森林と地下水

The forest is an ecosystem in which plants coexist with other living organisms, soil, and water. The flow of water through a forest ecosystem is examined in this paper, stressing on the role of forest soil in the hydrologic cycle.<BR>Hydrologic characteristics of forest soil.<BR>The role of forest soil in the hydrologic cycle is discussed by dividing soil porosities into four groups. The porosity of 0<Pf<0.6, which amounts to more than 10% in the A horizon, decreases sharply in the order of the A, B, and C horizons. This simultaneously provides high infiltration capacity of the surface of the A horizon and low permeability of the C horizon. The former reduces surface runoff and the latter accelerates saturated subsurface lateral flow.<BR>The porosity of 0.7<Pf<1.7 also decreases vertically. The water in this pore range moves quickly by gravity, being only slightly affected by capillary tension. These pores can store 150 mm of water in the whole soil depth of the A, B and C horizons. The storage capacity of the porosity of 1.7<Pf<2.7 is 120 mm in the whole soil depth. The water in these pores migrates slowly by gravity under strong capillary tensions. The water in these two kinds of pores contributes greatly to the slow release of soil water, which recharges shallow groundwater storage from which streamflow discharges.<BR>Seasonal variations of groundwater runoff depletion rates.<BR>Seasonal variations of groundwater runoff depletion rates are discussed for a small watershed in the University Forest of the University of Tokyo where streamflow has been measured since 1926. Monthly groundwater depletion curves were synthesized from streamflow data measured by a point guage at 10 : 00 a.m. The annual cycle is closely related to that of evapotranspiration rates. The depletion rates, in which the diurnal fluctuation of streamflow is taken into consideration, showed a great difference between the growing season and dormant season. A similar seasonal difference was observed in the lysimeter data of the Coweeta Hydrologic Laboratory (TSUKAMOTO 1986). SUZUKI (1984) verified that the differences of the depletion rates are caused by transpiration by using mathematical simulation that applied RICHARD'S equation to a sloping soil model. OHTA (1987) showed that a mathematical simulation produced diurnal fluctuations in discharge for a sloping soil model 30 m in length. This writer concludes that seasonal variations in streamflow depletion rates are brought about by trapping soil water slowly migrating downwards as unsaturated flow by transpiration.<BR>Subsurface water movement and discharge.<BR>Subsurface water discharge from a forested slope with an average soil depth is discussed based on the previous research on saturated and unsaturated soil water movement. The following three storm conditions are assumed.<BR>Case I. Ten days after a large storm : Most streamflow is supplied from unsaturated water migrating through the soils of the middle and lower slopes and from saturated storage of the lower slopes.<BR>Case II. Small storm (<50 mm) immediately after Case I : Quick response and quick discharge to streams occur through the phreatic zones of shallow groundwater storage. In the early stages of the storm, old water is pushed out in unsaturated and saturated forms. At the end of the storm, new storm water from the A horizon of the lower slopes participates the hydrograph.<BR>Case III. Large storm (>200 mm) occurring after Case II : Saturated lateral flow occurs in the A horizon along the whole slope profiles at the end of the storm. A large increase in new storm water would be recorded.