Recently NO₃-N concentrations of rivers in the agricultural area of Kasumigaura lake basin have increased. In spite of much eutrophication, this water of lake is used as a source of drinking water. In this area, unlined storage ponds are located in pig farms to store animal waste. However the effect of this type of pig farms on nitrogen concentration of rivers is not clear.
NO₃-N concentration of the rivers has increased, above 5 mg/l in this area, where average stocking density in the catchment area is about 300 head/km² (pig). A detailed investigation was carried out in two sub-catchment areas of this river. Near the pig farms high concentrations of NO₃-N were observed. Stocking density of pigs and NO₃-N concentration had a high correlation (R=0.80). In this analysis stocking density was predicted by the size of the pig house estimated by from aerial photographs. In this area, the stocking density had a larger effect on nitrogen concentration in rivers than the land use.

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Investigations were carried out in two watersheds in the Kasumigaura lake basin. Pig farms with the unlined storage ponds had a large effect on nitrogen concentration of rivers. The higher stocking density was, the higher NO₃-N concentration in stream water.
Measurements of water quality were carried out once a month. The change of nitrogen concentration of stream water was smaller than that of stream discharge. Nitrogen concentration was high and almost constant through out the year in this high stocking density area.
Nitrogen concentrations were lower in irrigation period than in non-irrigation period. It was considered that this decrease of concentration was caused by the nitrogen removal due to denitrification and plant uptake in paddy fields. The value of nitrogen removal increased corresponding to the increase of the rate of paddy field area and nitrogen concentration of stream water. But in the case of very high concentration, nitrogen removal was not so high, because farmers did not use the polluted water for paddy fields to prevent excessive application of nitrogen.

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The relation between nitrogen concentration of stream waters and stocking density in 6 districts where unlined storage ponds are used, and in 3 districts where manureslurry is applied in agriculture field were investigated.
Stocking density and NO₃-N concentration of stream waters had a high correlation in the unlined storage pond areas. Stocking density and specific NO₃-N load also had a high correlation.
In unlined storage pond areas, the effluent rate of nitrogen due to pig farming is estimated to be about 30%. At manure-slurry areas, NO₃-N concentrations are lower than those of unlined storage pond areas. Therefore the effluent rate in manure-slurry areas is lower than that in unlined storage pond areas.

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The authors designed a new technique to measure dielectric constant of a soil by frequency domain reflectometry (FDR) of coherent wave from the soil sample. In addition, they developed a technique to measure soil moisture at a specified soil depth by a combined use of 2 probes with different length. Furthermore, dependence of the dielectric constant on temperature change was also investigated. The measurements were made on 3 types of soils (a Toyoura standard sand, a Masa sand and a loam) with different types of probes (a one-rod probe and a three-rod probe). The obtained results verified that the technique is very promissing for nondestractive and continuous soil moisture measurement monitoring both in a laboratory and on a field. The important results obtained are as follows.
(1) Measurement by a three-rod probe and a one-rod probe agreed well each other. The one-rod probe was more sensitive at low soil moisture than the three-rod probe.
(2) The relationship between the soil moisture and the dielectric constant of the soil was expressed by a single regression curve independent of soil texture at a small experimental error. The derived regression curve coincided well with that obtained by Topp et al. A exception was seen on the standard sand and the Masa sand. At saturated condition of those soils, the measured dielectric constants showed slightly larger values than those of the loam.
(3) Measured dielectric constants did not show significant temperature dependence in the temperature range from 10 to 40°C. Below 10°C, some temperature dependence was detected, however, it was not so significant as any correction should be needed.
(4) Combined use of probes of different length enabled to measure the soil moisture at a specified soil depth. The soil moisture profiles measured by the combined probe method were compared with those of the single rod dielectric measurement method by probes buried at each depth as well as with profiles obtained by sampling and oven-dry method. The agreement was quite good.

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Field tests on soil moisture measurement by frequency domain reflectometry (FDR) were carried out at two upland fields with different soils (Ando soils (Kuroboku) and Residual regosols (Masa)). Calibration curves between dielectric constants and volumetric water contents were determined for each soil. Dielectric constants of the field soils were converted into volumetric water contents using the calibration curves.
Soil moisture profile from 0 to 50 cm was measured by two FDR methods: the direct FDR method and the combined probe FDR method. In the direct FDR method, soil moisture was measured by a probe inserted at every 5cm interval depth in the soil. In the combined probe FDR method, soil moisture was measured by a series of probes with varying lengths ranging from 5 to 50 cm. These probes were inserted from the soil surface, then the soil moisture at each depth which coincided with the difference in probe lengths was calculated from difference of propagation velocity in the probes. The soil moisture profile measured by the FDR method was compared to those obtained by the oven dry method and a tensiometer method. Spatial distribution of the surface soil moisture was also determined by the FDR measurement. Obtained results are summarized as follows:
(1) The calibration curve for the Masa soil agreed well with that of the Toyoura standard sand and the Topp's curve, while, dielectric constants of Kuroboku soil were smaller than those of sandy soils at the same water content. Therefore, another calibration curve was needed for the Kuroboku soil.
(2) Soil moisture profiles measured by the direct FDR and the combined probe FDR method agreed with those measured by the oven dry method within 5% difference of volumetric water content. On the other hand, those obtained by the tensiometer method gave 7-15% higher values than those of the oven dry method. The FDR methods were verified to give accurate and reliable soil moisture values in a short measuring time.
(3) Spatial distribution of field surface soil moisture was successfully measured by the FDR method. The quick and nondestructive measurement of FDR technique will be a powerful tool for analysis of spatial variability of soil moisture.

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Rill development process and relation of rill erosion to rainfall was studied on test plot under natural rainfall. The test plot used in the study, with a slope steepness of 10°, was 2.5m wide and 22.1m long in the direction of slope, and filled with Sirasu soil. The rill networks on the plot were photographed, the depth and section profiles of rill were also measured every 20cm after each rainfall. Invested data and one derived from photographs show that:
(1) The critical condition for rill occurring on Sirasu soil is rainfall of at least 50mm, and maximum 30-min intensity (I₃₀) more than 20mm/h.
(2) The critical distance for rill development on Sirasu soil was observed as 2m, and rill density reached a maximum of 2m/m². Interspace between rills is narrow compared with that of other soils.
(3) As the first step of rill development, there were many pairs of short rill to occur on the plot, and as rainfall continued, the short rills connected and deepened and widened.
(4) The scouring and side-wall sloughing of the rills reached a maximum of 10-11m and 6-10m from the end of plot. No relationship was found between the depth and width of the rills, but it was understood that at the beginning of rill erosion, the width of a rill is greater than the depth, and as rainfall continues, the depth becomes larger quickly than the width. Deep scouring is a dominant form of rill development on Sirasu soil.
Section profiles of rill was found to be narrow and deep rectangles with vertical side-walls. Areas of section profiles can be estimated with the depth and width. There was a linear relationship between the length and circumference of a rill. It suggested that rill on Sirasu soil is nearly linear on the base of the fact that circumference of rill is two times as long as length of rill. An exponential relationship between length of rill and areas of rill was also established.

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The settling characteristics of Ariake clay suspensions with different initial water content were investigated using cylinders of 1000cm³ and 100cm³ under different salt concentrations. The settling patterns of Isahaya Bay and Ariake-kantaku clays were similar to a marine clay at Tokyo Bay rather than kaolinite and bentonite. The settling rate of the solid-liquid interface linearly increased for Isahaya Bay clay while exhibited a smaller increase for Ariake-kantaku clay with increasing the initial water content of the clay suspensions. The settling rate of the solid-liquid interface was not affected by the salt concentration under the constant initial water content of the clay suspensions.
The settling rate of the clay particles was accelerated when they are coagulated into flocs. Smectite and iron oxide contents, and the percentage of the exchangeable calcium ion was assumed to be the major factors that differentiate the mode of coagulation between Isahaya Bay and Ariakekantaku clays. The time?settling curves for the two clays were unchanged by cation exchange treatment and iron oxide removal, where the settling rate in sea water, for the clays with initial water content of 4000%, was four times greater for Isahaya Bay clay than for Ariake-kantaku clay. On the contrary, organic matter removal from the natural clays decreased the difference in the settling rate between the two clays for the whole range of the initial water content of the suspensions. These results indicate that the difference in settling rate between Isahaya Bay and Ariakekantaku clays in sea water would be attributed to the difference in the constituents of the organic matter in the clays.

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Recently, in hilly (less favored) rural areas, regional resources such as farmland, forest, etc., suffered from difficulties in management by depopulation due to moving out of young peoples. In this paper, destruction of farmland caused by rainfall compared between two townswhere large amounts of depopulation observed, and two cities-where the population slightly increased or did not change. Runoff characteristics in the catchment of three different types of land utilization also investigated. These were (i) bench terraced paddy field ( “paddy field catchment”: including 75% of forest in the catchment), (ii) reclaimed upland field ( “upland field catchment”: including 50% of forest in the catchment) and (iii) “forest catchment”. The results of the study are as follows;
(1) The number of destruction caused by heavy rainfall on farmlands were far large amount in the two towns than that of in the two cities. On the contrary, there observed no differences in the total renovation costs between the two cities and the two towns.
(2) Though smaller in sizes, a large number of destruction taken place in the two towns, suggesting that daily management of agricultural lands seemed to be difficult in the depopulated areas.
(3) Paddy fields that occupied 25% of the catchment area, affected discharge of the catchment. The “paddy field catchment” showed similar runoff characteristics like “upland field catchment”.
(4) The amount of direct runoff and peak discharge in the “paddy field catchment”, and also in the “upland field catchment”, showed increasing tendency in comparison with those of the “forest catchment”. Discharge in the “paddy field catchment” after the peak decreased more quickly than that of in the “forest catchment”.
(5) Generally, paddy field area possesses the function of flood control by storing water in its dike. Considering the results of the study, it concluded that this “paddy field catchment” does not seem to perform this control function. It seems to concern with method of water management of this area.

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A method was developed for designing microirrigation submain units. Hydraulic analysis is implemented by combining the lateral discharge equation, the back step method, the forward step method, and the golden section search. When the required uniformity of water application, the required average emitter discharge, one parameter of the lateral length and diameter, one parameter of the submain length and diameter, and the field conditions are given, the unknown parameters of the lateral and submain, the best submain position (paired laterals), and the operating pressure head of a submain unit can be accurately designed using a personal computer. The calculations in design procedures are simple and fast. It was found that the lengths of a lateral and submain may have two or three solutions and the diameters may have two solutions for some values of a uniformity coefficient of water application in slope conditions.

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2 kinds of model for the paddy field with the stratified soil layers (the total layers are composed of an organic model, and the total layers are composed of an inorganic model) were formed, and then their percolation type (the closed system percolation, and the open system percolation) and the corresponding relation such as the soluble ions (Fe, Mn) concentoration and so forth in the precipitated percolating water were investigated. Allthe more, for a model, entire layers are composed of an inorganic soil, the paddy field soil (alluvial soil), which actually has the illuviation layers, was used.
As a consequence, the knowledges as follows were obtained.
(1) Also in any of 2 models for the paddy fields with the stratified layers, in case of a low drainage level (70-90cm under a surface soil), the closed system percolation layers (surface soil, plowsole, Kuroboku soil, and alluvial soil) were low DO (about 2ppm), and presented a reducing layer, in which Fe, Mn ions were detected. However the open system percolation layers (subsoil, compacted Kuroboku soil, and a soil sampled undisturbed subsoil of paddy field with illuviation layers) were high DO (about 6ppm), and were an oxidation layer, in which Fe, Mn ions couldn't be detected.
(2) When in the same soil columnar model, a drainage level was raised (8-20cm below a surface soil) and moreover the total layer closed system was made, any layers in both models as well become low DO and even in inorganic soil, they became the reducing layer (30 days). In the precipitated percolating water, a detection of Fe, Mn ion has become to be recognized from all layers.
(3) After dismembering of paddy field model with stratified soil layers, a detection depth of Fe and Mn ion in the precipitated percolating water, and an increase and decrease relation of these materials in the layers were investigated. Also in the both models, the soil color and concentration increase phenomena were not recognized.
Based on the observation results mentioned above, the percolation pattern of precipitated percolating water was proven to be a qualitatively important factor for the soluble ions to eluviate and illuviate corresponding to an oxidation and reduction.

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The asphalt mixtures used for asphalt facing of fill-type dams require the following properties:(1) impermeability, (2) conformity to settlement of basement, (3) stability against shear stress of the slope, and (4) durability against weathering action. Since asphalt facing is exposed to natural environment after its completion, resistance of asphalt to weathering action has a strong effect on these required properties.
In this research the durability of asphalt mixtures for asphalt facing was examined based on the results of mechanical tests on specimens taken from asphalt facing in use, those exposed to outdoor environment, and of physical tests on recovered asphalt.
It is confirmed that long-term durability can be expected for asphalt mixtures to be used for asphalt facing. Selection of asphalt mixtures of facing is an important factor for future construction of more durable asphalt facing.

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Images of soil macropores can be easily obtained by radiographing soil samples filled with contrast agent using soft X-ray. It is, however, difficult to recognize the intricated shade formation in these images by image processors. In this study, recognition of a conjugated pair of macropores shade and measurement of the pore width from stereo images were conducted using the naked eye, and the system for calculating the three dimensional expression of soil structure was developed by the image processor.
The soft X-ray stereoradiographies of grey lowland soil in paddy fields and andisol in upland soil were used to verify the effectiveness of this system. At first, it was confirmed that the threedimensional images obtained by the system were almost the same as those obtained using the naked eye. Then, the distribution of diameters and direction of macropores were calculated. In paddy soil, it was found that the main macropores were in the vertical direction and the others were in the horizontal direction. The tortuosity of main macropores were 1.15 in paddy soil and 1.31 in upland soil. The percentages of pore volume, which strongly affected the permeabilities, were less than 1% for total pore volume.

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In order to assess the erosion characteristics of bottom sediments on the tidal flats in Isahaya Bay, field observation and hydraulic experiments were performed. Tidal current velocity and suspended solids concentration were measured for 12 hours in the zone one meter above the sea bottom. Two time series of bed shear stresses were generated during a period from the flood tide to the ebb tide in an annular flume.
The velocity of tidal current on the observation station exhibited a logarithmic increase from the bottom toward a height of 0.5 meters. The suspended solids concentration indicated peaks both at the end of ebb tide and the beginning of flood tide. The increasing rate of the bed shear stress estimated from the measurement of the tidal current velocity during a tidal cycle was greater during the flood tide than during the ebb tide.
In the hydraulic experiments the seawater and the sediment surface mud obtained from an observation point in Isahaya Bay were used for hydraulic experiments. The mud sediment in the annular flume was prepared by allowing the suspension to settle for three hours.
The suspended solids concentration in the seawater was measured, and the density and the shear stress versus shear rate relationship were determined for the mud at the surface layer of the sediment.
The degree of erosion was greater during the flood tide than during the ebb tide. The generation and duration of the erosion were explained in terms of the comparison between the yield value and the bed shear stress at the surface layer of the sediment. The critical erosion shear stress for the sediment was almost equal to the yield value at the surface layer of the sediment.

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In water resources development projects, reservoir capacity requirements (CR) are based on the excess probability concept. This paper proposes to use CR as an index for expressing the characteristics of the river flow time-series. Ten reservoirs throughout Japan are selected under the condition that their inflow records indicate the natural discharge from the catchment areas with little influence from human activity. Using 21 to 35 year historical inflow records from these reservoirs, the authors studies the relationship between water use and CR (at the return period of ten years) for the paddy irrigation season (April to September) as well as for the whole year.
The water use level up to 70% of the average discharge is discussed to avoid over-year storage operation.
The main results obtained are as follows:
1) CR is recognized as an appropriate index which totally reflects change in discharge, and is a more stable index than the maximum storage requirement which is influenced by extraordinary droughts, when they occur, during record period.
2) Since the correlation between the CR for the paddy irrigation season and that for the whole year is not strong, the latter gives little information on the availability of water during the irrigation season, when most of the water resource shortages occur in Japan.
3) The square root of CR is proportional to the developed flow added to the low flow at each reservoir.
4) The proportional constants of the ten reservoirs are analyzed with the multiple-regression method adopting several predictor variables such as the average discharge, low flow discharge, recession coefficient during low flow, and so on. This suggests that only the average discharge is a major determinant of the proportional constants with the determination coefficient of 0.92.

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An upwind wedge-shaped intermediate layer between upper mixed and lower undisturbed layers, which was formed in a wind induced two-layered flow, was studied theoretically and experimentally.
Assuming the wedge as a mass body, equations governing for the wedge depth at the upwind end and the wedge density were lead from the two-dimensional volume and mass conservation equations.
The parameters which were related to the geometric configurations, density, expansion and contraction velocities and entrainment from the wedge to the mixing layers, were investigated by using experimental data.
The scale of the wedge formation was dependent on the overall Richardson numeber. The wedge density increased exponentially with time. In a growing state of the wedge formation the entrainment rate from the wedge to the mixing layer was much smaller than in a quasi-steady state.

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A time marching method for one-dimensional discontinuous open channel flows is discussed. A one-parameter family of explicit and implicit upwind second-order-accurate, total variation diminishing (TVD) schemes developed by Yee are introduced into the steady flow computations in open channels, two ways of linearizing the implicit operator are discussed. These TVD schemes have the property of not generating spurious oscillations when applied to one-dimensional nonlinear scalar hyperbolic conservation laws and constant coefficient hyperbolic systems. For boundary computations, a differential scheme derived from the theory of characteristics is proposed.
The results of numerical experiments are shown as follows. The unsteady flow computation for the dam-break problem shows that the calculated values by conservative linearized form agree well with the theoretical values, but the propagation speed lag and the wave height difference in the field of hydraulic bore are observed when nonconservative linearized form is used. Furthermore there is a numerical solution gap in the field of rarefaction wave. The steady flow computation for the open channel with some inner boundary conditions shows that nonconservative linearized form not only has a fairly rapid convergence rate, but also generates a high resolved approximation to the steady-state solution except the profile of hydraulic jump.

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The contact aeration process with anaerobic filter tanks is considered to be promising as a simple and convenient method for the simultaneous removal of organic matter and nitrogen in small scale plants treating domestic wastewater. In this process, wastewater is first led into an anaerobic filter tank and then it is led into a contact aeration tank before some of the treated wastewater is returned to the anaerobic filter tank.
In this study, plant site investigations on the contact aeration process with anaerobic filter tanks were made to find a suitable operation of the plants using this process.
As a result, the efficiency and stability of BOD and SS removal, and of denitrification were found to be satisfactory. Therefore, the important point of the plants' operation is how to obtain a high rate of nitrification, and then keep the return water ratio high. The intermittent operation of blowers was worthwhile in terms of saving energy, but the stopping period of aeration should be restricted in a few hours in order to make the nitrification satisfactorily. The abnormal growth of large invertebrate in the aeration tanks greatly deteriorated the progress of nitrification.
To exterminate the invertebrate, the method of stopping aeration for a few days was available. The intermittent aeration, and the increase of the BOD loading on the aeration tanks changed the dominant species of invertebrate, but could not prevent their abnormal growth. The periodical back washing of contactors probably was effective for prevention of their abnormal growth. In the situation of abnormal growth, the increase of aeration intensity was available for the promotion of nitrification.

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The contact aeration process with anaerobic filter tanks is considered to be promising as a simple and convenient method for small domestic wastewater treatment plants in rural areas.
In this study, data obtained in 15 plants which are in actual operation were analyzed by means of Quantification-I, a kind of multivariate analysis, to determine the factors affecting nitrogen removal in this process and suitable operation and design of the plants using that process.
As a result, the major factors for nitrogen removal were found to be the temperature, the DO concentration, specific surface area of contactors, and the propagation of large invertebrates in aeration tanks, the BOD loading on aeration tanks, and the daily aeration hours.
In order to achieve the higher rate of nitrogen removal, the DO concentration in the final aeration tank should be 5mg/l or higher, the return water ratio should be about 3, the daily aeration hours should be 20 h/d or longer. The lower the BOD loading is, the more satisfactorily nitrogen is removed. The larger the specific surface area is under 50 m²/m³, the higher nitrogen removal is achieved. However, the back washing might be an important operation in using the contactors having over 50 m²/m³. The abnormal growth of large invertebrates has a great effect on the nitrogen removal. Therefore, it is of urgent necessity to develop some appropriate method to exterminate them or prevent their abnormal growth.
It is very important to regulate these factors in winter, when the temperature of wastewater in aeration tanks is low, especially under 14°C

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[in Japanese]

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