Throughfall isotopic composition in relation to drop size at the intra-event scale in a Mediterranean Scots pine stand

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.

High‐resolution temporal dynamics of intra‐storm isotopic composition of stemflow and throughfall in a Mediterranean Scots pine forest

“…Mean rainfall intensities ranged from 0.6 to 17.7 mm/h, with 76% of the events having mean intensities below 5 mm/h. Event duration varied between 1.2 and 43.3 h, with 24% of the events classified as short‐duration events (≤7 h; following Pinos et al, 2020).…”

“…Evaporation causes isotopic enrichment whereas exchange, canopy selection, mixing of waters and sub‐canopy recycling cause either isotopic enrichment or depletion. Isotopic modification of throughfall can occur in both directions (enrichment and depletion) (e.g., Pinos et al, 2020; Saxena, 1986), whereas for stemflow, enrichment usually prevails, unlike both rainfall and throughfall isotopic compositions (e.g., Allen et al, 2014; Cayuela et al, 2018a). Therefore, in isotope‐based hydrology studies, it is preferable to use the isotopic composition of throughfall and stemflow instead of that of rainfall as the water input signal into a catchment with ample forest cover (Klaus & McDonnell, 2013; Stockinger et al, 2017).…”

Drivers of the circumferential variation of stemflow inputs on the boles of Pinus sylvestris L. (Scots pine)

“…For rainfall classification, a maximum 30‐min rainfall intensity threshold of 12 mm·h −1 was used to separate low‐ and high‐intensity events, and a rainfall duration threshold of 8.5 h was used to distinguish between short‐to‐medium and long rainfall events. These criteria are based on the Pinos et al (2020) rainfall event classification. However, here, we slightly widen the thresholds values (from 10 to 12 mm·h −1 and from 7 to 8.5 h) to cover the full range of events, keeping a similar number of events in the different rainfall classes.…”

Canopy Effects on Rainfall Partition and Throughfall Drop Size Distribution in a Tropical Dry Forest

“…We assumed that there was no canopy or trunk water storage at the onset of the rainfall events. Such an assumption might interfere with the stemflow quantification but is expected to have little influence on throughfall [40], as this is the major component of rainfall partitioning and usually much higher than the canopy storage capacity, estimated as 0.5-1.0 mm [5,26].…”