??Transport phenomena of solute in open channel networks are described in the framework of longitudinal dispersion. Modeling of the longitudinal dispersion in an open channel network in general requires the use of a solute transport equation coupled with an internal boundary condition to appropriately determine the dynamics of solute concentration around junctions. However, only a few researches have focused on dependence of the longitudinal dispersion on the internal boundary conditions. This paper carries out theoretical and numerical analysis on two internal boundary conditions that have different physical and mathematical basis with each other. A conforming Petrov-Galerkin finite element scheme is used to solve the solute transport equation. Computational results of a series of numerical simulations reveal that the dynamics of the solute concentration in open channel networks critically depends on the internal boundary conditions in particular for the diffusion-dominant cases that arise in typical situations, indicating their importance in practical analysis.
??In an attempt to counter heat damage to rice plants, the influences of geological conditions in paddy fields on the subsurface water flow and soil temperature for saturated irrigation are clarified by simulating the soil water movement and heat transport. Considering paddy fields located in a lowland or an alluvial fan as the geological conditions of the target area, the simulations of the water movement and heat transport in soil are carried out over 20 days, corresponding to the ripening period of rice grain. The results indicated that the time-varying soil temperatures and the average soil temperature of the lowland paddy field were lower than those of the alluvial fan, and thus the effect of saturated irrigation depends on the geological conditions of paddy fields under its implementation.
??Sediment yield and sediment grain size characteristics were investigated in a study of the effect of differences in geology on erosion from yarding roads. An experimental catchment with an area of 38 m2 and 8° average slope was established below a yarding road within an area of Shirasu (Ito pyroclastic flow deposits) in an abandoned clear-cut plantation forest about 0.5 km northeast of Kagoshima Airport, Kagoshima Prefecture. Sediment yield in the experimental catchment occurred via Hortonian overland flow. The yearly sediment yield rate in the experimental catchment was 6.1 mm, which was about 1.8 to 6.8 times greater than that measured in comparable sedimentary rock catchments (R1 and R2) in an abandoned clear-cut plantation forest. R1 has an area of 86 m2 and 8° average slope, and R2 has an area of 111 m2 and 6° average slope. This difference is because of greater soil erodibility in the Shirasu sediments than in sediment from the sedimentary rock catchments. Median grain diameters, coefficients of uniformity, coefficients of curvature and constituent grain diameters in sediments from the experimental catchment and R1 catchment showed similar trends, although the R2 catchment showed different trends in these properties.
??Analyzing macroscopic dynamics of surface water flows is one of the most important issues in agricultural and environmental engineering. The dynamics of surface water is described using the cross-sectionally averaged one-dimensional shallow water equations (1-D SWEs) with the assumptions of the incompressibility of water and hydrostatic pressure distribution. A typical surface water system involves an open channel network. To develop an accurate and efficient simulation method for the flows in open channel networks is therefore an important task. This paper proposes a new momentum flux evaluation scheme for the 1-D SWEs in locally 1-D open channel networks. The scheme evaluates the momentum flux considering the contact angles of the reaches at a junction as well as the discharge ratios, ensuring non-increase of the momentum variation. Numerical simulations of flows in an open channel network having a junction are carried out to compare the proposed scheme with the existing ones. Computational results of the discharge ratios show the validity of the proposed scheme, which in some cases is as accurate as the 2-D counterpart. Computational results of water surface profiles show the advantages of the proposed scheme over the existing ones, accurately reproducing the experimental flows that the latter cannot appropriately deal with.
??Understanding transport processes in surface water systems involving farmlands is a fundamental research topic in environmental hydraulics. Analysis of water flows in agricultural drainage canals, ubiquitous in the surface water systems in Japan, is an essential step to assess their environmental and ecological states. This paper carries out numerical analysis of steady water flows in a vegetated agricultural drainage canal that collects runoff from paddy fields. Water flows in the canal are described on the basis of the shallow water theory, which leads to the governing equations of the water surface elevation and discharge. Two shallow water models, the dynamic wave model and diffusion wave model, are applied to analyze steady flows in the canal. The parameters to specify the drag force caused by aquatic vegetation in the canal are determined so that the observed water surface profile is accurately reproduced using the dynamic wave model. The resulting dynamic wave model with the estimated parameters reasonably well reproduces the observed water surface profile. The computational results also suggest the validity of the diffusion wave model in simulating the steady flows in the canal.
??In this study, we aimed to examine the effects of hole-mulching culture on the growth circumstance and production of plum. As a result of three years’ experiment, the following effects of mulching were clarified. 1) The soil moisture content of hole-mulching plot increased greater by about 25% than the mulching plot due to the penetration of rainwater from multi-hole. 2) The evaporation from soil surface of hole-mulching plot was suppressed to about one-third of non-mulching plot. 3) Soil temperature of hole-mulching plot tended to be lower than mulching plot. 4) The production of hole-mulching plot was higher than non-mulching plot, but slightly lower than mulching plot. 5) The variation in individual weight of fruit of hole-mulching plot was as small as mulching plot and smaller than the non-mulching plot, and the percentage of fruits less than 15g with low commercial value was less than 10% of total fruits.
??The low-water design of a river in Japan, such as the dry-season discharge, is usually based on probabilistic river discharge data. In this paper, some hydrological and statistical analyses for the dry-season discharges between 1961 and 2010 in the Hii River basin were executed at 5 stations including 2 tributaries. The results showed that the probabilistic hydrologic dry-season discharges at the main stations in the Hii River and its tributaries have been arising as it nears to the present. It means the dry-season discharges in the Hii River basin have been intensified. At the same time, it also indicates that these indexes even for the same return period may fluctuate in the future.