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Isotope hydrology for estimating groundwater recharge in water-limited environments: potential and limits

Beyer, Matthias, Gaj, Marcel, Königer, Paul, Tulimeveva Hamutoko, Josefina, Wanke, Heike, Wallner, Markus, Himmelsbach, Thomas
Grundwasser 2018 v.23 no.1 pp. 73-90
climatic factors, deuterium oxide, groundwater, groundwater recharge, rain, soil water, stable isotopes, vegetation, water vapor, wet season, Namibia
The estimation of groundwater recharge in water-limited environments is challenging due to climatic conditions, the occurrence of deep unsaturated zones, and specialized vegetation. We critically examined two methods based on stable isotopes of soil water: (i) the interpretation of natural isotope depth-profiles and subsequent approximation of recharge using empirical relationships and (ii) the use of deuterium-enriched water (²H₂O) as tracer. Numerous depth-profiles were measured directly in the field in semiarid Namibia using a novel in-situ technique. Additionally, ²H₂O was injected into the soil and its displacement over a complete rainy season monitored. Estimated recharge ranges between 0 and 29 mm/y for three rainy seasons experiencing seasonal rainfall of 660 mm (2013/14), 313 mm (2014/15) and 535 mm (2015/16). The results of this study fortify the suitability of water stable isotope-based approaches for recharge estimation and highlight enormous potential for future studies of water vapor transport and ecohydrological processes.