BEHAVIOR OF SUBSURFACE WATER IN A HEADWATER BASIN IN THE TAMA HILLS

Abstract

The research on the three-dimensional behavior of subsurface water during and after a rainstorm throughout a small forested basin (0.56 ha) was carried out in the Tama Hills. The basin is located in the northeastern part of Kanagawa Prefecture, Japan and composed of flat valley bottom (mean gradient of 10°) and steep side slopes (mean gradient of 25°). Twenty four borings were practiced to know soil profiles and depths to the Tertiary bed rock (mudstone) from the ground surface. As the results, soil profiles were classified as A-horizon, B-horizon, transported soil layer, weathered zone of bed rock and bed rock. Depths to the bed rock were about 3 to 4m in the valley bottom and 1.5 to 2m in the middle parts of the side slopes. The ancient V-shaped valley on the bed rock and its weathered zone were buried by the transported soil, and the present geomorphic feature was formed over. The transported soil is composed of loam and masses of weathered bed rock and assumed to be carried from upper parts of the side slopes. Fifty-six observation wells, being classified into Wells D and Wells S, were installed through-out the basin. The Wells D were dug to the top of the bed rock and used to observe the behavior of the “deep groundwater body”. On the other hand, the Wells S were dug to about 1m below the ground surface and used to observe the behavior of the “shallow ground-water body”. In order to characterize vertical distributions of soil-water pressure head in response to rainstorms, forty tensiometers were equipped at five sites. Three rainstorms with a total rainfall of more than 100mm occurred during an observation period from September 28 to October 21, 1979. The major results clarified by this research are as follows: 1. The impermeable layer, the top of which is generally about 1m below the ground surface, is found almost over the basin. This layer plays important roles in building up the temporal shallow groundwater body and confining the deep groundwater body during and shortly after the rainstorms. 2. The impermeable layer is not found both in a lower part of the valley bottom and in vicinity of the ridge top. A lack of the impermeable layer in vicinity of the ridge top may affect the behavior of the deep groundwater body during the rainstorms. The rainwater brought on this part is expected to cause a quick response of the piezometric surface of the deep groundwater body to the rainstorms and make the piezometric surface rise about 10cm above the ground surface in some wells. 3. A temporal saturated zone is formed in the A-horizon during and shortly after the rainstorms and discharges into the stream channels, making an important contribution to stormflow generation. There is a strong possibility of application of the results to other headwater basins in the Tama Hills, and therefore more similar researches must be carried out from viewpoints of disaster prevention and geomorphological evolution.