RUNOFF - Fundamentals of Hydrology - page 82

Examples of low infiltration rates can be found
with compacted soils (e.g. from vehicle movements
in an agricultural field), on roads and paved areas,
on heavily crusted soils and what are referred to as
hydrophobic soils .
Basher and Ross (2001) reported infiltration
capacities of 400 mm/hour in market gardens in the
North Island of New Zealand and that these rates
increased during the growing season to as high as
900 mm/hour. However, Basher and Ross (2001)
also showed a decline in infiltration capacity to
as low as 0.5 mm/hour in wheel tracks at the same
site.
Hydrophobic soils have a peculiar ability to swell
rapidly on contact with water, which can create an
impermeable barrier at the soil surface to infiltrating
water, leading to Hortonian overland flow. The
cause of hydrophobicity in soils has been linked into
several factors including the presence of micro-
rhizal fungi and swelling clays such as allophane
(Doerr et al ., 2007). Hydrophobicity is a temporary
soil property; continued contact with water will
increase the infiltration rate. For example Clothier
et al . (2000) showed how a yellow brown earth/loam
changed from an initial infiltration capacity of
2 mm/hour to 14 mm/hour as the soil hydro-
phobicity breaks down.
In Hewlett and Hibbert's (1967) original hypo-
thesis it was suggested that contributing saturated
areas would be immediately adjacent to stream
channels. Subsequent work by the likes of Dunne
and Black (1970), Anderson and Burt (1978) and
others has identified other areas in a catchment
prone to inducing saturated overland flow. These
include hillslope hollows, slope concavities (in
section) and where there is a thinning of the soil
overlying an impermeable base. In these situations
any throughflow is likely to return to the surface
as the volume of soil receiving it is not large enough
for the amount of water entering it. This can be
commonly observed in the field where wet and
boggy areas can be found at the base of slopes and
at valley heads (hillslope hollows).

Subsurface flow

Under the variable source areas concept there are
places within a catchment that contribute overland
flow to the storm hydrograph. When we total up
the amount of water found in a storm hydrograph
it is difficult to believe that it has all come from
overland flow, especially when this is confined to a
relatively small part of the catchment (i.e. variable
source areas concept). The manner in which the
recession limb of a hydrograph attenuates the storm-
flow suggests that it may be derived from a slower
movement of water: subsurface flow. In addition to
this, tracer studies looking at where the water has
been before entering the stream as stormflow have
found that a large amount of the storm hydrograph
consists of 'old water' (e.g. Martinec et al ., 1974;
Fritz et al ., 1976). This old water has been sitting
in the soil, or as fully saturated groundwater, for
a considerable length of time and yet enters the
stream during a storm event. There have been
several theories put forward to try and explain these
findings, almost all involving throughflow and
groundwater.
Throughflow is a general term used to describe the
movement of water through the unsaturated zone;
normally this is the soil matrix. Once water infil-
trates the soil surface it continues to move, either
through the soil matrix or along preferential flow
paths (referred to as lateral or preferential flow). The
rate of soil water movement through a saturated soil
matrix is described by Darcy's law (see Chapter 4)
and the Richards approximation of Darcy's law
when below saturation. Under normal, vertical,
infiltration conditions the hydraulic gradient has a
value of -1 and the saturated hydraulic conductivity
is the infiltration capacity. Once the soil is saturated
the movement of water is not only vertical. With a
sloping water table on a hillsope, water moves down
slope. However, the movement of water through a
saturated soil matrix is not rapid, e.g. Kelliher and
Scotter (1992) report a K sat value of 13 mm/hour for
a fine sandy loam. In order for throughflow to
contribute to storm runoff there must be another
mechanism (other than matrix flow) operating.