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

Abstract

It is widely recognized that saturated hydraulic conductivity is dominatedby the micromorphology of soil pores rather than by the merely totalporosity or dry bulk density. Nevertheless, some researchers arereporting that the decrease in saturated hydraulic conductivity ofsubsoil is simply associated with the decrease in soil porosity orincrease in dry bulk density. Based on these understandings in publishedpapers and on our preliminary field investigation, we assumed thatmicromorphology of soil pores in topsoils is subjected to be destroyedwith continuous disturbance by frequent tillage while subsoils tendto be compacted without serious changes of micromorphology of soilpores. Thus, we focused on finding the dependence of saturated hydraulicconductivity on dry bulk density by separating the soils into tilledlayer and compacted layer. The objective of this study was to describethe relationship between saturated hydraulic conductivity and drybulk density using a theoretical model, the non-similar media concept(NSMC) model, capable of predicting saturated hydraulic conductivitiesof soils with different values of dry bulk densities. The study areawas located near the Tone River in Saitama Prefecture, Japan, wherethe soils were classified into Haplic Brown Lowland Soils accordingto the Classification of Cultivated Soils in Japan (Eutric Fluvisolaccording to FAO/UNESCO). Two sites, where the topsoils were seasonallytilled while the subsoils were sustained as it is, and another sitewhere 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 ofundisturbed soil cores from different depths were measured in thelaboratory. The NSMC model was carefully applied only when the soiltextures were the same among samples. The well-known conventionalequations formulated by Kozeny-Carman and by Campbell, were usedto compare the applicabilities with the NSMC model. The NSMC modelsucceeded in predicting the saturated hydraulic conductivities inthe compacted subsoils. On the other hand, the NSMC model was notapplicable to the tilled topsoils and to the deeply tilled subsoil.The saturated hydraulic conductivity of tilled topsoils and deeplytilled subsoil was always lower than that of compacted subsoils atthe same dry bulk densities. The Kozeny-Carman and Campbell equationsboth failed in the prediction of saturated hydraulic conductivityin subsoil. It was concluded that the saturated hydraulic conductivityof subsoils under compaction without extreme disturbance is wellrelated with its dry bulk density by the NSMC model.