Abstract
Abstract. The major fraction of water reaching the forest floor is throughfall, which consists of free throughfall, splash throughfall and canopy drip. Research has shown that forest canopies modify the isotopic composition of throughfall by means of evaporation, isotopic exchange, canopy selection and mixing of rainfall waters. However, the effects of these factors in relation to throughfall isotopic composition and the throughfall drop size reaching the soil surface are unclear. Based on research in a mountainous Scots pine stand in northeastern Spain, this study sought to fill this knowledge gap by examining the isotopic composition of throughfall in relation to throughfall drop size. In the experimental stand, throughfall consisted on average of 65 % canopy drip, 19 % free throughfall and 16 % splash throughfall. The dynamics of the isotopic composition of throughfall and rainfall showed complex behaviour throughout events. The isotopic shift showed no direct relationship with meteorological variables, number of drops, drop velocities, throughfall and rainfall amount, or raindrop kinetic energy. However, the experiment did reveal that the isotopic shift was higher at the beginning of an event, decreasing as cumulative rainfall increased, and that it also increased when the median volume drop size of throughfall (D50_TF) approached or was lower than the median volume drop size of rainfall (D50_RF). This finding indicates that the major contribution of splash throughfall at the initial phase of rain events matched the highest vapour pressure deficit (VPD) and, at the same time, corresponded to higher isotopic enrichment, which implies that splash droplet evaporation occurred. Future applications of our approach will improve understanding of how throughfall isotopic composition may vary with drop type and size during rainfall events across a range of forest types.
Highlights
Forests play an important role in the water balance of catchments by redistributing rainfall in throughfall, stemflow and interception loss
For 33 of the 98 pairs of samples, throughfall was higher than rainfall; one-third of these samples corresponded to the end of the rainfall event, after rainfall stopped, whereas the remaining two-thirds were distributed without any specific pattern at different time intervals during the rainfall events
Results indicated that throughfall showed a lower number of drops, slower drop velocity and larger drop diameter than open rainfall did
Summary
Forests play an important role in the water balance of catchments by redistributing rainfall in throughfall, stemflow and interception loss. It has been shown that the forest canopy modifies the isotopic composition of throughfall and stemflow in relation to open rainfall (Allen et al, 2017; Cayuela et al, 2018a). Isotopic fractionation can occur in both directions (enrichment and depletion), with enrichment being more frequent (Saxena, 1986). Throughfall isotopic shifts are mainly caused by four factors: evaporation, isotopic exchange, canopy selection and mixing of waters (Allen et al, 2017), subcanopy water recycling i.e. evapotranspiration and recondensation (Green et al, 2015), may exert an influence. Isotopic fractionation by evaporation occurs when rain water molecules achieve enough energy to change from liquid to the gas phase, resulting in an enrichment of heavy isotopes.
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