The role of stable isotopes in understanding rainfall interception processes: a review
Citation:
Allen, S., R. Keim, H. Barnard, J. McDonnell, AND J. Renee Brooks. The role of stable isotopes in understanding rainfall interception processes: a review. WIREs Water. John Wiley & Sons, Inc., Hoboken, NJ, 4(1):e1187, (2017).
Impact/Purpose:
Evaporation from forest canopies (interception loss) is a prominent factor in the catchment water balance, accounting for, on average 13-22% of precipitation1 (and sometimes much more at the stand scale2). However, interception processes other than evaporation are also important: vegetation redistributes precipitation from the atmosphere to the soil in space and time3 by a complex suite of within-canopy pathways. The details of water redistribution by the canopy have important implications for heterogeneities in water quality and quantity at the soil surface, and in the subsurface, and ultimately streams. Stable isotope composition of water (δw) is promising as a tracer to reveal new mechanistic understanding of interception processes. In this review, we aggregate previous stable isotope investigations in canopy interception to synthesize the intellectual progress made, the ambiguities that remain, and to identify ways to use stable isotopes to improve physical understanding of canopy interception. This paper is a deliverable under SSWR 3.01G.
Description:
The isotopic composition of water transmitted by the canopy as throughfall or stemflow reflects important hydrologic processes occurring in the canopy. A synthesis of the literature shows that complex spatiotemporal variations of isotopic composition are created by canopy interception. As a whole, the studies suggest a set of controlling factors including fractionation, exchange among liquid and vapor phase water, and spatiotemporal redistribution along varying canopy flowpaths. However, our limited understanding of physical processes and water routing in the canopy limits the ability to discern all details for predicting interception isotope effects. We suggest that the isotopic composition of throughfall and stemflow may be the key to improve our understanding of water storage and transport in the canopy, similar to how isotopic analysis contributed to progress in our understanding of watershed runoff processes. While interception isotope effects have largely been studied under the premise that they are a source of error, previous works also indicate a wide range of possible interactions that intercepted water may have with the canopy and airspace. We identify new research questions that may be answered by stable isotopes as a path forward in examining and generalizing small-scale interception processes that could facilitate integration of interception into watershed ecohydrological concepts.
