Nonrainfall water origins and formation mechanisms

Dryland ecosystems cover 40% of the total land surface on Earth and are defined broadly as zones where precipitation is considerably less than the potential evapotranspiration. Nonrainfall waters (for example, fog and dew) are the least-studied and least-characterized components of the hydrological cycle, although they supply critical amounts of water for dryland ecosystems. The sources of nonrainfall waters are largely unknown for most systems. In addition, most field and modeling studies tend to consider all nonrainfall inputs as a single category because of technical constraints, which hinders prediction of dryland responses to future warming conditions. This study uses multiple stable isotopes (2H, 18O, and 17O) to show that fog and dew have multiple origins and that groundwater in drylands can be recycled via evapotranspiration and redistributed to the upper soil profile as nonrainfall water. Surprisingly, the non–ocean-derived (locally generated) fog accounts for more than half of the total fog events, suggesting a potential shift from advection-dominated fog to radiation-dominated fog in the fog zone of the Namib Desert. This shift will have implications on the flora and fauna distribution in this fog-dependent system. We also demonstrate that fog and dew can be differentiated on the basis of the dominant fractionation (equilibrium and kinetic) processes during their formation using the 17O-18O relationship. Our results are of great significance in an era of global climate change where the importance of nonrainfall water increases because rainfall is predicted to decline in many dryland ecosystems. Fog and dew in the Namib Desert have multiple origins and their formation can be differentiated using stable isotopes. Fog and dew in the Namib Desert have multiple origins and their formation can be differentiated using stable isotopes

The Impact of Rainfall on Soil Moisture Dynamics in a Foggy Desert.

Soil moisture is a key variable in dryland ecosystems since it determines the occurrence and duration of vegetation water stress and affects the development of weather patterns including rainfall. However, the lack of ground observations of soil moisture and rainfall dynamics in many drylands has long been a major obstacle in understanding ecohydrological processes in these ecosystems. It is also uncertain to what extent rainfall controls soil moisture dynamics in fog dominated dryland systems. To this end, in this study, twelve to nineteen months’ continuous daily records of rainfall and soil moisture (from January 2014 to August 2015) obtained from three sites (one sand dune site and two gravel plain sites) in the Namib Desert are reported. A process-based model simulating the stochastic soil moisture dynamics in water-limited systems was used to study the relationships between soil moisture and rainfall dynamics. Model sensitivity in response to different soil and vegetation parameters under diverse soil textures was also investigated. Our field observations showed that surface soil moisture dynamics generally follow rainfall patterns at the two gravel plain sites, whereas soil moisture dynamics in the sand dune site did not show a significant relationship with rainfall pattern. The modeling results suggested that most of the soil moisture dynamics can be simulated except the daily fluctuations, which may require a modification of the model structure to include non-rainfall components. Sensitivity analyses suggested that soil hygroscopic point (sh) and field capacity (sfc) were two main parameters controlling soil moisture output, though permanent wilting point (sw) was also very sensitive under the parameter setting of sand dune (Gobabeb) and gravel plain (Kleinberg). Overall, the modeling results were not sensitive to the parameters in non-bounded group (e.g., soil hydraulic conductivity (Ks) and soil porosity (n)). Field observations, stochastic modeling results as well as sensitivity analyses provide soil moisture baseline information for future monitoring and the prediction of soil moisture patterns in the Namib Desert

Temporal dynamics of hot desert microbial communities reveal structural and functional responses to water input

8 páginas, 4 figuras. -- The first publication is available at https://www.nature.comThe temporal dynamics of desert soil microbial communities are poorly understood. Given the implications for ecosystem functioning under a global change scenario, a better understanding of desert microbial community stability is crucial. Here, we sampled soils in the central Namib Desert on sixteen different occasions over a one-year period. Using Illumina-based amplicon sequencing of the 16S rRNA gene, we found that α-diversity (richness) was more variable at a given sampling date (spatial variability) than over the course of one year (temporal variability). Community composition remained essentially unchanged across the first 10 months, indicating that spatial sampling might be more important than temporal sampling when assessing β-diversity patterns in desert soils. However, a major shift in microbial community composition was found following a single precipitation event. This shift in composition was associated with a rapid increase in CO2 respiration and productivity, supporting the view that desert soil microbial communities respond rapidly to re-wetting and that this response may be the result of both taxon-specific selection and changes in the availability or accessibility of organic substrates. Recovery to quasi pre-disturbance community composition was achieved within one month after rainfall.We gratefully acknowledge financial support from the National Research Foundation of South Africa (grant no.81779 and TTK2008052000003), the Research Council of Norway (grant No. 180352) and the University of the Western Cape. Partial support was also provided under the Laboratory Directed Research and Development Program at PNNL, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy under contract DE-AC05-76RL01830.Peer reviewe

Hypolithic and soil microbial community assembly along an aridity gradient in the Namib Desert

The Namib Dessert is considered the oldest desert in the world and hyperarid for the last 5 million years. However, the environmental buffering provided by quartz and other translucent rocks supports extensive hypolithic microbial communities. In this study, open soil and hypolithic microbial communities have been investigated along an East–West transect characterized by an inverse fog-rainfall gradient. Multivariate analysis showed that structurally different microbial communities occur in soil and in hypolithic zones. Using variation partitioning, we found that hypolithic communities exhibited a fog-related distribution as indicated by the significant East– West clustering. Sodium content was also an important environmental factor affecting the composition of both soil and hypolithic microbial communities. Finally, although null models for patterns in microbial communities were not supported by experimental data, the amount of unexplained variation (68–97 %) suggests that stochastic processes also play a role in the assembly of such communities in the Namib Desert.Web of Scienc

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

A Decade of Solar and Terrestrial Radiation Monitoring at Gobabeb for BSRN

The Gobabeb Baseline Surface Radiation Network (BSRN) location was established a decade ago as part of a global network to provide standardised, high quality surface observations of radiation fluxes under the auspices of the World Meteorological Organisation. The Gobabeb BSRN measures the incoming and outgoing shortwave and longwave radiation at two locations in the Central Namib Desert. With a suite of instruments, radiative fluxes of shortwave downward radiation (SWD), direct solar radiation (DIR), diffuse radiation (DIF), longwave downward radiation (LWD), shortwave upward radiation (SWU) and longwave upward radiation (LWU) are recorded at 1-min intervals, together with relevant meteorological variables directly at the locations. As logistical issues prevent frequent calibration of instruments against World Radiometric standards, the measurements of DIR, SWD and LWD are duplicated to replicate radiation measurements as primary and redundant datasets. The two datasets are compared to each other to identify and exclude questionable data before being deposited in an open-access repository. The Gobabeb BSRN dataset is of very high quality, with less than 1% data missing for downward radiation. The upward radiation dataset has more missing data due to its becoming operational at a later date. As expected, climatic variables have the greatest influences on radiation fluxes at Gobabeb. Due to its location in a hyperarid desert, radiation is generally high throughout the year. Reflected radiation from rare cloud walls during the austral summer may, however, result in very high downward fluxes. The frequent incidence of fog due to onshore advection of stratus cloud banks over the nearby South Atlantic Ocean from August to February is distinctly visible in the dataset. Variation in the radiative fluxes of the different elements provides more detailed information on seasonal and daily incidence of fog, as well as seasonal changes in atmospheric aerosols. We briefly illustrate how high-quality BSRN data are used globally for validating solar energy resource assessments and evaluating differences between modelled predictions and actual surface performance

The correlations between rainfall and soil moisture for different layers of gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG) and gravel plain at Kleinberg (GPK).

<p>The correlations between rainfall and soil moisture for different layers of gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG) and gravel plain at Kleinberg (GPK).</p

Rainfall regimes and volumetric soil moisture patterns for different depth of soil types in gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG) and gravel plain at Kleinberg (GPK).

<p>Rainfall regimes and volumetric soil moisture patterns for different depth of soil types in gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG) and gravel plain at Kleinberg (GPK).</p

Model sensitivity of the key parameters for gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG) and gravel plain at Kleinberg (GPK).

<p>Model sensitivity of the key parameters for gravel plain at Gobabeb (GPG), sand dune at Gobabeb (SDG) and gravel plain at Kleinberg (GPK).</p

Mean soil moisture, standard deviation, coefficient of variation (CV), rainfall depth (mm), rainfall frequency λ (unitless) and average rainfall depth α (mm) for different soil depths of Gravel plain (Gobabeb) (January 2, 2014 to July 28, 2015), Sand dune (Gobabeb) (July 28, 2014 to July 28, 2015) and Gravel plain (Kleinberg) (January 1, 2014 to August 3, 2015).

<p>Mean soil moisture, standard deviation, coefficient of variation (CV), rainfall depth (mm), rainfall frequency λ (unitless) and average rainfall depth α (mm) for different soil depths of Gravel plain (Gobabeb) (January 2, 2014 to July 28, 2015), Sand dune (Gobabeb) (July 28, 2014 to July 28, 2015) and Gravel plain (Kleinberg) (January 1, 2014 to August 3, 2015).</p