Freiburger Schriften zur Hydrologie
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Band/volume 16: KÖNIGER P. (2003):
Tracerhydrologische Ansätze zur Bestimmung der Grundwasserneubildung
The aim of this study was to estimate groundwater recharge at the Forest
Meteorological test site Hartheim during the investigation period from
November 1997 to October 2000. The Meteorological Institute of the Freiburg
University has conducted continuous micro-meteorological measurements
of water balance and radiation components at this test site for more than
thirty years. Small amounts of rainfall, high evapotranspiration and lower-ing
of the groundwater level lead to classifying the area as almost semi arid.
The soil is characterized by a low water storage capacity and a high permeability.
A distance of seven meters to the groundwater table suggests that capillary
rise and groundwater uptake are neglectable.
Tracer methods are known to be the most precise methods to estimate groundwater
re-charge. In this work, environmental tracer methods were applied in
the saturated and un-saturated zone, and artificial tracers were used
in the unsaturated zone (sprinkling experi-ments and dry powder deposition).
A main focus was the application of the light stable water isotopes deuterium
and oxygen-18. In this study they were used as environmental and artificial
tracers. The derived recharge results were compared with water balance
cal-culations using the Bowen Ratio Energy Balance method for calculation
Micrometeorological and hydrometeorological investigations of evapotranspiration,
pre-cipitation and soil water storage point out relatively wet conditions
during the three-year period of investigation. These were mainly caused
by high rainfall amounts during the year 1999. Water balance calculations
resulted in 5 mm recharge during the hydrological year 1998, 200 mm for
1999 and 100 mm for 2000. The water balance components showed a high temporal
variability. Isotope contents in precipitation measured at the test site
showed a clear seasonal variability and were thus used as input for studies
of the satu-rated and unsaturated zone.
Investigations carried out in the saturated zone showed that environmental
tracers could not be used for an estimation of direct recharge in this
case. The influence of the nearby Rhine river by causing bank filtration
during flood events dominated the isotopic signature in the flood plain
aquifer. However, isotopic variations in the groundwater were observed
over the whole period and were interpreted for bank filtration amounts.
Experiments with artificial tracers allowed an interpretation of penetration
depths and flow velocities in the unsaturated zone. Infiltrated water
at the study site reached a depth of 140 cm where soil water loss due
to transpiration can be neglected. The root depth is smaller than 40 cm.
The observed results for applied deuterium are similar to that of fluorescein
and showed that deuterium is a useful tracer for studies in the unsaturated
The isotope content of soil water in deep profiles usually reflects the
influence of infiltrated precipitation and evaporation and can be interpreted
for groundwater recharge using the signature method. Four profiles at
the Hartheim test site were excavated to a depth of 180 cm. Between two
profiles that were taken in August and November 1999 the water moved to
the entire depth of the profiles.
Continuous observations of isotope content in the upper 40 cm of the soil
layer allowed an estimation of the dampening depth of the isotope signature
and suggests a dampening to 1% within 200 cm. Flow velocities calculated
using a sine curve fit resulted in values of 1 to 2 cm per day. In contrast
to the artificial tracer experiments these flow velocities reflect slower
Finally, the analysis of oxygen-18 deuterium relation and deuterium excess
values revealed the influence of evaporation on the different water components
of the investigated area. The soil water of the uppermost layer is influenced
most strongly. Variations of the deuterium excess observed in the soil
profiles could not be definitively interpreted as seasonal variations
because these variations were not observable in precipitation. The parameters
of the local meteoric water line at the test site are close to those of
two other sta-tions in the upper Rhine valley.
During the investigation period a quantitative estimation of direct groundwater
recharge was possible using water balance calculations for the Hartheim
pine forest. Tracer methods proved that direct recharge occurred but could
not be evaluated quantitatively. How-ever as opposed to some earlier hypotheses,
it can be concluded that direct groundwater recharge is not neglectable
at the Meteorological test site. In fact, in relation to the variable
water balance components it even is considerable.
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