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water which was present in the soil profile prior to the rainfall event; if infiltration excess
overland flow had been the only mechanism, all the storm runoff would have been event
water, that is water furnished by the rainfall event. The ratio of event to pre-event water
in the streamflow was found to depend on the rainfall duration and intensity. Because
overland flow was so widespread, they concluded that inthis type of tropical rainforest
the variable source area concept does not apply. Elsenbeer (2001) subsequently surmised
that overland flow may be a common flowpath in tropical rain forest catchments with
“acrisol” profiles; these are soils, inwhich the clay content increases with depth, resulting
in a decreasing hydraulic conductivity.
11.2.2
Subsurface stormflow
In many catchments under natural conditions infiltration is never exceeded, and the
precipitation and other input can readily enter into the ground surface; thus the sub-
sequent flow to the stream channel takes place below the surface, presumably through
the soil mantle of the catchment. Lowdermilk (1934) and Hursh (1936) appear to have
been among the first to propose subsurface flow as the main streamflow generation
mechanism in forested hill slopes (see also Hewlett, 1974). It was later confirmed insev-
eral experimental investigations that subsurface flow can even be the only mechanism
under certain conditions (see Roessel, 1950; Hewlett and Hibbert, 1963; Whipkey, 1965;
Weyman, 1970).
The notion that subsurface flow isanimportant, and sometimes the only process
of water transmission, was resisted by many on the grounds that porous mediaflow
is generally much too slow compared with overland flow to be able to produce the
observed streamflows. One early explanation of this paradox was suggested by Hursh
(1944), who assumed that the transport takes place through secondary porosity of particle
aggregates, forming a three-dimensional lattice pattern, and through hydraulic pathways
consisting of dead root channels and animal burrows (see also Section 8.3.1). At the
time, this possibility of macropore flow and piping seems to have been largely dismissed
as unrealistic by experimentalists and mostly ignored by modelers. However, subsequent
experimental work in the field, some of itwith chemical and isotopic tracers, has produced
ample and incontrovertible evidence not only for macropore flow and its importance, but
for several other mechanisms enhancing subsurface flow as well. These are considered
more closely in what follows.
Macropores and other preferential flow paths
The concept of preferential flow paths or macropores is an old one; “little channels” and
“light soil, mixed with pebbles and roots of trees” were invoked as early as the 1680s
by Mariotte to explain infiltration and to refute the claims of Seneca and Perrault that
rain water cannot possibly penetrate the soil to be the source of springs. In general,
macropores can be defined as secondary, often pipe-like structures of the soil matrix,
that are the remains of purely physical processes, such as erosion initiated by desiccation
cracking, and different forms of biological activity, such as decaying plant root channels
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