Browsing by Author "Wanke, Heike"

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    An Analysis of Precipitation Isotope Distributions across Namibia Using Historical Data
    (Plos, 2016-05-04) Kaseke, Kudzai Farai; Wang, Lixin; Wanke, Heike; Turewicz, Veronika; Koeniger, Paul; Department of Earth Sciences, School of Science
    Global precipitation isoscapes based on the Global Network for Isotopes in Precipitation (GNIP) network are an important toolset that aid our understanding of global hydrologic cycles. Although the GNIP database is instrumental in developing global isoscapes, data coverage in some regions of hydrological interest (e.g., drylands) is low or non-existent thus the accuracy and relevance of global isoscapes to these regions is debatable. Capitalizing on existing literature isotope data, we generated rainfall isoscapes for Namibia (dryland) using the cokriging method and compared it to a globally fitted isoscape (GFI) downscaled to country level. Results showed weak correlation between observed and predicted isotope values in the GFI model (r2 < 0.20) while the cokriging isoscape showed stronger correlation (r2 = 0.67). The general trend of the local cokriging isoscape is consistent with synoptic weather systems (i.e., influences from Atlantic Ocean maritime vapour, Indian Ocean maritime vapour, Zaire Air Boundary, the Intertropical Convergence Zone and Tropical Temperate Troughs) and topography affecting the region. However, because we used the unweighted approach in this method, due to data scarcity, the absolute values could be improved in future studies. A comparison of local meteoric water lines (LMWL) constructed from the cokriging and GFI suggested that the GFI model still reflects the global average even when downscaled. The cokriging LMWL was however more consistent with expectations for an arid environment. The results indicate that although not ideal, for data deficient regions such as many drylands, the unweighted cokriging approach using historical local data can be an alternative approach to modelling rainfall isoscapes that are more relevant to the local conditions compared to using downscaled global isoscapes.
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    Precipitation Origins and Key Drivers of Precipitation Isotope (18O, 2H, and 17O) Compositions Over Windhoek
    (AGU, 2018) Kaseke, Kudzai Farai; Wang, Lixin; Wanke, Heike; Tian, Chao; Lanning, Matthew; Jiao, Wenzhe; Earth Sciences, School of Science
    Southern African climate is characterized by large precipitation variability, and model precipitation estimates can vary by 70% during summer. This may be partly attributed to underestimation and lack of knowledge of the exact influence of the Atlantic Ocean on precipitation over the region. The current study models trajectories of precipitation events sampled from Windhoek (2012–2016), coupled with isotopes (δ18O, δ2H, δ17O, d, and δ′17O‐δ′18O) to determine key local drivers of isotope compositions as well as infer source evaporative conditions. Multiple linear regression analyses suggest that key drivers of isotope compositions (relative humidity, precipitation amount, and air temperature) account for 47–53% of δ18O, δ2H, and δ17O variability. Surprisingly, precipitation δ18O, δ2H, and δ17O were independent of precipitation type (stratiform versus convective), and this may be attributed to greater modification of stratiform compared to convective raindrops, leading to convergence of isotopes from these precipitation types. Trajectory analyses showed that 78% and 21% of precipitation events during the period originated from the Indian and South Atlantic Oceans, respectively. Although precipitation from the Atlantic Ocean was significantly enriched compared to that from the Indian Ocean (p < 0.05), d was similar, suggesting significant local modification (up to 55% of d variability). Therefore, d may not be a conservative tracer of evaporation conditions at the source, at least for Windhoek. The δ′17O‐δ′18O appeared to be a better alternative to d, consistent with trajectory analyses, and appeared to differentiate El Niño from non‐El Niño droughts. Thus, δ′17O‐δ′18O could be a novel tool to identify drought mechanisms.