Groundwater-based isoscapes: a case study from Namibia

Groundwater-based isoscapes for δ18O and δ2H are often considered as proxies for precipitation; however, in drylands of sub-Saharan Africa, usually rain events of exceptional depth, intensity or frequency replenish the groundwater reservoirs. Here, groundwater-based isoscapes can contribute to the understanding of groundwater recharge processes, if combined with precipitation data. This study has thus compiled a groundwater-based database for Namibia. Typical isotope effects have been evaluated, and correlations with further variables tested. Explanatory variables of our data set correlate in general stronger with the traced source water than with the observed composition of the groundwater, highlighting the importance of considering both, groundwater-based and precipitation-based isoscapes. Mid-season precipitation during wetter than normal years has been identified as the main groundwater recharge source. Consequently, changes in the recurrency, frequency or amount of such rainfall events are likely to have the strongest impact on future availability of groundwater resources in Namibia.