Global Change Abstracts The Swiss Contribution - SCNAT

Global Change Abstracts – The Swiss Contribution | Oceans and Fresh Water Systems
08.1-246
Ecosystem expansion and contraction dynamics
along a large Alpine alluvial corridor
(Tagliamento River, Northeast Italy)
Doering M, Uehlinger U, Rotach A, Schlaepfer D R,
Tockner K
Switzerland
Hydrology , Ecology , Geomorphology
Riverine floodplains are pulsing ecosystems that
expand and contract with changing flow. In this
study we quantified large-scale expansion and
contraction dynamics of surface waters along
a 41.5 km braided section of the last remaining
semi-natural large Alpine gravel-bed river (Tagliamento
River; NE Italy). To assess surface-subsurface
exchange patterns We measured discharge and
vertical hydraulic gradients at multiple locations
along the corridor. We identified two river sections
delineated by distinct geomorphic knickpoints. In
the upper 29 km, Section I (losing zone), surface
flow decreased on average by 2.5 +/- 0.8 m(3) s(-1)
per river-km. In the downstream 12.5 km, Section
II (gaining zone), surface How increased on average
by 0.3 +/- 0.1 m(3)s(-1) per river-km. The losing
zone experienced frequent and extensive drying
and rewetting cycles. The length of the dry river
section was measured over a 1.5 year period using
differential GPS. Up to 23 km of Section I fell dry
at the surface. Frequent and irregular flow pulses
led to rapid expansions of the wetted channel at
velocities of up to 3 km h(-1), while the subsequent
contraction velocities were less than 0-5 km h(-1).
Water level was linearly regressed against the total
length of the dry river section (r(2) = 0.74; p
< 0.0001). This relationship, in combination with
a continuous stage record, was used to evaluate
expansion and contraction dynamics over a 4 year
period. Timing, frequency, magnitude (spatial extent)
and duration of expansion and contraction
dynamics reflected the flashy flow regime of the
Tagliamento River, including a high intra- and
inter-annual variability of surface drying and rewetting.
Our study emphasizes that even small
changes in How can cause major increases or
decreases of ecosystem size, thereby creating a
highly dynamic and harsh environment for both
terrestrial and aquatic organisms.
Earth Surface Processes and Landforms, 2007, V32,
N11, OCT 15, pp 1693-1704.
131
08.1-247
Field evidence of a dynamic leakage coefficient
for modelling river- aquifer interactions
Doppler T, Franssen H J H, Kaiser H P, Kuhlman U,
Stauffer F
Switzerland
Modelling , Water Resources , Hydrology
In groundwater flow modelling, the interaction
between rivers and aquifers is usually modelled
with spatially and temporally constant leakage
coefficients. We used conventional model calibration
techniques to investigate the time-varying
river-aquifer interactions in the sandy gravel
aquifer of the upper Limmat valley in Zürich
(Switzerland). The aim of the study was to determine
whether the leakage coefficients have to be
treated as time-dependent in order to adequately
model the dynamics of the groundwater flow. A
transient horizontal two-dimensional groundwater
flow model was established together with a
one-dimensional hydraulic model for river flow,
as well as a scheme calculating groundwater recharge
and lateral inflow from meteorological
data and a soil water balance model. The groundwater
flow model was calibrated using hydraulic
head data from May and June 2004 and July and
August 2005. The verification period covered 13
years using hydraulic head data from 90 piezometers.
The comparison of the model results with
the measurements in the verification period revealed
three phenomena concerning river-aquifer
interaction which all showed up as systematic deviations
between model and observations. (1) The
major flood event in May 1999 had a significant
and persistent influence on the river-aquifer interaction.
In an impounded river section upstream
of a weir, the infiltration of river water was enhanced
by the flooding probably due to erosion
processes. (2) Seasonal river water temperature
fluctuations influenced the infiltration rate, due
to the temperature dependence of hydraulic
conductivity of the river bed. (3) Depending on
geometry and hydraulic characteristics of the riverbanks
the leakage coefficient can be a function
of the river stage. With higher water levels, additional
areas can contribute to the infiltration of
river water. Therefore, in modelling groundwater
flow with strong river-aquifer interactions, it can
become necessary to consider dynamic leakage
coefficients and to recalibrate periodically.
Journal of Hydrology, 2007, V347, N1-2, DEC 15,
pp 177-187.