Temporal and spatial dynamics of nitrate (NO₃^--N) in groundwater are investigated in the southern part of Okinawa Main Island, Japan, where Ryukyu limestone is extensively distributed. Influent factors of NO₃^--N formation are also examined on the basis of observed data in groundwater of the study area. It is found that probability distribution of NO₃^--N concentrations agree with normal distribution and the 75 per cent probability of NO₃^--N concentration is a little higher than the upper value (10mgL^-1) of environmental quality criteria. It is considered that NO₃^--N concentrations are influenced by nitrogen load accompanied with groundwater flow from the upstream to the downstream of the study area. It is also found that NO₃^--N concentration formation is assumed to depend on two factors, that, mixing-inflow actions and denitrification-dilution actions under conditions of groundwater flow caused by completion of the subsurface dam. It is suggested that nitrogen load from agricultural land in this area influences the long-term fluctuation of NO₃^--N concentrations in groundwater. It is recognized that groundwater level participates annual fluctuation of NO₃^--N concentrations in groundwater.

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The shipment of lotus plants in Ibaraki Prefecture is the first of its kind in the entire country. About 90 % of the lotus plantation in Ibaraki Prefecture was distributed the town and villages along Lake Kasumigaura. In this study, we investigated the load that outflows to the lake from the irrigation and drainage pump station and gates via the lotus fields, and further inflows to the lotus fields from Kawajiri River. The mechanism of load and unit effluent load of the lotus fields were determined by estimating the water and mass balances. As a result, 95% of the drainage from the study area entered the lake, and 5% of that drainage was recirculated as irrigation water. The discharged loads of COD, T-N, and T-P in cultivating period were found to be maximum during the puddling and planting period. Based on the mass balances of COD, T-N, and T-P, deduction in inflow and outflow loads from Lake Kasumigaura were 45.59, 2.97, and 2.22 kg·km^-2·d^-1, respectively. It was concluded that the discharged load can be effectively reduced by improving the irrigation system during puddling and planting period.

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We developed resistivity fish counter unit, which is a resistivity fish counter with rectangle frame type electrodes installed inside a polyvinyl chloride tube. In this study four resistivity fish counter units were assembled in the fish way at the Tone-ozeki weir at upstream period for small fish. We counted the number of small fish passing through each fish counter units. The each fish counter units was captured by a video camera from directly above, for 100 minutes. The number of fish was counted from the video analysis. The accuracy of the number of fish counted by using the fish counter was evaluated by comparing the result obtained from the fish counter with that obtained from the video analysis. The accuracy of the number of passing fish counted by using the fish counter was about 90% when number of fish, which pass through a fish counter unit, was less than 50/min. However the accuracy of the number of fish was about 60% when number of fish was more than 50/min because a number of fish pass through a fish counter unit together. The correction method using a correction formula was proposed to improve the counting accuracy by the resistivity fish counter in case number of fish is more than 50/min. It was shown that number of fish per 10 minutes can be counted with an accuracy of nearly 100% using the correction method.

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Wire netting fence which is lighter than the conventional fences has been introduced as a countermeasure of protection of agricultural damage by wildlife recently. There are two reasons to be introduced wire netting fence. The one is farmers' aging and labor shortage, the other is that farmers in the affected areas must put the fence in the ground by themselves when they introduce the damage protection public works. Considering farmers' aging and the depopulation in rural areas, it should be important for the farmers to talk with the maintenance of the fence in the long term perspective. This paper focuses on the farmers' consciousness of maintenance of the fence, and aims to uncover the critical points to maintain the fence effect.
The questionnaire to 68 farmers in K district of Aridagawa Town, Wakayama prefecture is analyzed from two points of view of difference in the farming communities in which the fence is introduced or not and in the respondents are leaders or not. As a result, it becomes clear that most of the respondents recognize the importance of daily maintenance works. To the contrary, the respondents who live in the community which don't introduce wire netting fence yet and who are not leaders don't understand the importance of the road for fence maintenance and rationalization of the maintenance work load. Besides, even the leaders don't recognize the importance of the future alternative ways of maintenance, which are the communities' cooperation and volunteer participation. These results show that the opportunity to learn the importance of the road for fence maintenance, the change in age structure and the alternatives to the present is necessary in the fence introduction process.

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We develop a decontamination method using the stirring soil and water systems built-in a recovering slurry system for decontaminating radioactively contaminated soil in already cultivated paddy fields. The recovering slurry system consists of stirring soil and water by puddling and pumping the soil as slurry into a settling tank with three suction nozzles and vacuum pump. The two pilot-scale operation tests were carried out : Test 1 on a relation between the puddling times and decontamination effect on the condition of 50m suction hosepipe length (SHL) and Test 2 on applicability for a 30m×100m size field (SHL: 180m). The Test 1 showed drastic decrease in soil radiocesium concentration by 2 repetitive puddling and gradual decrease by 3 to 9 repetitive puddling, and 30-40% decontamination reduce ratio (DR) for 9 puddling times and 2.6 times enrichment factor (EF) of soil radiocesium concentration. The Test 2 shows 15 % DR for 4 repetitive puddling and 1.7 times EF. Lower DR and EF of test 2 than of test 1 were caused by reducing sucking force due to longer SHL. It is necessary to improve EF and separation ratio of fine fractions at the nozzle point and to increase in amount of recovery slurry before application of the systems to the actual field.

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A new soil water table management system called the Farm Oriented Enhancing Aquatic System (FOEAS) has recently received widespread attention in Japan. The system incorporates buried controllable porous tube irrigation and mole drainage. To evaluate the drainage and irrigation functions of FOEAS, we performed two-dimensional measurement of soil water content in field plots with two treatments: (1) FOEAS set to maintain a water table depth of 30 cm (FOEAS treatment), and (2) a control in which FOEAS (including drainage) was installed but irrigation functions were not activated (control). Measurements were conducted continuously for 2 years under soybean cultivation, and the fluctuations in soil water content were analyzed. Soil water contents of FOEAS treatment and control were same in 2011, and that of FOEAS treatment was higher than that of control in 2012. Both treatments performed well with respect to drainage ability even though above-average precipitation was received in both years. For the FOEAS treatment, irrigation was applied successfully and the water table was maintained at 30 cm. However, this did not result in a significantly higher soybean yield.

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Reliability analysis has become important to designs of all kinds of structures, since both international and Japanese standards of structural designs are determined to have their theoretical and practical backgrounds on the reliability analyses. However, to apply reliability analyses to actual structures, statistics such as mean values and standard variations of design variables are needed. Aiming to apply reliability analyses to RC cast-in-place open channels, investigations are conducted for the standard variation of effective depth, which is supposed to be an important design variable in reliability analyses. Thickness of sidewalls and reinforcement coverings in them are measured in RC open channels, and the obtained results are examined statistically. The result showed that the construction accuracy of reinforcement positioning is much inferior to that of side wall thickness, and several cases are found in which construction accuracies are different among sidewalls along the same channel. As the standard variation for effective depth of sidewall of RC open channels, a figure of 10.1mm is proposed as the result of this study.

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Recently agricultural pipelines used longer than their durable years are increasing. Appropriate evaluations of the structural safety of buried pipeline are required for effective preservation of aging pipelines to prevent failures by the deterioration. However, it is difficult to quantitatively evaluate the structural safety with a conventional method that measures pipe deflection which denotes simple cross-sectional deformation. Thus a new verification method is required to quantitatively evaluate structural safety. Therefore, a new method that estimates the bending strains by measuring changes in the curvature of pipes was developed. Experiments with a welded steel pipe were conducted to estimate the accuracy of the method. Results of the experiments show that structural safety can quantitatively and easily be evaluated with this method and its accuracy is more than 80 percent with strains greater than 1,000μ bending strains.

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The accuracy and precision of estimated solute loads from a small forested catchment are evaluated based on the 10 minutes resolution discharge data and 15 minutes resolution water quality data during twenty three-month period. The target indices are potassium, chloride, and sodium. Loads are calculated and evaluated using a number of proposed load calculation methods based on regular interval sampling. As a result, the best performance is attained with the load calculation method that combines 7-parameters model in USGS Load estimator, where parameters are determined by least squares method in log-scaled load space, and bias correction by composite method. However, with this method, the 95% confidence interval constructed by bootstrap method doesn't include a true value with expected confidence level for all the combinations of index and sampling interval. The load calculation method that gives more unbiased estimated value are required.

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This paper discusses the distribution properties of pan evaporation that is an index of the evapotranspiration in a small wetland. The evapotranspiration is very important to a small wetland. It is to influence the wet condition of the surface very much. Kawaminami wetland in Miyazaki Prefecture is the studied site. In this wetland, the values of pan evaporation at many locations were recorded. Furthermore, groundwater levels and meteorological elements were observed. As a result, distribution of pan evaporation has been related to distribution of surface solar radiation and surface moisture condition in wetland. Therefore relations of pan evaporation and evapotranspiration were considered and arranged a complementary relationship. The complementary relationship has accorded with distribution of pan evaporation in wetland. Finally evapotranspiration of the wetland was able to be estimated.

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It is important to evaluate tensile strength characteristics of foundation and embankment material of fill dam in order to examine conditions for crack development which is one of the causes for the hydraulic fracturing. While it is possible to evaluate tensile strength of hard rock by fixing both ends of the specimen with chucks and applying the force directly, it is not possible to use the same method for soft material like cohesive soil. In this study, we designed an I-shaped plate specimen by the FEM analysis so that it contains parts where stress acts almost uniformly. The direct tensile test was performed with the specimen made of volcanic ashes loam and the tensile strain was measured by image instrumentation using digital cameras. Our tensile test method enabled continuous measurement of tensile stress and strain with soft material like cohesive soil.

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There were the following typos in the article, “Irrigation, Drainage and Rural Engineering Journal Vol.81 (2013) No.3 p.263-270”

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