Freiburger Schriften zur Hydrologie
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Band/volume 17: ARMBRUSTER V. (2002):
Grundwasserneubildung in Baden-Württemberg
The objective ofthis study was the development of a method to determine
the lang term mean groundwater recharge from precipitation für large
scale areas. This method was supposed to account für the influence
of climate, land use, soil and groundwater level on the goveming processes
of evapotranspiration and runoff generation. Olle focus of the study was
the determination of groundwater recharge in hard rock regions. This implied
to improve the description of fast lateral runoff components in the macro
scale. The method was applied in the federal state of Baden-Württemberg
für the time period 1961-1990. Groundwater recharge GWR is determined
according to following equation:
GWR = (P-ET)' (Dbas/Dtot)
In a first step the sink term Evapotranspiration ET is subtracted from
Precipitation P. In a second step fast lateral runoff components are considered
by multiplying the resulting total discharge with the ratio of baseflow
divided by total discharge Dbas/Dtot.
The detailed evapotranspiration model TRAIN-GWN (TRAnspirationINterzeption-GrundWasserNeubildung)
was newly developed in this study. Its application provides precipitation
and evapotranspiration für the whole study area. The model is based
on the evapotranspiration model TRAIN (MENZEL 1997a, 1999). TRAIN-GWN
consists of conceptual and physically based modules. It works on a daily
time step on a 500 m x 500 m grid and simulates evapotranspiration and
percolation out of the rooted soil zone. The snow module is based on the
degree dar method. The soil module according to the HBV -model and capillary
rise according to the 'Bodenkundliche Kartieranleitung' (AG BODEN 1994)
are newly integrated. Another new module is the interpolation of meteorological
input data, based on a combination of Inverse Distance Weight and a regression
on altitude. Radiation calculation, interception according to the theory
of latent heat transfer over droplets, and evapotranspiration according
to the Penman-Monteith-relation are strongly physically based components.
Extended sensitivity analysis were conducted. The lang term mean groundwater
recharge is sensitive to different land use parameters, especially the
leaf area index and to the two soi! parameters. Among the meteorological
input data, precipitation hag a large impact on recharge.
For the land use agriculture and grass the model was validated on measured
evapotranspiration and percolation of Olle weighable and twenty non-weighable
lysimeters. A regionalization function für the empirical soil parameter
BETA was derived and groundwater recharge was simulated on a weekly and
monthly basis. The results confmn the concept of the soi! module, accounting
für percolation before maximum soil water content is reached. The
mean ready groundwater recharge could be determined without systematic
deviation, showing a mean deviation of 8%. In addition, the inter-annual
variation could be reproduced weIl. A comparison of the evapotranspiration
with results of the model VEKOS (KLÄMT 1988) showed good agreement
of ready totals, while considerable deviations occurred in winter times.
This might be due to the fact, thai TRAIN-GWN does not simulate soil evaporation
separately.
In a next step TRAIN-GWN was validated on measured discharge of tell mainly
forested, mountainous catchments. The systematic overestimation of the
modeled lang term discharge amounts to 8%. Furthermore, catchment evapotranspiration
and precipitation were compared to values from the Hydrological Atlas
of Germany. Evapotranspiration is weIl comparable, with TRAIN-GWN showing
only 5% higher values. Precipitation does not show systematic deviation,
while single catchments show differences ofup to 7%. This demonstrates
the difficulties of precipitation determination in mountains, which presents
a main source of erraT für groundwater recharge simulations. Overall
the results show, that precipitation is slightly overestimated.
The spatial distribution of the lang term mean evapotranspiration is influenced
by climate, land use, soils and groundwater levels. The mean evapotranspiration
für the whole study area amounts to 555 mm, the mean für agricultural
areas and grasslands is 533 mm, while it is 635 mm für forests.
In areas with fast lateral runoff components this sink term is accounted
für by the lang term mean ratio of baseflow divided by total discharge
(Dbas/Dtot). Base flow was determined from time series of discharge, using
a modified Wundt/Kille-method, the Demuth-method. This results in baseflow
during low flow conditions, which is not the same as baseflow, defmed
by source areas and flow paths. In the mountainous crystalline test catchment
Brugga in the Black Forest, für example, the baseflow according to
the Demuth-method originates from slow inter flow from the debris cover
of slopes in addition to groundwater from hard rock aquifers. Since the
used Demuth-method is based on discharge data, the data base is large
enough to allow the regionalization of the ratio with a multiple linear
regression. 105 test catchments were selected. Their determined Dbas/Dtot
ranges from 16 to 80% and is scale independent. The regression model was
developed based on a calibration data set of 70 catchments and their characteristics.
The model consists of eleven catchment parameters, which are all plausible
to describe runoff generation. They characterize soils, geology, hydrogeology
and drainage density. Parameters describing precipitation and topography
were statistically insignificant. Model performance is satisfying with
a coefficient of determination of 0.76. Its validation on an independent
data set of 35 catchments yield a coefficient of determination of 0.74,
confirming the developed model.
In a next step Baden- Württemberg was delineated into areas without
fast lateral runoff components, mainly weIl permeable alluvial aquifers,
and into areas with lateral components. The latter Olles were further
subdivided into 1669 sub catchments. On the basis of their characteristics
Dbas/Dtot was regionalized. The spatial distribution of Dbas/Dtot plausibly
reflects the physiographic differences of the regions.
Groundwater recharge results from the combination of the lang term mean
total discharge with the regionalized Dbas/Dtot. Precipitation and Dbas/Dtot
strongly influence the large scale variability of the groundwater recharge,
while the small scale variability is mainly govemed by evapotranspiration
and its influencing factors. The mean evapotranspiration in Baden-Württemberg
amounts to 237 mm, 86% of the area shows values between 50 and 400 mm.
Due to the high temporal resolution, TRAIN-GWN can simulate the inter-annual
distribution of groundwater recharge, in addition to values of single
years. As the latter vary strongly, the lang term mean groundwater recharge
can only be considered as a first estimate für single years.
The combination of the detailed evapotranspiration model with the lang
term mean Dbas/Dtot presents a new approach in the modeling of groundwater
recharge from precipitation. It contains an improved method to ac count
für fast ateral runoff components in the macro scale.. This method
is based on several catchment characteristics, being plausible für
runoff generation. Required input data sets are available für most
large scale study areas. The approach is applicable in alluvial aquifers
as weIl as in hard rock regions. With its application in BadenWürttemberg
spatially detailed groundwater recharge, determined with a unique method,
is available in the whole federal state für the fIrst time. The approach
presents an adequate tool für different practical questions ofwater
management. Long term mean groundwater recharge is the basis für
the sustainable management of groundwater resources, while monthly time
series are needed für transient groundwater models. Impacts of climate
change on groundwater recharge and its inter annual variation can also
be estimated.
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