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Figure 1.5 Dominant processes of hillslope response to rainfall (after Dunne, 1978, with kind permission of
Wiley-Blackwell).
within the soil at a permeability break, perhaps associated with a horizon boundary. This might lead to the
generation of a perched water table and even to saturation at the surface of a soil that may be unsaturated
at depth (see Figure 1.4e; Weyman, 1970). Attempts have been made to suggest which mechanisms
might be dominant in different environments (see Figure 1.5) but there may still be much to learn
from direct observations of runoff processes in a catchment of interest. A selection of classic papers on
runoff generation processes has been published by Beven (2006d), while Smakhtin (2002) has reviewed
early process studies in Russia and Bonell (2004) gives a summary of work in tropical catchments. An
interesting contribution is made by McGlynn et al. (2002) who discuss the way in which research has
changed the perceptual model of runoff processes in the small Maimai catchment in New Zealand.
1.5 Flow Processes and Geochemical Characteristics
One of the most influential factors in revising hydrological thinking in the last 30 years has been the use
of geochemical characteristics to provide additional information on flow processes. Some characteristics,
in particular the use of artificial tracers, can provide direct information on flow velocities; others, such
as the various environmental tracers, require a greater degree of inference. Even the results of artificial
tracers may be difficult to interpret since most tracers are not ideal for following water movement over
the wide range of time scales involved and it is difficult to apply artificial tracers at the catchment scale.
Thus, any experiment will tend to sample only some of the possible flow pathways.
The environmental isotopes of oxygen and hydrogen are often used in catchment scale studies (see
the review by Sklash, 1990). They have the advantage that they are part of the water molecule and will
therefore follow the flow pathways of water in the catchment directly. There remain some difficulties of
interpretation of the results due to spatial and temporal variations in the concentrations of the isotopes
 
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