Process realism: flow paths and storage - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 60

Figure 4.6. (Top) Ratio of runoff to precipitation minus evaporation (Qr:R) over Colorado River and lowland and coastal basins in
Texas. Catchments that are groundwater exporters (Qr:R

1) brown, groundwater importers (Qr:R 1) blue. (Bottom) Geological

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cross-section. From Schaller and Fan ( 2009 ).

different physical concepts related to the movement of
water and associated substances through a catchment.
These include: (i) mixing, (ii) advection and dispersion,
and (iii) decay. Each of the concepts can be used to infer
different pieces of information from the tracers.
Mixing: sources and pathways of water in the catch-
ment: The idea of end member mixing analysis (EMMA) is
that runoff consists of two or more sources that can be
distinguished in terms of their isotopic and/or chemical
characteristics, e.g., subsurface water and surface runoff.
The isotopic contents of groundwater and surface runoff
need to be sufficiently different for the method to give
meaningful results. Also, the isotopic contents should vary
only to a limited extent with time, which is not always the
case (Hooper and Shoemaker, 1986 ). Finally, the method
assumes perfect mixing of the sources, which is not con-
sistent with the occurrence of preferential flow. The mass
balance equations for water and the tracer(s) then give the
relative contribution of the sources, e.g., the proportion of

event runoff that comes from rain through rapid flow paths
(surface and preferential flow paths) and the contribution
of pre-event water in the subsurface that is mobilised by
pressure transfer. The fraction of pre-event water is usually
larger than the fraction of event water and changes dynam-
ically with time (e.g., Harris et al., 1995 ; Buttle and Peters,
1997 ; Laudon, 2002 ). Figure 4.7 illustrates the EMMA for
the San Pedro River, Arizona (Baillie et al., 2007 ). Basin
groundwater (far away from the stream) is isotopically
light, and primarily comprises winter precipitation that
has recharged in the mountains. In contrast, riparian
groundwater is isotopically heavier, due to groundwater
recharge by isotopically heavy summer (monsoon) precipi-
tation during floods. The baseflow sources of the San
Pedro River vary along the river reach (triangles in Figure
4.7 ). In the losing reaches of the river, stream recharge
from the riparian aquifer (recharged by monsoon floods)
dominates, while in the gaining reaches of the river, basin
groundwater plays a significant role in baseflow. For the