Resolving seasonal and diel dynamics of non-rainfall water inputs in a Mediterranean ecosystem using lysimeters

Abstract

Abstract. The input of liquid water to terrestrial ecosystems is composed of rain and non-rainfall water (NRW). The latter comprises dew, fog, and the adsorption of atmospheric vapor on soil particle surfaces. Although NRW inputs can be relevant to support ecosystem functioning in seasonally dry ecosystems, they are understudied, being relatively small, and therefore hard to measure. In this study, we apply a partitioning routine focusing on NRW inputs over 1 year of data from large, high-precision weighing lysimeters at a semi-arid Mediterranean site. NRW inputs occur for at least 3 h on 297 d (81 % of the year), with a mean diel duration of 6 h. They reflect a pronounced seasonality as modulated by environmental conditions (i.e., temperature and net radiation). During the wet season, both dew and fog dominate NRW, while during the dry season it is mostly the soil adsorption of atmospheric water vapor. Although NRW contributes only 7.4 % to the annual water input, NRW is the only water input to the ecosystem during 15 weeks, mainly in the dry season. Benefitting from the comprehensive set of measurements at our experimental site, we show that our findings are in line with (i) independent measurements and (ii) independent model simulations forced with (near-) surface energy and moisture measurements. Furthermore, we discuss the simultaneous occurrence of soil vapor adsorption and negative eddy-covariance-derived latent heat fluxes. This study shows that NRW inputs can be reliably detected through high-resolution weighing lysimeters and a few additional measurements. Their main occurrence during nighttime underlines the necessity to consider ecosystem water fluxes at a high temporal resolution and with 24 h coverage.

Interpretability of negative latent heat fluxes from eddy covariance measurements in dry conditions

“…Each station has a lower-boundary control system, consisting of a heat exchange system and porous ceramic bars at the bottom of each column to adjust soil temperature and water content to the conditions of the surrounding soil at the same depth (Groh et al, 2016;Podlasly and Schwärzel, 2013). More details on the technical specifications are given by Paulus et al (2022) and on the excavation method by Reth et al (2021). Within each column, SWC and soil temperature (T soil , °C) (UMP-1, Umwelt-Geräte-Technik GmbH) are measured at 0.1 m soil depth at a resolution of 0.1 % SWC and 0.02 °C, according to the manufacturer.…”

“…This aspect is particularly crucial for the precise detection of minor flux events such as dew or SVA. The details of the AWAT filter are given in Peters et al (2014Peters et al ( , 2016Peters et al ( , 2017 and its application to lysimeter raw data in Paulus et al (2022)…”

“…In another case study in a semi-arid region, SVA accounted for up to 40 % of diel evaporation during the crop growth period (Zhang et al, 2019). Furthermore, in a Mediterranean tree-grass ecosystem, SVA served as the sole water input for several consecutive weeks in the dry season (Paulus et al, 2022).…”

“…Measurements of SVA in natural and managed ecosystems with the perspective to quantify its role as a water input have traditionally been performed using cloth plates (Kidron, 1998), weighing lysimeters (Kidron and Starinsky, 2019;Verhoef et al, 2006;Uclés et al, 2013;Feigenwinter et al, 2020;Paulus et al, 2022), and sampling campaigns (McHugh et al, 2015). Although uncertainties can emerge due to temperature differences between the (micro)lysimeter and the surrounding soil (Kidron and Kronenfeld, 2020) when temperature control is lacking, the latest generation of large high-precision weighing lysimeters now features sen-sor arrays.…”

“…Previous studies have reported simultaneous measurements of downward (negative) latent heat fluxes (λE, W m −2 ) using the eddy covariance (EC) method alongside independent SVA measurements (Qubaja et al, 2020;Paulus et al, 2022). Florentin and Agam (2017) compared SVA from an EC measurement system with microlysimeter measurements over a 7 d period in the Negev and found that while the EC method accurately captured the dynamics of SVA, it did not fully capture its magnitude.…”