Analysis of water vapor adsorption in soils by means of a lysimeter and numerical modeling

Abstract

Daily temperature oscillations can cause adsorption (and desorption) of atmospheric water vapor by soils. The resulting daily fluctuations in the amount of liquid water in the soil can be measured by high-precision weighing lysimeters. We analyzed the data of a lysimeter in a sandy dune sediment in southern Spain using Codebright, a thermohydraulic numerical model for unsaturated flow that takes into account water, vapor, and heat transport in the soil, as well as soil-atmosphere interactions such as precipitation, evaporation, and solar radiation. The analysis shows that daily temperature oscillations, psychrometrics, and soil water retention can explain the fluctuations of the amount of liquid water in the soil. The retention curve, especially its driest part, is essential for the existence of these fluctuations. The fluctuations could not be reproduced by a model using the van Genuchten retention curve with a constant residual saturation. On the other hand, satisfactory results could be obtained by models using retention curves that at their driest part still show a change of saturation with suction. Moreover, the models suggest within the top few decimeters of the soil a pattern of alternating bands of condensation and evaporation, which follows the daily temperature oscillations that fade out deeper in the soil.

Lysimeter based evaporation and condensation dynamics in a Mediterranean ecosystem

“…At our study site Majadas, the largest annual NRWI contribution is adsorption with 21.2 mm year −1 . This value is relatively low, compared to sites close to the sea, where values of 81 mm year −1 (Saaltink et al, 2020), and 26 mm to 110 mm between April and October have been reported (Kosmas et al, 2001). But it matches well with observations from Qubaja et al (2020) that measured annual adsorption of 14 mm year −1 using flux chambers in a semiarid pine forest in Israel.…”

“…In contrast, for adsorption, reported observation times differ. Kosmas et al (1998) observed the flux to occur mainly between 0:00 and 6:00 h and also Saaltink et al (2020) found suitable night-time conditions for adsorption through a reversed gradient of vapor concentration between soil and atmosphere from lysimeter observations and confirmed it with a fully coupled numerical model. Yet, Verhoef et al (2006) found adsorption occurring during the afternoon and ceasing at night.…”

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

“…At our study site of Majadas, the largest annual NRW contribution is adsorption (21.2 mm yr −1 ). This value is relatively low, compared to sites close to the sea, where values of 81 mm yr −1 (Saaltink et al, 2020) and 26 and 110 mm between April and October have been reported (Kosmas et al, 2001). But it matches well with observations from Qubaja et al (2020), who measured the annual adsorption of 14 mm yr −1 using flux chambers in a semi-arid pine forest in Israel.…”

“…The respective NRW is the adsorption of atmospheric vapor. However, despite being well understood at pore and laboratory scales (Tuller et al, 1999;Arthur et al, 2016), this process was underrepresented in NRW studies (Zhang et al, 2019b;Saaltink et al, 2020;Kohfahl et al, 2019).…”

Comparing precision lysimeter rainfall measurements against rain gauges in a coastal dune belt, Spain

“…This means that vapour adsorption, measured by the lysimeter but not by TP1, can be neglected during rainfall events. Saaltink et al (2020) estimated vapour adsorption of 0.22 mm/day for the same site, which indeed is very low with respect to rainfall.…”