In-situ unsaturated zone stable water isotope (²H and 18O) measurements in semi-arid environments using tunable off-axis integrated cavity output spectroscopy.

Stable isotopes (deuterium, ²H, and oxygen-18, ¹⁸O) of soil pore water were measured directly in the field using tunable off-axis integrated cavity output spectroscopy (OA-ICOS) and commercially available soil gas probes in a semi-arid region of the Cuvelai-Etosha-Basin, Namibia. 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). After drift correction of the isotope data, mean precision for over 140 measurements of two consecutive field campaigns in June and November 2014 were 1.8 and 0.46 ‰ for δ²H and ¹⁸O, respectively. Mean Accuracy using quality check standards was 5 and 0.3 ‰ for δ²H and δ¹⁸O, respectively. Results support the applicability of an in-situ measurement system for the determination of stable isotopes in soil pore water. Spatio-temporal variability could be deduced with the observed data in an extremely dry evaporation dominated environment which was sporadically affected by intermittent rainfall. [ABSTRACT FROM AUTHOR]