Stable Isotopes Indicate Within-Canopy Processes During Interception of Rainfall

Abstract:

Stable isotopes of water have been used to gain process-level understand of mixing, storage, and transport in all components of the hydrological cycle. Canopy interception processes remain some of the least understood because of the relatively small storage pool, rapid turnover, and variability at short intervals relative to, for example, soils. Stable isotopes provide a look into the ‘black box’ of canopy processes that control interception storage and throughfall generation. Several recent studies have compared throughfall isotopic composition to open rainfall; canopy effects vary in direction and magnitude but are ubiquitous. We present findings from three studies using isotopes of throughfall. In all cases, common patterns and persistence of patterns in variability of throughfall amount (e.g., correlation with canopy characteristics, correlations with precipitation characteristics, and geostatistical relationships) were infrequently apparent for isotopic composition. Data consistently support the so called ‘selection’ effect, that throughfall composition is a product of spatially and temporally varying transmission of rainfall that has temporally varying isotopic composition. There is little evidence of isotopic fractionation by wet-canopy evaporation. Additionally, isotopic composition of storm-total throughfall is generally less variable than is amount or solute content, suggesting two possibilities: (1) high spatial homogeneity in the selection effect, or (2) rapid exchange and equilibration of droplets with vapor in the canopy airspace, lending support to the hypothesized role of splash droplet evaporation. These results suggest a need to re-examine conceptual models of the progression from interception to evaporation and throughfall generation.