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In-Depth Information
seasonal changes and seasonal constants in the flow and transport parameters. The
emphasis of this chapter is on the transient storage parameters.
2 A Unique Natural Hydraulics Laboratory
The study watershed is situated in south-west Iceland (64
05
0
N, 21
30
0
W) on the
mid-Atlantic ridge between the North American and Eurasian tectonic plates and is
characterised by intense volcanic and geothermal activity (Arnason et al.
1969
;
Franzson et al.
2005
). Heating of the stream water is by steam from boiling
geothermal water reservoirs, which heats up the upper cold groundwater that
feeds the streams (Arnason et al.
1969
). The water chemistry is very similar
between streams despite large temperature differences. Precipitation, which
exceeds 3,000 mm per year, infiltrates the porous volcanic bedrock (Einarsson
1984
) and numerous small permanent streams, mostly groundwater fed, emerge
from the valley side and discharge into the River Hengladals
ยด
(Friberg et al.
2009
).
Before human settlement in Iceland (900 AD), birch woodland and scrub covered
the area with unbroken heathland vegetation continuing up to 500-600 m elevation.
The catchment comprises a higher plateau region connected to a lower flood-
plain by a narrow river gorge. The lower floodplain is surrounded on all sides by
steep valley sides. Apart from the moss cover and sparse grassland of the plateau
and plain, there are extensive areas stripped of vegetation and soil where rocks of
volcanic origin protrude. Allochthonous organic matter input to the streams is
therefore considered minimal beyond the dissolved organic carbon coming from
the groundwater. Long-term landscape change is the only known anthropogenic
pressure on the streams investigated (Simpson et al.
2001
).
Sixteen streams in the same watershed were available for experiments with flows
ranging from 1 to 16 L/s, typical stream widths varied from 0.25 to 1.25 m and
depths were of the order of 0.1-0.4 m.
3 Tracer Experiments
For each experiment, YSI600xlmmulti-parameter sondes (YSI, Yellow Spring, USA)
were placed at two longitudinal stations (typically 60 m apart) in the study stream
and set to record conductivity at fixed 10 s intervals. Pre-weighed NaCl was fully
dissolved in 2 L of streamwater and then immediately released into the stream at some
distance upstream of the upper station. The mixing zone was generally sufficiently
long (~11 m) for complete cross-sectional mixing to take place before the upper
station. In the shortest streams, additional deflectors and pools were created upstream
of the upper station to increase mixing. Any naturally occurring background conduc-
tivity signal was subtracted from the observations prior to the modelling. A typical
conductivity versus time graph is shown in Fig.
1
. Although there were 16 streams in
