The effects of mowing with grass cutters at paddy levees on food habit of the Indian Rice Frog, Fejervarya limnocharis, were evaluated to establish preferable weeding methods for conserving habitats. The prey animals at the levee along with the stomach contents of the frogs were compared between before and after mowing sections. The numbers of prey animals per square meter were not different for both sections, while the species compositions were different. Not only the total numbers but also the gross wet mass of stomach contents per frog in after mowing section was more than that in before mowing section. The frog in after mowing section can eat the prey animals in the plants mowed down. These results suggest that leaving the mowed plants may serve prey animals to the frog.
An inverse modeling to reproduce a variably saturated water flow in non-isothermal soil based on field observation is proposed. Since the water movement in the surface soil is significantly affected by the soil temperature, the governing equations system is composed of the mixed form Richards equation for the water movement and heat conduction equation for the thermal transport. To complete the water flow model of interest, unknown model parameters are determined with inverse technique. The major unknown parameter is the relative hydraulic conductivity (RHC) described as a free-form parameterized function which is a sequential piecewise cubic spline function and therefore can express the flexible functional form of the parameter. The inverse problem is defined as the minimization of errors between the observed and computed pressure heads to determine the coefficient values of the free-form function, and solved through a simulation-optimization method. To validate the water flow model developed, its practical application to in-situ soil is implemented. The results show that the functional form of RHC is successfully identified, and that both water movement and thermal transport models can produce the forward solutions which are good agreement with observed data for desorption period.
Low-lying agricultural reservoirs under the pressure of urbanization in Japan are subject to increasingly higher risk of inundation disaster due to the change in the runoff characteristics by decreasing the area of paddy fields and the recent increase in the frequency and magnitude of heavy rainfall events. The objective of this study is to develop an inundation analysis model for the evaluation of the flood mitigation effect of a Paddy Field Dam project, specifically aiming at low-lying agricultural lands. The model developed in this study is composed of three modules: runoff calculation module in which the runoff from each landuse cell is calculated using water balance analysis, one dimensional unsteady flow module in which flood routing is performed to calculate flow in the channel network, and inundation flow module in which the behavior of flooded water is demonstrated using topographically adjustable cells. One of the unique characteristics of this model is that the runoff from landuse with different attributes are reflected in a realistic but concise manner in the calculation as the origin of runoff. As a result of applying the model to two type of study field, inundation process was verified to be reproduced well.
In this study, we clarified socio-economic background and geographical characteristics of the heavy metal contamination of farmland in Taiwan and identified limits of an existing institution to cope with contaminated farmland. Main cause of heavy metal contamination of farmland is ill-treated wastewater discharged by factories into agricultural canals, which contaminates water quality and sediments. Thus, the contaminated farmland can often be observed in urban and sub-urban areas. Such phenomena are linked to weak-enforceability of land use regulation and factory effluent standard. Also, decontamination method can only reduce contamination level of farmland without eliminating sources of heavy-metal contamination, causing gradual expansion of contaminated farmland. Finally, we proposed that, in order to cope with heavy metal contamination of farmland in Taiwan, 1) decontamination method should be improved so that it can deal with the contaminated sites and surrounding areas as a whole, 2) land use regulation and factory effluent standard should be improved both by tightening the control over illegal activity and by setting more stringent conditions to assure better water quality for agriculture, and 3) location of factories should be spatially, well-coordinated making best use of the opportunity of farmland consolidation projects.
In the Tedori river alluvial fan area, which has gravely sand beds and very steep gradient, return flow analysis of irrigation water in paddy area for dual-purpose of irrigation and drainage main canal systems was carried out. Research was conducted at test paddy area having 179 ha useing new procedure for return flow of irrigation water. The main result was as followers: 1)The methods of the return flow of irrigation water for very complicated irrigation and drainage systems was proposed. The accuracy of this method was verified by comparison with the calculated and observed discharge. 2)Using this results, shortage of irrigation water was analyzed before reservoir construction and land consolidation, which was remarkably decrease percolation because of soil compaction. 3)Reasons of many intake weir being in this paddy area would be traditional customs before reservoir construction and land consolidation, in spite of the study area has lot of irrigation water. 4)At present, the reuse of irrigation water systems was very well functioning for enlarging paddy management area because of change from private farmer to company or village management.
Effects of intermittent irrigation with different intervals on greenhouse gas emissions and rice yield were investigated in rice paddy fields. Three treatments for water management practices were prepared as: (1) continuous flooding except during the midseason drainage, (2) intermittent irrigation with two-day drainage, and (3) intermittent irrigation with four-day drainage. The longer the drainage period in intermittent irrigation, the less the CH4 was emitted whereas the more the N2O was. Two-day drainage treatment had the smallest cumulative CO2-equivalent fluxes for CH4 and N2O. Decreases in a drainage period resulted in the reduction of the net greenhouse gas emission. Brown rice yields were the highest for the continuous flooding treatment. The yields of two-day and four-day drainage treatments were smaller than that of the continuous flooding treatment by 13% and 18%, respectively. The two-day drainage treatment achieved the smallest net greenhouse gas emission per brown rice yield, thus mitigated the emission of CH4 and N2O with maintaining yield.
Unionid mussels have very low mobility. Therefore, expansion of their habitat and gene flow between their local habitats are dependent on movement of their host fishes. In this study, moving distance of Nipponocypris sieboldii, which is host fish species of unionid mussels, was investigated in Kudase River and a drainage canal of the paddy fields in Gifu Prefecture. Moving distance of N. sieboldii is described by the formula: y=exp(6.50+0.06x). Explanatory variable is days until recapture. If the duration of glochidial infection is 7days, moving distance of N. sieboldii estimated by this formula is 1,040m. Therefore, in this investigation area, if the local habitats of Unionidae are located within about 1,000m, it will be effective for the expansion of habitat and the gene flow between local habitats.
Effect of ‘crashing and compaction method’ by using farming machineries on reduction of water percolation was examined in paddy fields with volcanic ash soil. After removing plow layer, 27 experimental plots, each of which has a combination of treatments: tillage depth (0, 10, 20cm), compaction intensity (L, M, H) were allocated in the paddy field following split-plot experimental design. Saturated hydraulic conductivity was measured by using closed-type infiltrometers just after and 3 months after the construction of the layer for percolation controlling, to compare the effect of the factors by ANOVA. The study revealed clear effect of tillage before compaction. Although the hydraulic conductivity of hardpan kept no less than 140mm/d by the compaction treatment alone, it decreased to 15 to 22mm/d in average by the combination of tillage and compaction. However the 10cm of tillage was too thin to make a layer for the stable percolation control under the two months of drying and freezing circumstance. Tillage to 20cm and moderate compaction was evaluated to be adequate for controlling excess seepage under normal farming management.
Using plowing and compaction, field surface compaction methods to reduce the water-requirement rate were investigated in a direct-seeding paddy field used for rice cultivation. The water-requirement rate, percolation rate, and soil hardness were surveyed on paddy plots that had been compacted under various soil water conditions with various compaction strengths. They were compacted in three steps, resembling assumed preparations for on-site seeding work. Overpercolation at the levee side and corners of a field increased the water requirements on the direct-seeding paddy field. Three-step compaction of the field surface steadily reduced the water requirement rate to 20mm/d, from 100mm/d, suggesting that gradual compaction provides useful functions for the direct-seeding paddy field at each step: drainage functions for seeding, water retention and drainage functions for emergence of rice, and water storage functions after seedling establishment. To reduce the water-requirement rate to about 20mm/d, it is desirable to compact the field surface with high soil water contents as much as possible during work with a tractor. After field surface compaction, the soil hardness at 5cm under the field surface must be about 20mm, as measured with a soil hardness meter.
The duration required to meet target cadmium (Cd) concentrations in soils or foods is an important factor to determine feasibility of phytoextraction systems. In addition, changes in Cd concentrations by repeated cropping with a phytoextraction plant in fields is required to estimate this duration. Therefore, we developed a method to predict Cd concentration changes by repeated cropping with Arabidopsis halleri ssp. gemmifera in Cd-contaminated Andisol fields using pot experimental data. The method was composed of a balance equation of Cd in the field and a relational expression between soil Cd concentrations and Cd concentrations of the plant (above-ground part) obtained from pot and field experiments. Then, the validity of the method was confirmed by comparison with values measured in the field. Cd concentration changes calculated by this method agreed well with the values measured in the field. Therefore, this method makes it possible to predict the duration required to remediate Cd-contaminated Andisols using Arabidopsis halleri ssp. gemmifera.
We restudied the stability indices of refuse landfill indicated by Tokyo Metropolitan Government in 2001, with arranging the recent data of observation (quantity of gas, ground subsidence, underground temperature, etc.) and considering the new report. When 25 years after the end of refuse landfill work passed, the observed value was under the all stability indices with modifying the evaluation of underground temperature. In conclusion, we recognized to change from the refuse landfill condition to the natural ground condition.
Radiocesium (Cs) decontamination of land has been required as one of the restoration measures after the Tohoku-Pacific Ocean Earthquake disaster. The Ministry of Agriculture, Forestry and Fisheries (MAFF) of Japan plans to carry out decontamination measures of farmland in response to the situation of land contamination. However, in order to achieve effective restoration, strategic and comprehensive measures including land readjustment are necessary. In this paper, we examined characteristics of Cs contamination from the view of the spatial extent contamination, land use, land improvement and working environment, and made the following proposals: (1) Implementation of Cs decontamination works ought to follow the principle of “containment”, “isolation”, and “securing long-term safety” of Cs, and improve both production environment and working environment of farmlands. (2) Strategic approaches should be carried out in a comprehensive and unified manner for the effective implementation of Cs decontamination.