Abstract
Coastal mangroves are increasingly recognized as valuable natural resources and important sites of water and carbon exchange. In this study, we examine atmospheric water cycling in the boundary layer above a coastal mangrove forest in southern China. We collected site observations of isotopic ratios in water vapor and precipitation along with core meteorological variables during July 2017. Our evaluation of these data highlights the influences of large-scale atmospheric transport and rain–vapor exchange in the boundary layer water budget. Rain–vapor exchange takes different forms for different types of rainfall events. The evolution of isotopic ratios in water vapor suggests that substantial rain recycling occurs during the passage of large-scale organized convective systems, but that this process is much weaker during rainfall associated with less organized events of local origin. We further examine the influences of large-scale transport during the observation period using a Lagrangian trajectory-based moisture source analysis. More than half (63%) of the boundary layer moisture during the study period traced back to the South China Sea, consistent with prevailing southerly to southwesterly flow. Other important moisture sources included mainland Southeast Asia and the Indian Ocean, local land areas (e.g., Hainan Island and the Leizhou Peninsula), and the Pacific Ocean. Together, these five regions contributed more than 90% of the water vapor. The most pronounced changes in isotopic content due to large-scale transport during the study period were related to the passage of Tropical Storm Talas. The outer rain bands of this tropical cyclone passed over the measurement site on 15–17 July, causing a sharp reduction in the heavy isotopic content of boundary layer water vapor and a substantial increase in deuterium excess. These changes are consistent with extensive isotopic distillation and rain–vapor exchange in downdrafts associated with the intense convective systems produced by this storm.
Highlights
The water cycle in coastal regions links the atmosphere to the ocean and biosphere, playing important roles in both atmospheric and surface hydrology
We have presented high-resolution measurements of isotopic ratios in surface-layer water vapor collected at the Gaoqiao Mangrove Reserve in southern China over multiple weeks in July 2017
These measurements of the stable isotopic composition of water vapor are supported by observations of isotopic ratios in precipitation, measurements of core meteorological variables collected at a nearby flux tower, and back trajectory analyses
Summary
The water cycle in coastal regions links the atmosphere to the ocean and biosphere, playing important roles in both atmospheric and surface hydrology. Krklec and Domínguez-Villar [13] used back trajectories together with isotopic observations to analyze the long-term water vapor source distribution for Eagle Cave in central Spain, another source of paleoclimate proxy data, from 2009 to 2011 Their results indicated that local recirculation can only explain 12% of monthly variability in the isotopic composition of precipitation, while local climatological conditions (such as temperature, rainfall, and water vapor transport pathways) can explain 74%. Previous studies on atmospheric water cycling in mangrove ecosystems have focused mainly on local processes such as evapotranspiration [21,22] We supplement these studies by using measurements of isotopic composition in water vapor and precipitation collected at the Gaoqiao Mangrove Reserve during July 2017 together with meteorological data and Lagrangian back trajectories to explore the influences of large-scale transport and convective rainfall on the water budget of the atmospheric boundary layer above the mangrove forest. This study helps to constrain the role of large-scale atmospheric processes relative to local land–atmosphere exchange in the isotopic water budgets of mangrove forests at this and other nearby measurement sites
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