Geoscience Reference
In-Depth Information
that are necessary in the different analysis methods presented below. As ever, in using such techniques,
start by evaluating the assumptions that are required against the perceptual model of the processes.
11.2
Advection and Dispersion in the Catchment System
The residence time of a water particle in the catchment will be related to both its origin on entry (in space
and time) and to the net effect of all the different velocities experienced by that particle in its pathway to
the catchment outlet. This might involve laminar flows in different pore sizes in the soil matrix; laminar,
transitional or turbulent flows in macropores and over the soil surface; and turbulent flows in rills, gulleys
or the stream channels. It might also involve periods of immobility or loss as evapotranspiration (see the
perceptual model of Section 1.4). The description of the transport process of water and tracers through
the system, for both surface and subsurface flow processes has traditionally been in terms of
advection
and
dispersion
. Advection is the movement associated with the mean velocity of the flow at any point;
dispersion is the spread resulting from the distribution of velocities around the mean at that point.
The theory of advection and dispersion can be found in Box 11.1, including descriptions based on
the advection-dispersion equation (ADE), and the aggregated dead zone (ADZ) model. In steady flows,
both can be used in the form of linear transfer functions (i.e. similar to the unit hydrograph method
for predicting hydrographs in Section 2.1). In many catchment systems, of course, we are not so much
interested in steady flows (though it can be a good approximation, for example, in predicting transport in
larger river channels during recession periods). What happens to the transport during hydrographs, and
the consequent effects on residence time distributions, is much more interesting (e.g. Figure 11.1).
Figure 11.1
Predicted contributions to individual hydrographs from different precipitation inputs (grey shad-
ing); this hypothetical simulation uses the MIPs model of Davies et al. (2011) on a 160 m hillslope with a soil
depth of 1.5 m and a hydraulic conductivity profile similar to that found at the G ardsj on catchment, Sweden
(see Figure 11.8).
