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

Abstract

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

Spatio‐temporal variability of the isotopic input signal in a partly forested catchment: Implications for hydrograph separation

“…Our results confirmed the importance of taking into account the effect of forested areas on the isotopic input signal, especially in events with less than 20 mm of rainfall depth, as forest affects both the incident volume of water reaching the soil and its isotopic composition, as shown in this catchment (Cayuela et al, ) and elsewhere (Allen et al, ; Kubota & Tsuboyama, ; Stockinger, Lücke, Vereecken, & Bogena, ).…”

What have we learnt about Mediterranean catchment hydrology? 30 years observing hydrological processes in the Vallcebre Research Catchments

“…The modification of rainfall isotopic signature by throughfall and stemflow is attributed to the interaction of three factors: evaporation, isotopic exchange between vapour and liquid and selective canopy storage (Allen et al, 2017). The analysis of these processes that take place in the canopy is of paramount importance when tracing the source and movement of water through forested or partially forested catchments (Klaus and McDonnell, 2013).…”

Modification of the isotopic composition of rainfall by throughfall and stemflow: The case of Scots pine and downy oak forests under Mediterranean conditions

“…However, Kubota and Tsuboyama (2003) observed that stemflow samples were in general more enriched in δ 18 O than throughfall samples, although no specific reasons for such differences were discussed by the authors. Offsets can be different depending on the canopy characteristics, usually being greater in coniferous forests than in broadleaf forests, possible due to their higher storage capacity (Allen, Keim, Barnard, McDonnell, & Brooks, 2017). All these processes occur in the canopy and result in isotopic offsets of throughfall and stemflow from rainfall.…”

“…Ikawa, Yamamoto, Shimada, and Shimizu (2011), based on the different isotopic dynamics of stemflow rather than open rainfall and throughfall, suggested that the isotopic composition of stemflow could be affected more by mixing with rain water previously stored in the canopy and the stems. Until now, research efforts have tried to understand the factors that produce the modification of the isotopic composition of water that falls through the canopy, but no clear temporal or spatial patterns have yet been found (Allen et al, 2017). Offsets can be different depending on the canopy characteristics, usually being greater in coniferous forests than in broadleaf forests, possible due to their higher storage capacity (Allen, Keim, Barnard, McDonnell, & Brooks, 2017).…”

“…(a) Scots pine stand and (b) downy oak stand. Therefore, there is still an important challenge to understand how and why the isotopic composition of rain is modified during rainfall partitioning processes (Allen et al, 2017;Hsueh et al, 2016) and what implications this has for the identification of water sources and paths through a forested or partly forested catchment. The size is proportional to the diameter at breast height Kubota & Tsuboyama, 2003;Qu et al, 2014) and these have no strong concluding remarks.…”