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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.
Fog deposition, precipitation, throughfall, and stemflow were measured in a windward tropical montane cloud forest near Monteverde, Costa Rica, for a 65-day period during the dry season of 2003. Net fog deposition was measured directly with the eddy covariance method and amounted to 1.2 ± 0.1 mm day−1 (mean ± standard error). Fog water deposition was 4–7% of incident rainfall for the entire period. Stable isotope concentrations (δ18O and δ 2H) were determined in a large number of samples of each water component. Comparisons between direct fog deposition measurements and the results of a mass-balance model using the stable isotopes as tracers indicated that the latter might be a good tool to estimate fog deposition in the absence of direct measurement under many (but not all) conditions. At 506 mm, measured water inputs over the 65 days (fog plus rain) fell short by 46 mm compared to the canopy output of 552 mm (throughfall, stemflow, and evaporation). The discrepancy is attributed to underestimation of rainfall during conditions of high wind.
Montane cloud forests are widely believed to receive significant extra amounts of water to the water budget by the capture of water from passing fog and low cloud.
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