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We showed that the amount of irrigation water was 34.4 m3/s during irrigation period in the kinokawa river, Japan. This is very large compared to the river discharge in a normal flow condition. To take water use into account a runoff analysis using a distributed hydrological model, we proposed a new model that consider the water-intake from head works, inflow, storage and flood control by dams. It was found that the simulated river discharge using this model agreed with the observation data, especially for the low water flow in the irrigation period. In addition, the reduction rate of irrigation water from paddy fields to kinokawa river was inversely estimated. The rate was low in a small precipitation year and was high in a high precipitation year. This characteristic agrees with a traditional and empirical water uses of irrigation water.

This paper evaluated the effects of land use on river water quality in the Kishi and Zakuro Rivers. Both basins are mainly covered by forest and orchards, and especially about 30% of the Zakuro River watershed is orchard. In the field observation, the concentrations of NO3- and Mg2+ in the Zakuro River were observed to be higher than those in the Kishi River from autumn to spring. However, there were no differences in the concentrations of the other ions and COD between both rivers. In order to examine these results, especially the effects on the concentrations of NO3- and COD, nitrogen and COD loadings were calculated using pollutant load factors. The calculation result of nitrogen loading implied that the discharge of nitrogen from the orchards played the most important role in the change in the NO3- concentration of the Zakuro River. In the meantime, there were no differences in COD loadings between both basins and therefore the COD concentrations would be approximately equal.

Soluble substances of the Kinokawa-river increased down the stream. Excluding weathering of limestone, soluble substances in the upper stream of the catchment covered with forest were very low and main components were HCO-3, SO2-4, Na+, and Ca2+. Soluble substances in the upper stream of the forest catchment with limestone were high and the main components were Ca2+ and HCO-3 Ca2+ and HCO-3 concentrations increased from the upper stream to the middle stream with forest, but Na+, Cl- and SO2-4 concentrations remained unchanged because of weathering of limestone and feldspar. In the lower stream of the catchment with farm land and housing site, the concentrations of Na+, Cl-, NO-3 and SO2-4 were very high. It was concluded from the relation between land uses and water qualities that Na+ and SO2-4 origin were farmland, and Cl- origin was sewage respectively, and NO-3 was derived from both origins. Isotopic study of oxygen and hydrogen indicated that the river water originated from precipitation and isotopic ratios depended on mainly precipitation altitude.

The flux of NO-3 and BOD values in the Yamato river water directly increased with precipitation. Therefore, the NO-3 was considered to originate mainly from fertilizer (A) because it was reported that the flux of NO-3in farmland drainage increased with precipitation, although domestic sewage (B) also contains a lot of NO-3. As sediments in the river channels and drains containing organic compounds migrated on rainy days, river BOD values are considered to increase after rain. On the other hand, NO-3origins in branch rivers of the Yamato river are various combinations of (A) and (B). The NO-3origins of branch rivers were determined by whether the flux of NO-3 increased with the flow rate or not. On the other hand, CF concentration in the Yamato river water is considered to strongly depend on the amount of domestic sewage. Although the flow rate in the river changed remarkably due to heavy rains, the flux of CF remained unchanged because the volume of domestic sewage did not change even on a rainy day.

BOD指標の第1位の清流な河川である札内川 (平成7年度) とBOD指標ワースト1の大和川 (平成7年度) の支流石川について, 土地利用特性を考慮して河川水質の比較を行った. その結果, 札内川では中流域の農地において河川水は硝酸イオンに富んでいた. 硝酸イオンの単位面積負荷量は札内川の方が高い値を示し, 施肥等の影響が強いものと考えられる. また, 流域人口の違いは塩素イオン濃度に大きな影響を与えることが分かった. 酸素・水素同位体比より札内川の方が蒸発速度の速い気団が供給源となっていることが推定された. さらに, 両流域の比較によって酸素同位体比の緯度効果が確認された.

Sources of water and soluble substances in the river and groundwater were studied. As the δ D andδ18O values of river and groundwater were on the meteoric line and these values varied with sampling altitudes, these waters were thought to have arisen from precipitation. CO2 pressure in river and groundwater were estimated to be high from pH values and HCO 3- and were equilibrated with CO2 pressure in soil. The δ 13C values of river and groundwater were in agreement with that of soil. High CO2 pressure derived from soil was thought to cause weathering of feldspar in rock and to increase HCO3- and Ca2+ concentration in waters. As the δ 15N values of NO3- in river water varied with land use of sampling pints, the source of nitrogen contamination could be estimated from δ 15N values of NO3- in waters.

It is important that it makes clear cyclic process of water and water balance in urban river. But, the process of water cycle and water balance have not been made clear in the hydrologic system, because many factors are each other influenced complicatedly and that it is not elucidated in real basin.In this study, we attempt to clarity the process of water cycle in a watershed of the Ishikawa river with a discharge area of 220 km2 by environmental isotope tracers. Both geochemical data which are EC, δ18 O etc. and river discharge are directly measured at many points.As result, it is obvious that cyclic process of river water and water balance are grasped through geochemical data and field observation from macroscopic point.

Chlorinated hydrocarbons (CHCs) are highly volatile and little soluble in water, and can exist in unsaturated zone as vaporized, dissolved and undiluted non-aqueous phases. Therefore, it is of great importance to understand transport mechanism of CHCs with gas advection and diffusion in unsaturated zone, and then numerical analysis for vertical transport of CHCs in unsaturated zoneincluding effect of rainfall was conducted. Vertical one-dimensional fluid flow equations and mass transport equations for water and gas phases, and two layer model for mass transfer between the phases were employed in the model presented. The results showed that, though gas phase transport is usually dominant during dry period in unsaturated zone, transport in water phase overcomes that in gas phase during rainwater percolating period.

In the present paper, the simulation results of infiltration of trichloroethylene from the natural river bed to groundwater region is shown. The method of characteristics (MOC) and iterated alternating direction implicit method (IADI) was applied to the computation of two-dimensional convection-dispersion equation and groundwater flow equation. It was found that the infiltration from the river bed when the groundwater level was high in the steady state becomes large while small when the level was low. In the transient period of the groundwater table from high to low or from low to high levels, spread of trichloroethylene showed an opposite tendency due to the unsteady velocity field which contributes to the convection of mass transport.