H122-10
Water Vapor Adsorption May Provide as Much Water as Rainfall into the Hyperarid Soils of the Atacama Desert

Friday, 11 December 2020: 10:57
Virtual
Donald Glaser1, Hilairy Ellen Hartnett2, Damien Finn3, Hinsby Cadillo-Quiroz1, Saul Perez-Montano4 and Steven J Desch5, (1)Arizona State University, Tempe, AZ, United States, (2)Arizona State University, School of Earth and Space Exploration and School of Molecular Sciences, Tempe, AZ, United States, (3)Arizona State University, School of Life Science, Tempe, AZ, United States, (4)San Jose State University, San Jose, CA, United States, (5)Arizona State University, School of Earth and Space Exploration, Tempe, AZ, United States
Abstract:
The Atacama Desert on the west coast of Chile and Perú is considered among the driest environments on Earth and is thought to be at or near the limit for primary production. Rainfall events are minor and rare; accounting for ≤ 2 mm yr-1. Rain is generally assumed to be the largest water input to this system. Here, we present results that suggest water vapor adsorption (WVA) to soil particles may be an important source of water in this region and could provide additional moisture for soil microbes. WVA is a water-surface adhesion phenomenon governed by Van der Waals forces that can increase soil water content. To our knowledge WVA has not been quantified in hyperarid soils.

We used a 1-D diffusion-reaction model to calculate an instantaneous water vapor flux in and out of two Atacama soils with different nighttime and daytime humidity profiles. Field-collected profiles of soil temperature and relative humidity from 0-30 cm were used to inform the model. We also conducted a laboratory simulation experiment to measure changes in soil mass as a function of changes in temperature and humidity that mimic diurnal cycles at the Atacama sites.

The field data exhibit a diurnal pattern in absolute humidity with a distinctive nighttime absolute humidity minimum at 5 cm depth followed by a profile inversion to a midday absolute humidity maximum at 5 cm. The results of the diffusion model indicate an integrated nighttime flux of water vapor into the soil from the surface of +3.4 µmol cm-2, and an integrated midday flux of water of -3.1 µmol cm-2.

The laboratory simulations show significant soil mass changes in response to changes in temperature providing physical evidence for water vapor adsorption and desorption. The measured soil mass changes correspond to an aeral flux into the soil of +4.0 µmol cm-2 under nighttime conditions and -2.3 µmol cm-2 under midday conditions. The experimental and modelling approaches both predict a measurable water vapor flux as a function of temperature and the values are in agreement to within a factor of 0.3.

Our combination of modelling and laboratory approaches suggests WVA may provide a soil water input of ~0.5 mm yr-1 to 1.2 mm yr-1. These values approach that of the average rainfall of ~2 mm yr-1, suggesting that WVA may be a key soil water process in hyperarid systems.

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