Abstract

Recent studies using water stable isotopes (δ2H and δ18O) have described an ecohydrological separation of water in soils: a mobile water compartment (groundwater and streams) and a less mobile or static water compartment (used by plants); known as the “two water worlds hypothesis”. In this study, we used a dual isotope approach (δ2H and δ18O) to test the two water worlds hypothesis in highland ecosystems of central Argentina. The study area is characterized by seasonal rain inputs during summer time, while fog occurs all year round. Therefore we also assessed the role of rain and fog (meteorical water inputs) on groundwater recharge, stream flow and plant xylem water. Our results showed that rain and fog water isotopic composition differed; fog being more enriched than rain, though are in equilibrium. Groundwater and stream water isotopic signatures corroborate that the Local Meteoric Water Line (LMWL), using unweighted rain and fog samples, is in place. This preliminary assessment indicated that plant xylem water has a depleted isotopic signature compared to that of rain and fog, and plotted to the right of the LMWL, supporting the two water worlds hypothesis. This suggests a separation of a less mobile or static water compartment, used by plants; and a mobile water compartment feeding groundwater and stream flow. Based on our results we can also suggest that fog might be an important meteoric contribution for groundwater and stream flow. We conclude that we have supporting evidence for the two water worlds hypothesis, adding evidence from a seasonal ecosystem where rain inputs and transpiration outputs are in phase. We highlight the need of a deeper assessment and longer time series of δ2H and δ18O data and meteoric water inputs, which would help to clarify water sources contribution to groundwater recharge.