A field experiment was carried out to investigate the effects of compost, sawdust and rice straw biomass on soil three-phase composition, soil resistance to penetration, bulk density, nearsaturated hydraulic conductivity, K(h), and soil water retention characteristics. The experimental design involved ten split blocks such that the non-amended one plot was considered as control and other nine plots were under each of compost, sawdust and straw treatments at application rates of 0.1,0.2 and 0.3 m3m-3 of apparent soil volume. Addition of compost, sawdust and straw showed potential for improvement of surface soil physical and hydraulic properties, then its effectiveness was partly dependent on amendment types and application rates. Three-phase composition of all amended soils showed solid-phase reductions and increase of total porosity. Generally, soil resistance and bulk density at all amendment plots were decreased, which was likely due to reduction in soil solid phases. A good correlation between soil resistance and bulk density was also observed. Except for sawdust applied at higher rate, the K(h) generally increased at any level of compost and straw incorporations, and this was attributed to the of
reduction in solid phase of amended soils. Soil water content was relatively high at higher suction for compost amended soils, while improvement in soil water retention was limited at lower suction for sawdust, and gradually increased from low to high suction for straw amended soils, respectively.
In a series of agricultural land uses over a toposequence comprised of upland crop fields and lowland paddy fields, hydrological aspects are essential to an understanding of impacts of agricultural managements on groundwater environment. The objective of this study was to evaluate amounts and flow paths of shallow groundwater discharging from an agricultural catchment using a physical model. About 61% of the shallow groundwater percolated into the deeper aquifer and exited the catchment without passing through the paddy fields. Only the rest of the shallow groundwater, which was within 70 m of the paddy fields, reached the paddy fields after 14 years of travel. These results indicated that in a convergent-shaped catchment a large part of leached nutrients from upper crop fields can leave the catchment without being consumed in lowland paddy field area. We defined a risk of excess nutrient discharge from the catchment as a probability that 95% of the recharged groundwater bypasses the paddy fields, assuming that the most influential parameter falls within an uncertainty range of two orders of magnitude around the value giving the modeFs deterministic solution. The evaluated risk came to 44.8% even when the uncertainty range would include 99% of the whole population of the parameter.
Partitioning Interwell Tracer Test (PITT), which is used for the chromatographic separation of Break through curves (BTCs) of tracers, is useful to quantify the volume of Non-Aqueous Phase Liquids (NAPL) in contaminated aquifer. Since this separation is based on the partition law, it is most important to determine the partitioning coefficients of the tracers. However, the accuracy of the PITT would be influenced not only by the partitioning coefficient, but also by the tracer velocity. The effect of the latter factor on the accuracy has not been confirmed sufficiently yet, due to the lack of the basic research compared with the practical requirement. In this study, the effect of the tracer velocity on the accuracy of PITT was examined in laboratory column experiments. As a result from the experiments where five tracer velocities were set up, the accuracy of PITT was more improved as the velocity was lower. It would be caused by non-equilibrium partition of partitioning tracers into NAPL when the tracer velocity was higher, on the other hand, the more sufficient partition was achieved when the velocity was lower.
Effects of imogolite addition on colloidal stability of montmorillonite and kaolinite were examined to clarify the colloidal properties of imogolite which coagulates in alkaline condition. One to one mixture of montmorillonite and imogolite coagulated at any pH. Observation through transmission electron microscope revealed that montmorillonite and imogolite, including the mixture with lower imogolite concentration, mutually flocculated (formed a domain) at any pH. Non-deferrated kaolinite coagulated below pH 5.9 and dispersed above pH 6.0, whereas 5% imogolite added sample showed the opposite stability. Deferrated kaolinite dispersed above pH 4.2, whereas 10% imogolite added sample coagulated at any pH. All the imogolite added samples microscopically flocculated both under acidic and alkaline conditions. In acidic conditions, it was attributed to the electrostatic attraction between permanent negative charge of crystalline clay minerals and variable positive charge of imogolite. In alkaline conditions, it was considered to be resulted by the neutralized surface charge of imogolite.
In this study, we examined a groundwater control scheme to increase soybean yield in multipurpose paddy fields. In a multi-purpose paddy field with a box connecting the irrigation canal to underdrain system and a flexible overflow relief well (denoted “the experimental plot”), the groundwater level was aimed to keep at 10 cm below the soil surface of the field during the blossom and maximum luxuriance stages, i.e. in August when much water is required for soybean growth. On the other hand, in another multi-purpose paddy field with a horizontal relief well (denoted “the conventional plot”),the outlet of the relief well was kept at 80 cm below the soil surface and was always opened. The study was conducted for two years on these plots. The actual groundwater level in the field, soil water content in the topsoil and the soybean yield were measured. In the first year, the actual groundwater level in the experimental plot in August was kept higher (13.4+5.0 cm below the soil surface) as compared to that in the conventional plot (22.9 ± 7.5 cm). Soybean yield in the former was 25% higher than that in the latter, indicating the effect of the groundwater level control scheme. In the second year, the actual groundwater level in the experimental plot in September was kept higher (from 30 to 40 cm below the soil surface) as compared to that in the conventional plot (from 60 to 70 cm), and soybean yield in the former was 16% higher than that in the latter. Hydraulic conductivity in the plow layer and subsoil was found to become higher through the soybean cultivation. These results showed the possibility of higher yield in the soybean cultivation in multi-purpose paddy fields by adapting the groundwater level control scheme. It was also suggested that the groundwater level should be managed properly throughout the year to maintain low permeability of the subsoil.
The Fayer and Durner models, which modified the water retention curve of the van Genuchten (VG) model, combined with the statistical pore-size distribution model of Mualem were used to predict unsaturated hydraulic conductivity functions of Tottori dune sand and aggregated Kumamoto Andisol. Parameters of the Fayer and Durner models, as well as the VG model, were inversely estimated with the evaporation method. The Fayer model well described unsaturated hydraulic conductivity for the dune sand reflecting pore water retention for higher water contents and absorbed water retention on the surface of soil particles for lower water contents. The Durner model was suitable to the Andisol whose hydraulic properties were strongly affected with the distinct the aggregated structure. The Mualem model can predict unsaturated hydraulic conductivity for a wide range of soil water pressure when an appropriate model is used for the water retention curve and the pore-connectivity coefficient, I is estimated. Furthermore, it was also suggested that we need to use an appropriate soil water retention function to predict unsaturated hydraulic conductivity for much smaller pressure heads than the minimum pressure head observed during the evaporation experiment.