Using cotton pads to sample the stable water isotopes of throughfall inside tree canopies

Stable water isotopes (δ¹⁸O, δ²H) are used as tracers in hydrology to study the components of the terrestrial water cycle. The stable water isotopes of precipitation are affected by the passage of rainfall through tree canopy, resulting in a change of the tracer signal. Several processes within the canopy are thought to be responsible for this, including evaporation, liquid-vapor equilibration, redistribution, and legacy effects. However, it is currently not clear which processes dominate under which conditions, and predictions of these changes are not yet possible. This is partly due to a lack of high resolution throughfall data, as previous studies usually sampled throughfall in evaporation-reducing bulk containers placed under canopy. Here we propose to hang commonly available cotton products in tree canopy, let them soak up rainfall water, and subsequently measure the stable water isotopes directly from the wet cotton products using the direct liquid-vapor equilibration method in the laboratory. First, four products (two types of tampons, two types of cotton pads) were evaluated in terms of the minimum amount of water drops necessary for a reliable measurement, their price, and ease of handling. Cotton pads had the overall best rating and were therefor hung in a coniferous tree placed in a rainfall simulator. With a fixed rainfall intensity, we tested how long the cotton pads can be left hanging before significant isotopic changes due to evaporation occurred. While cotton pads that were on the outer edge of the canopy showed significant deviations after only half an hour, cotton pads inside the canopy as well as close to the stem could be left hanging for one hour. As a comparison, throughfall was also collected using a bulk sampler under the canopy, and this sample showed no significant changes even after four hours. It can thus be assumed that due to the comparatively low amount of water in the cotton pads (even if soaking wet), evaporative changes of isotope values had a stronger impact on the remaining water compared to the bulk throughfall sampler. This study presents first laboratory results and further tests, in the laboratory or in the field, are called for.