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The role of stable isotopes in understanding rainfall interception processes: a review

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The isotopic composition of water transmitted by the canopy as throughfall or stemflow reflects a suite of processes modifying rainfall. Factors that affect isotopic composition of canopy water include fractionation, exchange between liquid and vapor, and selective transmittance of temporally varying rainfall along varying canopy flowpaths. Despite frequent attribution of canopy effects on isotopic composition of throughfall to evaporative fractionation, data suggest exchange and selection are more likely the dominant factors. Temporal variability in canopy effects is generally consistent with either exchange or selection, but spatial variability is generally more consistent with selection. However, most investigations to date have not collected data sufficient to unambiguously identify controlling processes. Using isotopic data for improved understanding of physical processes and water routing in the canopy requires recognizing how these factors and processes lead to patterns of isotopic variability, and then applying this understanding toward focused data collection and analysis. WIREs Water 2017, 4:e1187. doi: 10.1002/wat2.1187 This article is categorized under: Science of Water > Methods
Expected effects of canopy interception on δ18O and δ2H: (a) general isotope concepts including the global mean water line (GMWL), deuterium excess (d), and effects of kinetic and equilibrium fractionation on precipitation (δprecip); (b) isotopic effects of evaporation on throughfall (TF) and the resulting theoretical local mean water lines of TF (LMWLTF) and open precipitation (LMWLOP); (c) isotopic effects of exchange on TF; and (d) isotopic effects of intra‐event selection on TF.
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Greatest individual‐event ranges of δ18O for each study: (a) between plot means in throughfall, (b) between throughfall and stemflow, and (c) among individual throughfall collectors; open bars indicate that mean spatial range across all events is also included.
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Study‐mean differences in δ18O (a) among throughfall collectors within plots and (b) between throughfall and stemflow. Open bars are mean differences and shaded bars are mean absolute value of differences. An ‘X’ indicates data are not available. Numbers of intervals measured are indicated above the bars.
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Differences between δ18O in throughfall and open precipitation (TF–OP): (a) range of single‐event differences, and (b) study‐mean differences. The number of measurement intervals (events) by study is between panels. Some values are arithmetic means and others are volume‐weighted; see Table . An ‘X’ indicates data are not available.
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