A Stable Isotope Approach to Investigative Ecohydrological Processes in Namibia

Abstract

Drylands cover 40% of the earth’s terrestrial surface supporting over 2 billion people, the majority of whom reside in developing nations characterised by high population growth rates. This imposes pressure on the already limited water resources and in some dryland regions such as southern Africa, the origins and dynamics of rainfall are not well understood. Research has also tended to focus on factors limiting (e.g., rainfall) than sustaining productivity in drylands. However, non-rainfall water (NRW) e.g., fog and dew can supplement and/or exceed rainfall in these environments and could potentially be exploited as potable water resources. Much remains unknown in terms of NRW formation mechanisms, origins, evolution, potability and potential impact of global climate change on these NRW dependent ecosystems. Using Namibia as a proxy for drylands and developing nations, this dissertation applies stable isotopes of water (δ2H, δ18O, δ17O and d-excess), cokriging and trajectory analysis methods to understand ecohydrological processes. Results suggest that locally generated NRW may be a regular occurrence even in coastal areas such as the Namib Desert, and that what may appear as a single fog event may consist of different fog types co-occurring. These results are important because NRW responses to global climate change is dependent on the source, groundwater vs. ocean, and being able to distinguish the two will allow for more accurate modelling. I also demonstrate, that fog and dew formation are controlled by different fractionation processes, paving the way for plant water use strategy studies and modelling responses to global climate change. The study also suggests that current NRW harvesting technologies could be improved and that the potability of this water could raise some public health concerns related to trace metal and biological contamination. At the same time, the dissertation concludes that global precipitation isoscapes do not capture local isotope variations in Namibia, suggesting caution when applied to drylands and developing nations. Finally, the dissertation also reports for the first time, δ17O precipitation results for Namibia, novel isotope methods to differentiate synoptic from local droughts and suggests non-negligible moisture contributions from the Atlantic Ocean due to a possible sub-tropical Atlantic Ocean dipole.