| Title |
In-situ unsaturated zone stable water isotope (2H and 18O) measurements in semi-arid environments using tunable off-axis integrated cavity output spectroscopy |
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| Author |
Gaj, M., Beyer, M., Koeniger, P., Wanke, H., Hamutoko, J., Himmelsbach, T. |
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| Abstract |
Stable isotopes (deuterium, 2H, and oxygen-18,
18O) of soil water were measured in the field using a liquid
water isotope analyzer (tunable off-axis integrated cavity
output spectroscope, OA-ICOS, LGR) and commercially
available soil gas probes (BGL-30, UMS, Munich) in the
semi-arid Cuvelai–Etosha Basin (CEB), Namibia. Results
support the applicability of an in situ measurement system
for the determination of stable isotopes in soil pore water.
High spatial and temporal resolution was achieved in the
study area with reasonable accuracy and measurements were
in agreement with laboratory-based cryogenic vacuum extraction
and subsequent cavity ring-down laser spectroscopic
isotope analysis (CRDS, L2120-i, Picarro Inc.). After drift
and span correction of the in situ isotope data, precision for
over 140 measurements taken during two consecutive field
campaigns (June and November 2014) was 1.8 and 0.48‰
for 2H and 18O, respectively. Mean measurement trueness
is determined using quality check standards and was 5 and
0.3‰for 2H and 18O, respectively. The isotope depth profiles
are used quantitatively to calculate a soil water balance.
The contribution of transpiration to total evapotranspiration
ranged between 72 and 92 %. Shortly after a rain event, the
contribution of transpiration was much lower, at 35 to 50 %.
Potential limitations of such an in situ system are related to
environmental conditions which could be minimized by using
a temperature-controlled chamber for the laser spectrometer.
Further, the applicability of the system using previously
oven-dried soil material might be limited by physicochemical
soil properties (i.e., clay minerals). Uncertainty in the in situ
system is suggested to be reduced by improving the calibration
procedure and further studying fractionation effects influencing
the isotope ratios in the soil water, especially at low
water contents. Furthermore, the influence of soil-respired
CO2 on isotope values within the root zone could not be deduced
from the data. |
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| Citation |
Gaj, M., Beyer, M., Koeniger, P., Wanke, H., Hamutoko, J., Himmelsbach, T. (2015): In-situ unsaturated zone stable water isotope (2H and 18O) measurements in semi-arid environments using tunable off-axis integrated cavity output spectroscopy, Hydrol. Earth Syst. Sci. Discuss., 12, 6115-6149, doi:10.5194/hessd-12-6115-2015, 2015. |
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