This paper describes a model-based methodology for optimally applying fertilizer to upland crop fields. An optimization problem is formulated as an environmentally-sound fertilizer application scheduling problem with the objective of controlling rainfall-and/or irrigation-induced nitrate leaching to groundwater while not causing a poor crop. To assess change in leaching nitrate in terms of changes in rainfall (and/or irrigation water supply) and applied fertilizer during a cropping period, a currently available well-formulated nitrate leaching model is employed which serves as a simulation module in the optimization model. This model-based methodology is applied to a farm plot, Kumamoto Prefecture, and optimum fertilizer application schedule is obtained under several maximum daily fertilizer conditions. The obtained schedules achieve effective control of nitrate leaching without causing nitrate deficit by avoiding rainy season and adding supplement application. It is thus concluded that the optimization model developed could be a useful decision-making support model for the agricultural sector to alleviate groundwater nitrate contamination because the model can advise timing and volume of fertilizer application taking into consideration rainfall properties.

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In this paper, the runoff properties in a forest and a terraced paddy filed catchment were compared and discussed according to the analysis by a long-term runoff model. By the comparison of observed discharge, it was found that storm runoff in the terraced paddy field catchment responds rapidly to rainfall and the peak discharge is greater than that of the forest catchment. It was also concluded that the amount of low flow discharge in the terraced paddy field catchment decreases remarkably comparing to the forest catchment during irrigation season, while no specific differences were identified during non-irrigation season. As the result of application of a simple long-term runoff model which is composed of single row of tanks, the model performance was not so good for the terraced paddy field catchment. Therefore, a distributed model which was composed of parallel row was introduced to represent the properties of soil and water management in the terraced paddy filed and the performance of the model was discussed. Then, better results were obtained, especially in low flow discharge during the irrigation season. Moreover, it was confirmed that the identified parameters reflected the hydrologic properties in the terraced paddy field.

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Population density of five fish species was investigated at five valleys with different environmental characteristics. The investigation was carried out during non-irrigation period (January and February), irrigation period (May and June), and transitional period (August and September). As a result, it was found that distribution pattem of Numamutsu, Tamoroko, Simadojyo, Dojyo, Hotokedojyo were different By using analysis of quantification theory I, environmental factors for each fish were identified. The factors obtained were summarized as follows; 1) distance of ditch from Tanigashira (upper most of hill-bottom valley) to each station, and from Tanijiri (lower most of hill-bottom valley) to each station were extracted as environmental factor of Hotokedojyo; 2) From Tanijiri to each station, maximum depth, water velocity, rate of sand riverbed area in May and June for Simadojyo; 3) From Tanijiri and Tanigashira to each station, maximum depth, water velocity, rate of undercut bank area, rate of silt and sand riverbed area in January and February for Numamutsu; 4) rate of undercut bank area in January and February for Tamoroko; and 5) distance of ditch from Tanijiri and Tanigashira to each station, water velocity, rate of overhanging vegetation in August and September for Dojyo one year old or over.

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A multipoint survey of water quality was conducted as part of fundamental research aiming at preserving and improving the water environment of Lake Togo, in Tottori Prefecture, Japan. The focus was on the zooplankton and phytoplankton, as well as on other factors related to eutrophication. The water environment of the lake was evaluated from the perspective of their seasonal variation and distribution. The results of this survey indicated that oxygen-deficient water frequently occurs at the lake bottom during the summer in spite of shallow lake. Furthermore, increase in salinity resulting from seawater inflow affected the nitrogen and the phosphorus levels. Measurement results for TN, TP, and chlorophyll-a showed that hypertrophication had taken place, particularly in the southern part of the lake. Phytoplankton growth was limited apparently due to phosphorus, because the TN/TP ratio at the water surface, averaged over all observation points, was higher than 29.0 during the summer, and there was a good correlation between the TP level and the blue-green algae population. These results indicated that reducing the phosphorus level was particularly important in controlling the growth of blue-green algae.

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In this paper, new evaluation method of flood mitigation function was proposed. It defined that with this method the flood mitigation function means averaging the rainfall-runoff. Moreover, it was shown that the amount of averages is related to the amount of peak discharge reduction. Therefore. this relation was defined as averaging time. The averaging time can be found by the accurated rainfall method and the moving average method. Furthermore, it is applicable to evaluate of the amout of peak discharge. Therefore, the concept of this averaging time can evalutate the flood mitigation function. It was shown that this evalutation method is a method effective in evaluation of the flood mitigation function in small cathcments and irrigation pond basins.

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The emissions of methane and nitrous oxide from paddy fields to the atmosphere were observed at two practically used rice paddy fields in the northern part of Honshu Island in Japan. The observation was conducted by the closed chamber method basically once a week for about 6 months from June to December 2005. Intermittent irrigation was introduced from the beginning of July to mid-August to analyze the effect of the water management on variation in the gas emissions. The methane emission and the nitrous oxide emission showed opposite tendency to each other, in agreement with the previous studies. During the intermittent irrigation, the gas emissions cyclically fluctuated, responding to soil redox conditions which are governed by the water regime. However, the peaks of the gas emission did not precisely coincide with the irrigation and drainage actions. The gas emission often lagged several days behind the water management. Spatial variation in the gas emissions was greater during the intermittent irrigation than in other periods. The spatial variation was greater in the nitrous oxide emission than in the methane emission. After harvest, the considerably high methane emission was observed, while such high emission of nitrous oxide was not detected after the harvest.

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In this study we assessed the effectiveness of artificial zeolite and hydrated lime, as amendments, to reduce surface runoff (SR) and sediment generation from acid soil taken from Yamaguchi prefecture in Japan. Air-dried soil aggregates (≤2 mm) were amended with 10 % and 15 % zeolite and 0.5 % and 5 % lime. The amended and unamended aggregates were packed to an average dry-bulk density of 1.30 Mg m^-3 in small soil plots and subjected to simulated rain intensities of 30 and 60 mm h^-1. Time incremented SR samples were collected for sediment concentration (SC) determination. The data indicated that both SR and SC were characterized by three phases. The amendments' impact was most significant during the first two phases. During these two phases both SR and SC were decreased by amendments and the reduction increased with the rates of incorporation. The largest reductions in SR and SC were found for zeolite at 15 % and the lowest for lime at 0.5 %. The amendment incorporation led to increases in wet aggregate stability, soil water retention, infiltration, and sub-soil water transport, and to decreases in dispersible clay. We suggest that SR and SC in acid soils can be substantially reduced by zeolite or lime, and this is attributed to amendment induced increase in wet aggregate stability and decrease in clay dispersion, and in this regard zeolite is more effective than lime.

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In the present study, the occurrence situation of hypoxic water which has bad influences on the ecosystem and fishery in the interior parts of the Ariake Sea, and the relationships between its occurrence and density stratification in the sea water were investigated on the base of field observation data.
As a result, it was clarified that the hypoxic water in the bottom layer occurred frequently at the neap tide during summer when wave height was low and the sea was calm in the interior western parts with high COD and mud content, and the decrease of supply of oxygen from the upper layer to the lower one owing to the fall of vertical diffusion coefficient between the upper layer and the lower one with the formation of density stratification contributed to the occurrence of hypoxic water greatly. Moreover, seasonal variations of degree of density stratification, vertical diffusion coefficient and consumption rate in the lower layer ware grasped by the analysis of field observation data using a two-layer box model.

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Wide cracks in RC structures cause reinforcement corrosion and affect the durability of the structures. To determine the criteria of crack width to assure the durability for agricultural RC open channels, investigations for cast-in-place RC open channels are carried out. Concrete cores containing both cracks and reinforcements are collected from RC open channels which have been in service for a long period, and they are used to study the relationship between crack width and reinforcement corrosion. It is shown that harmful corrosion tends to occur with cracks wider than 0.4mm by summary table with crack width and by discriminant analyses with crack width and reinforcement covering. The results of discriminant analyses show that reinforcement covering have little influences on reinforcement corrosion, since most of cracks in agricultural RC open channels are not bending cracks. It is indicated that corrosion does not progress in water and under the ground as badly as in the air, but the number of specimens is not sufficient to determine their particular criteria.

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Fish that live in and around paddy fields are impacted by agricultural practices, such as water drainage and pesticide application associated with rice farming. This study clarifies the behavior of fish when they leave paddy fields and their relationship to environmental factors. For example, water levels and pesticides. This study makes clear the following:(1) the fries of Misgurnus anguillicaudatus, Gnathopogon elongatus elongatus, and Carassius spp., left paddy fields 20 to 40 days after the start of the intake of water.(2) Standard lengths of the M. anguillicaudatus fries was about 30mm, and standard lengths of the G. elongatus elongatus fries was about 20mm.(3) The G. elongatus elongatus fries and Carassius spp. fries leave more quickly than the M. anguillicaudatus fries at the onset of water drainage.(4) Research shows that we must carefully manage the levels of water within 20 days of the initial intake of water from the irrigation ditches. For the pesticide application, 40 days is recommended.

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The objective of this study is to recover and to preserve the ecological corridor composed by the irrigation and drainage channels. During paddy rice growing season, water channel networks are divided into various sections by the many weirs for irrigation.
Specially designed flume type small fishway was developed for helping migration of fishes and other aquatic animals through the channel networks. The advantages of this fishway are as follows, 1) light weight and strong structure with made of aluminum, 2) easy to set up and to remove in normal channels, 3) seasonal migration of aquatic animals are controllable by the farmers, 4) shapes of pool are deformable to fit the target animals.
The hydraulic conditions of water flow in the fishway were carefully investigated by laboratory experiment. From the field's observations, it was proved that this fishway did not only impede usual agricultural water management and was but also suitable for the migration of aquatic animals.

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The heat budget of Lake Ikeda, which has a lake surface area of 10.62km² and a mean water depth of 125m, at Satsuma peninsula in Kagoshima was investigated based on field data observed from August 2004 to September 2005 and a heat budget equation considering the heat fluxes advected to and from the lake surface due to precipitation and evaporated water. The advected heat fluxes through the lake surface have no significant effect on the monthly heat budget of the lake. The small annual mean sensible heat flux of 14 W m^-2 and the large annual mean latent heat flux of 75 W m^-2 of Lake Ikeda in a warm-temperate region resulted in a relatively small Bowen ratio of 0.19. The latent heat flux of the lake is large from August to January and small from February to July. The seasonal change in the latent heat flux is small and different from that of deep lakes in the northern part of Japan. The calculated latent heat flux by the Priestley-Taylor method was in good agreement with that by the Bowen ratio method.

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Increasing concern over the human impact on the flora and fauna of rivers and their surrounding environment has produced a strong demand for operational tools to assess these issues and to develop mitigation procedures. To realize sustainable development in agriculture, it is essential to grasp ecology of organisms dwelling in and around paddy environment. To date, many researchers have proposed various models to quantify the habitat preference of aquatic and terrestrial animals. Although some models have achieved accurate prediction of spatial distribution of target species, only the standard habitat suitability index (HSI), based on resource selection functions, has been applied in a practical level for its simplicity in the modeling procedure. However, only a few studies have verified accuracy and the difference among standard type indices in predicting habitat selection. It is very important to clarify their ability to capture the habitat preference of target species. The present study has verified the prediction ability of the HSIs using field study focusing on Japanese medaka (Oryzias latipes), one of the most common fish dwelling in paddy environment in Japan. Consequently, Vanderploeg and Scavia's relativized electivity showed the best accuracy among the indices.

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Methane fermentation is a key technology to promote biomass use in rural areas. It is essential to develop a system that enables adequate use of the entire quantity of digested liquid produced by the methane fermentation process. Research on the use of digested liquid in agriculture sectors has progressed greatly these past five years. The application of digested liquid has been studied not only for pastures but also for paddies, upland crops, vegetables and horticulture products.
In this paper we first review recent trends and achievements of the research on digested liquid, and then present the prospects for the development of a biomass utilization system based on methane fermentation technology. The digested liquid should be used directly or after conversion for improvement of the quality. To promote the use of digested liquid, the following factors are important; 1) adequate design for fertilization, 2) improvement of handling, and 3) countermeasures to minimize environmental loads. It is desirable to develop a total system by cooperation among the many organizations concerned, especially in areas where methane fermentation is advantageous in terms of demand potential for the digested liquid and high rates of nitrogen and energy recovery.

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