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  • Print publication year: 2011
  • Online publication date: May 2011

23 - Understanding the role of fog in forest hydrology: stable isotopes as tools for determining input and partitioning of cloud water in montane forests

from Part III - Hydrometeorology of tropical montane cloud forest



Understanding the hydrology of tropical montane cloud forests (TMCF) has become essential as deforestation of mountain areas proceeds at an increased rate worldwide. Passive and active cloud water collectors, throughfall and stemflow collectors, visibility or droplet size measurements, and micrometeorological sensors are typically used to measure fog water inputs to ecosystems. In addition, stable isotopes may be used as a natural tracer for fog and rain. Previous studies have shown that the isotopic signature of fog tends to be more enriched in the heavier isotopes 2H and 18O than that of rain, due to differences in condensation temperature and history. Differences between fog and rain isotopes are largest for synoptic-scale rain storms vs. local fogs or orographic clouds. Isotopic differences have also been observed between locally generated rain and fog on mountains with orographic clouds, but only a few studies have been conducted. Quantifying fog deposition using isotope methods is more difficult in forests receiving mixed precipitation, due to limitations in the ability of sampling equipment to separate fog from rain.

This chapter describes the various types of fog most relevant to MCF and the importance of fog water deposition in the hydrological budget. A brief overview of isotope hydrology provides the background needed to understand isotope applications in cloud forests. A summary of previous work explains isotopic differences between rain and fog in different environments, and how monitoring the isotopic signature of surface water, soil water, and tree sap can yield estimates of the contribution of fog water to streamflow, recharge, and transpiration.

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