Fusetsu Takagi

Hydrograph separation and flow analysis by specific electrical conductance of water

The use of the specific electrical conductance of water to separate flow hydrographs into event (new) and pre-event (old) waters has been studied through the analysis of background events and flow process phenomena in a study basin. Runoff ratio analysis shows that background events, such as periods of no rainfall or no flow, influence the percentage of old water occurring in the hydrograph. Comparison of one result with a separation using δ18O as tracer shows that the separation model of Matsubayashi et al. (1991) produces reasonable results. The analysis of the conductance-discharge (C-Q) relationship showed that there were fast flows in a majority of the wet season hydrographs.

Characteristics of the dispersion coefficient in miscible displacement through a glass beads medium

Abstract The importance of understanding the process of solute transport in soils is well documented in several fields of study, including water supply, environmental protection and agriculture. However, researchers still disagree on some topics, especially the nature of the unsaturated dispersion coefficient. Miscible displacement experiments, carried out using glass beads as the porous medium show that while the saturated dispersion coefficient increases linearly with pore water velocity, under unsaturated conditions, the dispersion coefficient increases linearly but more rapidly with pore water velocity up to a limiting moisture content. Beyond this moisture content, the unsaturated dispersion coefficient decreases and approaches that of the saturated condition. These phenomena are discussed in relation to variation in pore water velocity and the mixing length of flow paths inside the porous medium. Mixing length in this sense is characteristic of the porous medium within which the concentration of the solution conforms to the surrounding concentration. The dispersion coefficient is expressed as the product of mixing length and standard deviation of the pore water velocity which is estimated from the ψ-θ and K -θ relationships. The mixing lengths are found to be almost constant (about 0.23 cm) in the lower moisture content range (0.16–0.27 cm 3 cm −3 ), but decrease at moisture contents greater than 0.27 cm 3 cm −3 . Mixing lengths are also smaller in saturated than in unsaturated conditions. Despite the apparent increase in pore water velocity and its variance at higher moisture content, the decrease in the dispersion coefficient can be attributed to a decrease in the mixing length. This study shows that the dispersion phenomenon, both in saturated and unsaturated conditions, can be described successfully using mixing length theory.

ON THE EFFECT OF THE GEOGRAPHICAL FEATURES TO THE RUN-OFF IN MOUNTAINOUS FOREST AREA

Understanding of the terrain effects on hillslope runoff processes is important to formulate the accurate runoff model. In this study, observations were, therefore, made to understand the spatial distribution of volumetric water contents, thickness and hydraulic conductivities of various soil layers, and response of volumetric water contents considering both concave and convex configurations of land surface in Kanedaira catchment. Observations showed that the response of volumetric water content of all points in concave part and surface of the convex part were similar to runoff response. However concave configuration had produced faster runoff than that by convex one.

On the Simulation of Rainfall Based on the Characteristics of Fourier Spectrum of Rainfall

Design rainfall is usually determined by magnifying historical data to an amount corresponding to a certain return period. However, the spatial distribution of the precipitation is usually not considered in the design rainfall computation. With this point of view, in this paper we aim to discuss the spatial characteristics of rainfall.