Geoscience Reference
In-Depth Information
undisturbed soil brought back to the laboratory. It has generally been found in such studies that more
detailed investigation reveals greater complexity and variability in the flow pathways. The same has
generally been true of adding different types of information, such as the use of artificial or environmental
tracers. Figure 1.1 is a good example of this (see also Section 1.5). Such complexity can be made part of
the perceptual model. As noted above, it is not necessary that the perceptual model be anything more than
a set of qualitative impressions, but complexity inevitably creates difficulty in the choice of assumptions
in moving from the perceptual model to a set of equations defining a conceptual model. Choices must
be made at this point to simplify the description and, as we will see, such choices have not always had a
good foundation in hydrological reality.
Consider, briefly, one hydrologist's perceptual model. It is based on an outline set out in Beven (1991a),
with some revision based on additional experience since then. In recession periods between storms,
storage in the soil and rock gradually declines (Figure 1.3a). If there is a water table, the level and
gradient will gradually fall. Storage will often be higher and water tables closer to the surface in the
valley bottom
riparian areas
, partly because of downslope flow, particularly where there is convergence
of flow in hillslope hollows. Storage in riparian areas may be maintained by return flows from deeper
layers (e.g. Huff
et al.
, 1982; Genereux
et al.
, 1993), but also because soils tend to be deeper in valley
bottoms (e.g. Dietrich
et al.
, 1995; Pi nol
et al.
, 1997). Loss of water by evapotranspiration will have a
greater or lesser effect on the profile of storage depending on season, climate and vegetation type and
rooting depth. Many plants, however, may extract water from considerable depth with roots penetrating
up to tens of meters into the soil and bedrock fractures and root channels also acting as pathways for
infiltrating water (for example the Jarrah trees of Western Australia). Plants that are
phreatophytes
(such
as the Cottonwoods of the western United States) will extract water directly from beneath the water
table. These evapotranspiration and drainage processes will be important in controlling the
antecedent
conditions
prior to a storm event.
The antecedent conditions, as well as the volume and intensity of rainfall (or snowmelt), will be
important in governing the processes by which a catchment responds to rainfall and the proportion of
the input volume that appears in the stream as part of the hydrograph (Figure 1.3b). Unless the stream is
ephemeral, there will always be a response from precipitation directly onto the channel and immediate
riparian area. This area, although a relatively small area of the catchment (perhaps 1-3%), may be an
important contributor to the hydrograph in catchments and storms with low runoff coefficients. Even in
ephemeral streams
, surface flow will often start first in the stream channels. The extent of the channel
network will generally expand into headwater areas as a storm progresses and will be greater during wet
seasons than dry (e.g. Hewlett, 1974).
Rainfalls and snowmelt inputs are not spatially uniform, but can show rapid changes in intensity and
volume over relatively short distances, particularly in convective events (e.g. Newson, 1980; Smith
et al.
,
1996; Goodrich
et al.
, 1997 ). The variability at ground level, after the pattern of intensities has been
affected by the vegetation canopy, may be even greater. Some of the rainfall will fall directly to the
ground as direct
throughfall
. Some of the rainfall will be intercepted and evaporated from the canopy
back to the atmosphere. Some evaporation of intercepted water may occur even during events, especially
from rough canopies under windy conditions, when the air is not saturated with vapour. Differences of
up to 30% between incident rainfall and throughfall have been measured in a Mediterranean catchment
(Llorens
et al.
, 1997). The remaining rainfall will drip from the vegetation of the canopy as throughfall
or run down the branches, trunks and stems as
stemflow
. The latter process may be important since, for
some canopies, 10% or more of the incident rainfall may reach the ground as stemflow resulting in local
concentrations of water at much higher intensities than the incident rainfall. Some plants, such as maize,
have a structure designed to channel water to their roots in this way.
Snowmelt rates will vary with elevation and aspect in that they affect the air temperature and radiation
inputs to the snowpack. The water equivalent of the snowpack can vary dramatically in space, due
particularly to the effects of wind drifting during snow events and after the snowpack has formed, as
