Geoscience Reference
In-Depth Information
This empirical relation is a function of summer and winter precipitation, soil mois-
ture and potential evapotranspiration after Haude (
1955
). The coefficients (
a-e
)
differ depending on land use and soil types. Relevant data are provided by weather
services and publicly available databases, e.g. Corine Landcover (
2000
) or Global
Landcover (
2000
). Equation (
5
) is implemented in a GIS scheme thus combining
the input data and resulting in aerially differentiated maps of groundwater recharge
(Meyer and Tesmer
2000
). Since groundwater recharge is calculated on an annual
basis seasonal variations of groundwater recharge are not considered in this study.
19.2.3 Numerical Groundwater Models Feflow and Modflow
Groundwater recharge, as a function of precipitation, is the key parameter when
considering submarine groundwater discharge. Groundwater discharge to the sea is
estimated by means of two different groundwater flow modelling codes.
By means of the finite-element code Feflow Wasy
R
(Diersch
2005
) a three-
dimensional high-resolution model of a coastal catchment was established for recent
conditions by Darsow (
2004
). This model ('Catchment') serves as the base model,
which is then run for different groundwater recharge and sea-level conditions.
The FE model 'Catchment' is run for steady-state conditions only, thus neglect-
ing the temporal response to varying boundary conditions, namely groundwater
recharge change due to precipitation variation and sea-level rise.
The temporal response is therefore analysed by means of a simplified finite-
difference model ('Simple') using the Modflow code (Harbaugh et al.
2000
). Here a
rectangular catchment area is represented by one layer only (two-dimensional) and
subdivided into finite-difference cells. The aquifer is assumed to be homogeneous
in space. The hydraulic impact of groundwater recharge and surface water bodies is
modelled by second and third kind boundary conditions, respectively. However, sea
level is modelled by a modified prescribed head boundary, which allows for tempo-
ral variation of a first kind boundary condition. Thus the simplified model is able to
reflect transient conditions.
19.3 Test Site: Subcatchment at Wismar Bay
The test area is located at the Wismar Bay, at the southwestern coast of the Baltic
Sea, just across Poel island. The total area is 140 km
2
. The coastline extends about
23 km from SW to NE and is mostly straight, being only interrupted by very few
small creeks (Fig.
19.1
). The area is generally flat, ranging from 0 m asl at the coast
up to 101.6 m asl in the southeast.
The catchment is drained by three gaining streams that flow towards northwest
to the coast with a total length of 38.5 km (Fig.
19.1
). A reservoir of 1.2
10
6
m
3
is
situated in the centre of the area. The climate is humid with an average temperature
of 8.4
◦
C. The annual precipitation is 600 mm (non-corrected), of which 275 mm
occurs in winter and 325 mm in the summer half year, respectively. The potential
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