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Höhenmodell, Gewässer (nicht ganzjährig fließend) und Konturlinien des mittleren Jahresniederschlages im Cuvelai-Etosha Basin (CEB). Die blauen Punkte stellen die Hauptuntersuchungsstandorte dar. Schraffiert hervorgehoben ist die Eastern Sand Zone
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Zusammenfassung
Die Quantifizierung der Grundwasserneubildung in Trockengebieten stellt aufgrund klimatischer Gegebenheiten, mächtiger ungesättigter Zonen sowie hochangepasster Vegetation eine besondere Herausforderung dar. In dieser Arbeit wurden Methoden, die auf stabilen Bodenwasserisotopen basieren, detailliert untersucht: (i) die Auswertung na...
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Citations
... As previously underlined, the difference in results between the two different methods may be related to difficulties in applying the piston displacement method under arid climate conditions, i.e. the observed scattered distribution of isotopic signatures makes it difficult to trace the line that would be parallel to the LMWL. Beyer et al. (2018) as well as Koeniger et al. (2016) have pointed out that the empirical nature of the piston displacement method must be taken into account, and that this might be a site-specific effect. Barnes and Allison (1988) indicated that the piston displacement method has not been corroborated by other data so far, but they recommend that it be validated under a wide variety of climatic conditions ). ...
Considering three sites under different climate conditions (arid, semiarid, and subhumid), this study aims to use the vadose-zone water stable isotope profiles to estimate the groundwater recharge rate. High-resolution vertical subsurface soil sampling along the vadose zone of the investigated sites was conducted. The collected samples were analysed to determine their stable isotopes ratios (δ2H and δ18O) that were used in the piston displacement method for estimating recharge. Annual recharge rates of 0.2% (±0.1%), 2.5 %, and 18% of the total annual precipitation were obtained for the arid, semiarid, and subhumid sites, respectively. Recharge rates at the semiarid and subhumid sites are comparable to those previously estimated using water balance-based methods. The recharge rate at the arid site is lower than that previously estimated for that site using the water budget-based method, revealing difficulties in applying the piston displacement method under an arid regime.
Groundwater recharge is widely recognized as being the most important parameter for the sustainable management of water resources. In semiarid environments, groundwater recharge can be quantified using the piston displacement method (PDM). From a single soil sampling campaign, the PDM relies on linking the deeper vadose zone soil pore water stable isotope composition (δ2HH2O and δ18OH2O) to the local meteoric water line (LMWL). However, the isotopic composition of precipitation changes seasonally, influencing the water isotope composition of the vadose zone over time. Thus, it is important to test whether the PDM is sensitive to seasonal soil sampling and whether the assessed recharge rate is independent of the time of sampling. This study investigates the effect of seasonal soil sampling on the distribution of vadose zone stable isotope composition to determine whether the sampling time influences the estimate of recharge rate from PDM. Soil samples were obtained along vertical profiles through the vadose zone in a semiarid region during the spring, summer, and autumn seasons. Specifically, the δ2HH2O and δ18OH2O of the soil pore water were determined along vertical profiles, and the PDM was applied to quantify the annual recharge. The δ2HH2O and δ18OH2O values range from −7.3 to −3.5 ‰ and from −54.4 to +7.41 ‰, respectively, and plot along a continuum with a slope less than the LMWL. Samples from deeper in the vadose zone profile had distinct ranges in isotopic composition between the three soil sampling campaigns, with isotopic composition of spring sampling dominated by lower values and those from autumn with higher values. Despite these differences, the resulting annual recharge rates from the different sampling campaigns are comparable (1.5 to 2 % of annual precipitation). Even though the pore water isotopic composition changed over time, the shift between the deeper vadose zone isotopic compositions and the LMWL remained relatively constant, leading to a similar recharge estimate over time. Therefore, the PDM-based recharge assessment in the tested semiarid environment is independent of the sampling time, which indicates that sampling for assessing groundwater recharge can be undertaken during any season.
