Geoscience Reference
In-Depth Information
et al
., 1991). On account of the different soil water characteristics in the two layers, for
agiven water pressure at the interface, at equilibrium the soil water content in the upper
layer is normally larger than in the lower coarse-textured layer. As a result, the hydraulic
conductivity in the upper layer may be considerably larger than in the lower layer; this
is illustrated inFigure 8.26. In such a case infiltrating rain water will not readily enter
into this lower layer but will tend to be diverted laterally and may cause a rapidrise
in water table further down the slope if the water in the upper layer is already close
to suction-saturated. Field observations within the source region of the Tama River in
east-central Honshu by Marui (1991; Tanaka, 1996) on a hillslope unit, characterized
bya4mthick fine-grained loam layer underlainby15mthick gravel layer, were con-
sistent with this sequence of events. He observed a large-scale groundwater ridge along
the steep hillslope. In addition, the air in the underlying partly saturated gravel seemed
to be confined by the surrounding groundwater body, and by the saturated zone in the
loam layer. In a separate study, Onodera (1991; Tanaka, 1996) inferred that the result-
ing air pressure increase may have led to increased groundwater outflow at the slope
surface.
In conclusion, it stands to reason, that mechanisms related to capillarity can lead to
so-called groundwater “ridging” not only inriparian areas, but also along hillslopes,
wherever the capillary fringe is already close to the ground surface. However, untilnow
no experiments have demonstrated that by itself this type of phenomenon is related
to the hydrograph; thus, whether or not this mechanism can explain large subsurface
stormflows remains to be answered.
11. 3
SUMMARY OF MECHANISMS AND
PARAMETERIZATION OPTIONS
11.3.1 General considerations
The brief review in Section 11.2 has shown that on the Earth's land surfaces one can
encounter a bewildering range of hydrologic, climatic, topographic and soil conditions,
which will favor widely different storm generation mechanisms. These mechanisms can
be overland flow due to infiltration excess precipitation, or to saturation excess near the
soil surface, resulting either from return outflow from the subsurface, or from rapidly
mobilized capillary fringe water in the soil profile to full saturation. On steep slopes
overland flow is more likely on converging sections in hollows. The mechanisms can
also be subsurface flow of water in a number of different ways. Especially during large
rainfall events, this can involve different types of macropores and preferential flow paths,
namely as vertical bypass flow to some depth, and then as lateral flow through pipes or
through a shallow porous soil layer with high organic content or at the soil bedrock
interface. At the same time a slower and less localized throughflow takes place in the
soil matrix. Several of these mechanisms have been found to be more than adequate to
produce high-intensity runoff events. It is also striking that these mechanisms are not
mutually exclusive and that in many situations they coexist and operate interactively in
the production of streamflow; their relative importance then depends on the prevailing
