Technical note: High-accuracy weighing micro-lysimeter system for long-term measurements of non-rainfall water inputs to grasslands

Abstract

Abstract. Non-rainfall water (NRW), defined here as dew, hoar frost, fog, rime, and water vapour adsorption, might be a relevant water source for ecosystems, especially during summer drought periods. These water inputs are often not considered in ecohydrological studies, because water amounts of NRW events are rather small and therefore difficult to measure. Here we present a novel micro-lysimeter (ML) system and its application which allows us to quantify very small water inputs from NRW during rain-free periods with an unprecedented high accuracy of ±0.25 g, which corresponds to ±0.005 mm water input. This is possible with an improved ML design paired with individual ML calibrations in combination with high-frequency measurements at 3.3 Hz and an efficient low-pass filtering to reduce noise level. With a set of ancillary sensors, the ML system furthermore allows differentiation between different types of NRW inputs, i.e. dew, hoar frost, fog, rime, and the combinations among these, but also additional events when condensation on leaves is less probable, such as water vapour adsorption events. In addition, our ML system design allows one to minimize deviations from natural conditions in terms of canopy and soil temperatures, plant growth, and soil moisture. This is found to be a crucial aspect for obtaining realistic NRW measurements in short-statured grasslands. Soil temperatures were higher in the ML compared to the control, and thus further studies should focus on improving the thermal soil regime of ML. Our ML system has proven to be useful for high-accuracy, long-term measurements of NRW on short-statured vegetation-like grasslands. Measurements with the ML system at a field site in Switzerland showed that NRW input occurred frequently, with 127 events over 12 months with a total NRW input of 15.9 mm. Drainage-water flow of the ML was not measured, and therefore the NRW inputs might be conservative estimates. High average monthly NRW inputs were measured during summer months, suggesting a high ecohydrological relevance of NRW inputs for temperate grasslands.

A Method Proposal for Throughfall Measurement in Grassland at Plot Scale in Temperate Climate: ‘Interception Tubes’

“…However, large lysimeters might not allow easily to observe small scale variation induced by grassland canopy. Further micro lysimeters in temperate climate needs to be controlled for how natural canopy and root growth might be altered due to the restricted root structure (Riedl et al, 2022) since, as we observed, the amount of net precipitation components, thereby interception, changes through seasonal plant development.…”

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

“…Since clear-sky conditions and the amount of humidity present in the air is a key factor for both dew and radiation fog formation, their distinction is challenging. Measurements from visibility sensors are therefore used in addition to RH t to classify fog unambiguously (e.g., Feigenwinter et al, 2020;Riedl et al, 2022). Given that at the experimental site visibility sensors are not available, we use LW radiation to benchmark fog occurrence in our analysis.…”

“…But current research that is quantifying such ratios is too scarce for generalization (Y. . Some studies, therefore, do not distinguish between fog and dew (Groh et al, 2018) or add a category of combined dew and fog (Riedl et al, 2022). Disentangling individual NRW fluxes is nevertheless important because of different respective (i) controlling factors and (ii) implications for the ecosystem.…”

“…But there is also great overlap of the LW ratio between the periods classified as dew and fog in moments where the surface cools and LW↑ is larger than LW↓. However, Feigenwinter et al (2020) used visibility sensors for fog detection in addition to micro-lysimeters and reported fog deposition continuing throughout several nights below the commonly used threshold of visibility < 1000 m. The problem of distinguishing between dew and fog in particular has received great attention in the literature (Xiao et al, 2009;Price and Clark, 2014;Groh et al, 2018;Riedl et al, 2022), but practical difficulties concerning the distinction between dew and adsorption (Kidron and Kronenfeld, 2020a;Price and Clark, 2014;Feigenwinter et al, 2020) and fog and adsorption (Kidron and Kronenfeld, 2020a) were also mentioned. In our study, however, the ranking of the individual NRW contributions was not affected by the parameter thresholds, e.g., in all tested cases, adsorption had the largest contribution to annual NRW input.…”

“…This has long been known for arid ecosystems (Duvdevani, 1964;Agam and Berliner, 2006;Kidron and Starinsky, 2019), but an increasing number of studies suggest ecological relevance also in the context of seasonal dry spells (Jacobs et al, 2006;Y. Li et al, 2021;Groh et al, 2018;Dawson and Goldsmith, 2018;Riedl et al, 2022). As leaf wetting events, dew and fog can affect leaf water status directly and alter the surface water and energy balance by changing leaf temperature, albedo, and water vapor deficit (Dawson and Goldsmith, 2018;Gerlein-Safdi et al, 2018;Aparecido et al, 2017).…”