Abstract
One of main research subjects in hydrology is to make clear runoff mechanism or mechanism of streamflow generation. A knowledge of this interest is significantly important for better elucidation of the mechanism of the hydrological cycle and of the material cycle in a basin. Because of complexity of basin response to rainfall events, previous studies have tended to analyze the basin response as a black box unit.
In recent years, however, there has been a burst of research activities aiming at obtaining better insight into the mechanism of streamflow generation. Three types of flow have been recognized as main sources of storm runoff, that is saturated overland flow, subsurface stormflow and groundwater flow. It is now commonly accepted that in many drainage basins in humid areas streamflow is controlled dominantly by subsurface flow. However, there remain some major questions concerning causal mechanism of feeding water into stream channels by subsurface flow.
The purpose of this study is to elucidate the main source of storm runoff during a typical storm event and to clarify the runoff mechanism of the specified main component.
The study was conducted in a small forested drainage basin with an area of approximately 2.2 ha in the headwaters of the Tama River system located in the western suburbs of Tokyo (Fig. 1). The basin is located in the Tama Hills which are underlain by the Pliocene Miura group and Pleistocene Narita group. The former is composed of sand, mud and gravel and the latter of gravel and volcanic ash soil, the so-called Kanto Loam. The topography is typical of a dissected diluvial hill having a valley floor slope of about 12% andd steep hillside slopes of about 50%. The upper 2m of the soil are broadly classified as clay loam and silty clay (Fig. 2). The vegetation consists of dense deciduous trees approximately 15 m in height and sparse bamboos 1_??_2 m high with a dense ground cover of ferns and small shrubs.
Within the drainage basin, the valley floor was instrumented for the intensive study (Fig. 3) . Precipitation was measured by a tipping bucket recording raingage located in the middle part of the valley floor. Discharge from the basin was continuously recorded at three sites as shown in Fig. 3 using a 90° V-notch weir and Parshall flumes. To analyze the dynamic response of the basin during a storm event, tensiometer and piezometer nests were utilized. Contributing areas of overland flow to the storm hydrograph were ascertained by field observations during storm events.
Intensive field observations were carried out from July to October 1981. During this period, one of the major storm events occurred on October 22, caused by the Typhoon No. 8124, which provided a total rainfall of 172.5 mm. Discharge began within a few minutes of the onset of rainfall and the peak discharge occurred within 10 minutes of the rainfall peak (Fig. 4).
In the drainage basin, overland flow occurred from restricted areas on the valley floor and no significant overland flow was produced on the steep hillside slopes during storm events. Discharge due to overland flow generating on these restricted areas was too small to account for the total discharge from the basin. This means that the saturated overland flow which has been suggested as a main source of streamflow generation in humid drainage basins on the basis of the variable source area concept does not explain the runoff mechanism of the basin.
On the basis of observations of groundwater discharge around the main weir (Fig. 7) and of the hydrograph separation using specific conductance (Fig. 9), we clarified discharge ratios and flow components between the gaging stations (Fig. 10).
