The moisture origin of dew: Insights from three sites with contrasting climatic conditions

Dew is one of the important moisture sources in many arid and semiarid regions. The knowledge of moisture origin of dew under various climatic conditions is still lacking. Isotopic variations can preserve information about moisture origin and formation mechanisms. Therefore, the isotopic compositions of dew and precipitation (delta H-2, delta O-18, delta O-17, d-excess, lc-excess and O-17-excess) were investigated at three sites with different climatic conditions (i.e., Gobabeb with extremely dry climate, Nice with Mediterranean climate and Indianapolis with humid continental climate). The results showed that there were three types of dew at Gobabeb: advective dew, groundwater-derived dew, and shallow soil water-derived dew, accounting for 27.3%, 45.4% and 27.3% of the dew events, respectively. The ultimate moisture sources of advective dew and the other two types of dew at Gobabeb were from the South Atlantic Ocean and a mixture of the Indian and South Atlantic Oceans, respectively. Dew in Nice included ocean-derived dew from the North Atlantic Ocean with local evapotranspiration replenishment, and local-derived dew, mainly from the continental Europe and Mediterranean Sea, accounting for 39.1% and 60.9% of the dew events, respectively. All the Indianapolis dew were likely local-derived dew. Based on the moisture origins, the future dew trends were speculated under global warming. Dew frequencies at Gobabeb and Indianapolis under future climates are uncertain due to the concurrent increases in atmospheric water vapour and temperature. The local-derived dew in Nice would likely decrease due to the decreasing precipitation and increasing drought, and the ocean-derived dew under future climates is uncertain. This study provides a practical method to distinguish dew moisture sources, and such information is useful for future prediction of dew trends under climate change.