The hydraulic properties of peat are well known to be different depending on the depth from the ground surface. Conventionally, the groundwater movement in a fixed point on peatland has been analyzed by the method of Tank Model, for instance, and has attained fairy good results. However, when we need to apply this method to the extensive area, there are issues of solving a vast number of parameters to be determined at each point. Using the motion equitation based on the Darcy's law and the continuity equation is another conventional solution for calculating the groundwater flow in extensive area, but the properties of peat is scarcely reflected to this model. In this study, first we formulated the hydraulic properties of peat such as porosity and permeability from the observed fluctuations of groundwater table and amount of rainfall. Then we carried two-dimensional flow model for unconfined groundwater in two methods. One is conventional method based on Darcy's law, and the other is proposed method considering hydraulic properties of peat. By using newly defined T-parameter, which has homogeneous element with the coefficient of transmissivity, the peat hydraulic properties have been represented as the factor of depth, and this derived good result in simulation of groundwater fluctuations in the latter method.
The purpose of this study is to investigate the water level manipulation in Lake Biwa paying attention to the spawning environment of the indigenous and endemic cyprinid fishes. We constructed the model analyzing the relationship between environmental condition (such as water level and precipitation) and spawning of Cyprinus carpio, Carassius sp., and two endemic species, C. buergeri grandoculis and C. cuvieri. Using this model, the number of eggs spawn on the reed beds is calculated in 3 monitoring points (Enshoji, Hariye, and Shin-hama) from 2004 to 2008, and this model calculates fairly well the variation in seasons and years. Predicting the effect of measures such as the change of water level manipulation on spawning, it is recommended to maintain water level for long-term higher in June and July, supposed the peak period of their spawning, and reproduce the stock laying eggs in higher water temperature.
Penthorum chinense (Saxifragaceae) is a wetland species whose conservation status is ranked as 'nearly threatened (NT)' in the national red data book of Japan. To determine ecological characteristics of its habitats as the basis for future restoration work, we studied microtopography and plant communities of two natural sites in Kanagawa prefecture, where the species grows in retarding basins. The highest frequency and dominance of P. chinense was recorded in the quadrates located at the water level of 0-20 cm below the ground. Such sites have likely experienced frequent flooding disturbances. In addition, we conducted artificial growth experiments to compare vegetative growth and sexual reproduction of P. chinense under different water-level conditions (i.e., 0 cm, 20 cm, and 40 cm below the ground) and competing vegetation (presence and absence). Penthorum chinense exhibited the most vigorous growth and capsule production when water level was set at 0 and 20 cm below the ground. In contrast, individuals planted at the water level of 40 cm exhibited the much lower production of capsules than those at the 0 and 20 cm levels regardless presence or absence of competing vegetation. With competing vegetation, growth and seedling establishment of P. chinense were extremely suppressed at the 40 cm-level due to the shades created by the perennial tall herbs. We concluded that P. chinense grows best under natural conditions at the open sites where water level was 0-20 cm below the ground and that potential competing species, such as perennial tall herbs, were likely disturbed by frequent flooding. Preservation and restoration of such sites are important for conservation of P. chinense.
To elucidate the effect of inundation on the habitats of various forms of Alnus japonica in wetlands, the tree height, base level of the stem, and upper level of the root system of each A. japonica tree were investigated at 15 sites near the gauge stations of groundwater level in the Kushiro Mire, northern Japan. The base level of the stem was found to be approximately 17 cm higher than the ground surface, and the upper level of the root system was also higher than the base level of the stem. The upper level of the root system was higher than the mean water level, suggesting that the root system may affect the habitat of A. japonica. The frequency of inundation in various periods was calculated for each A. japonica tree and indicated that the population of A. japonica dramatically decreases in the areas inundated from 12 hours to 1 week. In the areas inundated for over 1 week, the population of A. japonica trees in sprouting form was higher than those in single stem form. These results indicate that A. japonica can adapt to the longer inundation environment by germinating in higher ground surface, sprouting roots higher than base level of stem, and growing in sprouting form.
Hitomi-numa Pond is situated in the lowland areas in the central region of Sarobetsu Mire. This small pond (area, 0.7 ha) contains a relatively large floating islet which is composed of peat. This paper reports objective data that describe the current state of the pond and considers the processes underlying the formation of the pond and the floating islet. The islet was formed within only a few decades of the 20th century, under the influence of agricultural development in the surrounding mire. An excavated drain was constructed in this region for agricultural purposes. Through this drain, sand and mud flowed into the lowland area, eventually forming a natural levee. This levee altered the flow of ground- and surface water, and the water level in the lowland area gradually rose. Owing to the buoyancy of peat, the peat deposits in the lowland area surrounding the pond floated to the top of the pond as the water level elevated. A fragment of the floating peat presumably separated from the peat matrix and drifted on the surface of the pond. The floating islet of Hitomi-numa Pond is peculiar since it was formed because of an artificial act and since the time required for its formation can be accurately determined. Moreover, the process of its formation was complex, involving multiple factors.
Wet grasslands that occur on mineral soils are widely distributed in Japan. Because these wet grasslands serve as habitats for many indigenous and/or endangered species, their preservation is important for maintaining biodiversity of Japan. However, these wet grasslands have been studied very little. This study surveys the distribution of wet grasslands on mineral soils in Japan by referring to previous reports and discusses the classification of these grasslands on the basis of the physical characteristics of their substrata. Most wet grasslands on mineral soils are located in the Pacific side of the Chubu district or on the Seto Inland Sea sides of the Kinki and Chugoku districts. However, some are located in other, wider areas, including in regions with cold climates. This suggests that the formation of wet grasslands on mineral soils is attributable not only to a temperate climate that promotes the decomposition of plant residues, but also to hydrologic and geomorphologic systems that minimize the accumulation of organic matter. In addition, even in peat-based wetlands, there may be short periods during which, for some reason, no peat covers the ground surface. In consideration of these formation mechanisms, wet grasslands on mineral soils can be roughly classified into three types. The first is the hillside-slope type that is formed on hillside-slopes by seepage water spreading out onto, and down, the ground surface. The second is the valley-floor type that is formed on valley floors, or on the swales between beach ridges, by convergence of groundwater. This type may exist only in warm areas, because they tend to accumulate plant
residue. The third type is a pseudo type: that is, a peat-based wet grassland in which the peat is temporarily covered by mineral soil.