Geoscience Reference
In-Depth Information
stormflow component of a hydrograph. Although a
groundwater contribution to stormflow had been
suggested before, it was not until Sklash and
Farvolden (1979) provided a theoretical mechanism
for this to occur that the idea was seriously con-
sidered. They proposed the capillary fringe hypo-
thesis to explain the groundwater ridge, a rise in the
water table immediately adjacent to a stream
(as observed by Ragan, 1968). Sklash and Farvolden
(1979) suggested that the addition of a small
amount of infiltrating rainfall to the zone imme-
diately adjacent to a stream causes the soil water to
move from an unsaturated state (i.e. under tension)
to a saturated state (i.e. a positive pore pressure
expelling water). As explained in Chapter 4, the
relationship between soil water content and soil
water tension is non-linear. The addition of a small
amount of water can cause a rapid change in soil
moisture status from unsaturated to saturated. This
provides the groundwater ridge which:
not only provides the early increased impetus for the
displacement of the groundwater already in a discharge
position, but it also results in an increase in the size
of the groundwater discharge area which is essential
in producing large groundwater contributions to the
stream.
(Sklash and Farvolden, 1979: 65)
An important point to stress from the capillary
fringe hypothesis is that the groundwater ridge is
developing well before any throughflow may have
been received from the contributing hillslope
areas. These ideas confirm the variable source areas
concept and provide a mechanism for a significant
old water contribution to storm hydrographs.
Field studies such as that by McDonnell (1990)
have observed groundwater ridging to a limited
extent, although it is not an easy task as often the
instrument response time is too slow to detect
the rapid change in pore pressure properly.
Case study
THE MAIMAI RUNOFF GENERATION STUDIES
The Maimai catchment study (near Reefton on
West Coast of the South Island of New Zealand)
was established in 1974 for research into the
effects of logging native beech forest ( Nothofagus )
and replanting with different non-indigenous
species (Figure 5.4). The installation of hydro-
logical measuring equipment and the fact that
rainfall and stormflow are frequently observed
made it an ideal place for studying stormflow
generation mechanisms in depth. The knowledge
gained from detailed hydrological process studies
at Maimai have played a major part in shaping
thinking on stormflow generation mechanisms.
The Maimai catchment is characterised by
short, steep slopes (approximately 300 m with
angles of around 35°), covered in thick vegetation,
with incised channels and very small valley
bottoms. Annual rainfall is approximately 2,600
Figure 5.4 Maimai catchments in South Island of
New Zealand. At the time of photograph (1970s) five
catchments had been logged and are about to be
replanted with Pinus radiata .
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