Process realism: flow paths and storage - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 61

Figure 4.7. Stable isotopes collected
in the San Pedro River study area,
Arizona. End members are winter
and summer precipitation shown as
squares. Groundwater and baseflow
isotopic characteristics vary between
these two end members, depending
on the source of water. Basin
groundwater recharged from winter
precipitation, riparian groundwater
recharged by summer precipitation.
From Baillie et al.( 2007 ).

Riparian wells

-40 Stream baseflow

Basin wells
Summer precipitation
Winter precipitation

-50

-60

-70

-80

-11 -10 -9 -8 -7 -6

18 O

δ

Figure 4.8. Observed longitudinal
temperature profile of the Maisbich
stream in Luxembourg at different
times on 26 April 2006. Clear
temperature jumps can be seen at the
location of the groundwater inflows
(sources). From Westhoff et al.
( 2007 ).

Longitudinal Temperature Profile (hourly average)

20

Source 1

18

Source 2

16

Source 3 Source 4

Cable out of the water

15:00h

14

12:00h

18:00h

12

10

9:00h
6:00h

8

6

0 100 200 300 400 500 600

Distance from most upstream point (m)

San Pedro River as a whole, this result means that roughly
50% of the water observed in the river is from groundwater
locally recharged during the summer monsoon.
Water temperatures are also excellent natural tracers to
identify stream/aquifer interactions (Becker et al., 2004 ;
Conant, 2004 ;Constantzet al., 2003 ; Selker et al., 2006 ).
An example is shown in Figure 4.8 , where the longitudinal
distribution of stream temperatures has been observed by
distributed temperature sensing (TDS) with fibre optic cables
(Westhoff et al., 2007 ). During the day time (e.g., 12:00
noon) the stream water is warmer than groundwater, so the
subsurface inflow sources into the stream are indicated by
sudden decreases of the stream temperature along the stream
course. Conversely, during the night or early in the morning
(e.g., 6:00 AM) the stream water is colder than groundwater,

so there are sudden increases in the temperature. Based on
these observations and a number of assumptions on the
thermal characteristics of the system, the exchange fluxes
can be estimated (e.g., Westhoff et al., 2011 ).
Advection and dispersion: transit times of the water in
the catchment: The transit time distribution (TTD) charac-
terises the time the water molecules need to travel from the
surface where they fall (as rainfall) to the catchment outlet
(or any other point in the catchment) (Maloszewski and
Zuber, 1982 ; Jury and Roth, 1990 ; McGuire and McDon-
nell, 2006 ). The mean and the variance of TTD are related
to advection and dispersion, respectively, which in turn are
related to the mean and variance of hydraulic resistances
along the flow paths as well as the lengths of the active
flow paths. If the average distance to the stream and the