosen for the measurements. The saturated hydraulic conductivities and dry bulk densities of undisturbed soil cores from different depths were measured in the laboratory. The NSMC model was carefully applied only when the soil textures were the same among samples. The well-known conventional equations formulated by Kozeny–Carman and by Campbell, were used to compare the applicabilities with the NSMC model. The NSMC model succeeded in predicting the saturated hydraulic conductivities in the compacted subsoils. On the other hand, the NSMC model was not applicable to the tilled topsoils and to the deeply tilled subsoil. The saturated hydraulic conductivity of tilled topsoils and deeply tilled subsoil was always lower than that of compacted subsoils at the same dry bulk densities. The Kozeny–Carman and Campbell equations both failed in the prediction of saturated hydraulic conductivity in subsoil. It was concluded that the saturated hydraulic conductivity of subsoils under compaction without extreme disturbance is well related with its dry bulk density by the NSMC model.