Geoscience Reference
In-Depth Information
11
Beyond the Primer: Water Sources
and Residence Times in Catchments
Every theory is based on physical concepts expressed through mathematical idealisations.
They are introduced to give an adequate representation of the physical phenomena. No
physical concept is adequately defined without the knowledge of its domain of validity.
Leon Rosenfeld (quoted in Prigogine, 1997)
L'augmentation du debit des eaux souterraines, constate lors des crues, ne s'accompagne
pas d'une variation de leur teneur en tritium ...Tout se passe comme si la lame d'eau
infiltree chassait, plus vite, devant elle les eaux des episodes pluvieux anterieurs.
Edith Crouzet
et al.
, 1970
11.1
Natural and Artificial Tracers
There are many purposes for which it is not sufficient just to predict the discharge from a catchment area.
It would also be useful to have predictions of flow pathways and residence times of the water within the
catchment especially when concerned about water quality, sediment and pollutant transport. It would also
be nice to be able to have a more direct understanding of water sources and pathways when it is so difficult
to observe the subsurface. As noted in Section 1.5, the use of
environmental tracer
data was fundamental
in causing a re-evaluation of flow pathways in catchments by revealing that in many situations the bulk
of the hydrograph was made of
pre-event
water displaced from storage rather than from precipitation
in a storm. This meant that the simplistic Hortonian concept of the soil surface acting as a “separating
surface” between “storm runoff” and infiltration was no longer tenable for many catchments (though it
is worth noting that Robert Horton himself had a more complex view of infiltration and runoff processes
(see Beven, 2004b). It is also worth noting that the concept of displacement is also not new, Horton was
in correspondence with other hydrologists in the 1930s about ideas of “translationary flow”, a phrase
