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Clogging of streambeds due to clay deposition influences the stream‐subsurface exchange flux and thus directly modulates hyporheic ecological and biogeochemical processes. Clogging of sandy streambeds has previously been studied under losing and gaining flows and during streambed movement, but not when these two flow conditions coincided. We conducted flume experiments to quantify the combined effect of moving bedforms and losing or gaining flows on kaolinite deposition and streambed clogging. The experiments were conducted by adding pulses of kaolinite in a flume packed with sand under a stream water velocity of 25 cm/s. We measured the deposition rates, dynamics of hyporheic exchange flux (HEF) and vertical hydraulic conductivity (Kv), and the vertical distribution of kaolinite at the end of the experiments under two losing and two gaining flows (Darcy velocity of 10 and 20 cm/day). Kaolinite deposition led to clogging and reduction in Kv and HEF under all flow conditions. Deposition occurred faster under losing flow conditions than under gaining flow conditions. However, the changes in Kv and HEF were similar under losing and gaining flow conditions for similar kaolinite concentrations in the bed. Our results indicate that the deposition patterns of kaolinite were more influenced by bedform movement than by losing or gaining flow conditions, which is markedly different from the behavior observed under losing and gaining conditions for stationary bedforms. This implies that bedform morphodynamics control local‐scale clogging of sandy streambeds and should be accounted for when studying the hydrology of catchments at larger scales. Key Points: Bed movement had a greater influence on kaolinite deposition patterns than either losing or gaining flow conditionsKaolinite accumulated in the bed much more quickly under losing conditions than gaining conditions, causing rapid clogging of the bedHydraulic conductivity and hyporheic flux depended on kaolinite content in the bed and were similar under losing and gaining conditions [ABSTRACT FROM AUTHOR]

Streams and rivers play an important role in the global carbon cycle. The origins of CO2 in streams are often poorly constrained or neglected, which is especially true for CO2 originating from heterotrophic metabolism in streambeds. We hypothesized that sediment movement will have a direct effect on stream metabolism, and thus, the aim of this study was to quantify the effect of moving bedforms on the production of CO2 in sandy streambeds. We conducted flume experiments where we used planar optodes to measure the distributions of O2 and CO2 under various streambed celerities. We combined these measurements with an assessment of bed morphodynamics and modeling to calculate O2 consumption and CO2 production rates. Our results indicate that sediment transport can strongly influence streambed metabolism and CO2 production. We found that bedform celerity controls the shape of the hyporheic zone and exchange flux, and is directly linked to the spatial and temporal distributions of O2 and CO2. It was also found that the most pronounced change in CO2 production occurred when the bed changed from stationary conditions to a slowly moving bed. A more gradual increase in O2 consumption and CO2 production rates was observed with further increase in celerity. Our study also points out that bedform movement causes hydraulic isolation between the moving and the non‐moving fraction of the streambed that can lead to a transient storage of CO2 in deeper sediments, which may be released in bursts during bed scour. Plain Language Summary: Streams play an important role in the global carbon cycle. Carbon is transported in streams and rivers toward the oceans, stored in the streambed, or emitted as the greenhouse gas CO2 to the atmosphere. Many studies try to link CO2 in streamwater and CO2 emission to flow conditions and CO2 import via groundwater while ignoring the role of microbial processes in sediments that also produce CO2. We performed lab experiments using an artificial stream channel and special sensors in the streambed to investigate the influence of moving sandy sediments on the distribution and production of CO2. We found that sediment movement controls the distribution of CO2, and that those distributions are influenced by the flow paths of the water flowing through the sediment. We also found that the production of CO2 and the flux of stream water entering the sediment increased with the speed of the sediment movement. Moreover, our results point out that the movement of the sediment disconnects the lower, non‐moving part of the sediment from the upper, moving part of the sediment. This can build up a storage of high CO2 concentrations in the deeper sediment, which may be released as bursts during events of streambed erosion. Key Points: Bedform celerity controls the spatial and temporal distribution of CO2 in stationary and moving sandy bedformsProduction of CO2 in sediments increases abruptly as the bed starts to move and more gradually with increasing bedform celerityBedform movement causes isolation of the non‐moving fraction of the streambed and increase in transient storage of CO2 in deeper sediments [ABSTRACT FROM AUTHOR]

Communications and range estimation between formation members are essential functions of formation flying. The inherent features of laser free-space link: small weight and mass, power efficiency, broadband, high precision, availability of state-of-art components, make it a very attractive candidate for micro-satellite formation flying missions. A single two-way laser link, performing simultaneously communication and range estimation functions between formation spacecraft is being investigated in the framework of Broadband Laser Inter-Satellite Link (BLSIL), a joint Israeli-German applied research project. A unique technique of pointing control system for an inter-satellite laser communication and ranging link is presented. In this system, the same laser is used both as a beacon and as a transmitter. To maximize the traffic capacity of the link, the beamwidth is controlled from broad in the acquisition stage, to narrow in the tracking stage. Laser link pointing control presents difficult demands from the beam pointing precision. To establish and maintain the laser link between the satellites in orbit, pointing control must be fast and precise. A hierarchic two-level system control is proposed. The lower level controls the gimbals of the optical head of the electro-optical transceiver; the higher level is a fast closed loop that simultaneously controls the beamwidth and direction. It is the primary subject of the paper. Initial pointing acquisition assumes exchange of position-related information between the satellites to establish the initial line of sight (LOS). After initial acquisition a cooperative extremum-seeking algorithm is used for the fast higher level closed loop control. The paper gives a detailed description of the control algorithms. Representative simulations are presented.

Fine particles (0.1–100 microns) are ubiquitous within the water column. Observations on the interactions between suspended fine particles and sediment beds remain limited, reducing our ability to understand the interactions and feedbacks between fine particles, morphodynamics, and hyporheic flow. We performed laboratory experiments to explore changes in bedform morphodynamics and hyporheic flow following the progressive addition of kaolinite clay to the water column above a mobile sand bed. We characterized these interactions by taking high‐frequency time series measurements of bed topography and freestream clay concentration combined with solute injections and bed sediment cores to characterize subsurface properties. Deposition of initially suspended clay resulted in a decrease of bedform height, celerity, and sediment flux by 14%, 22%, and 29% when 1000 g was accumulated within the bed (equal to clay/sand mass ratio of 0.4% in the bed). The hyporheic exchange flux decreased by almost a factor of 2 for all clay additions, regardless of the amount of clay eventually deposited in the bed. Post experiment sediment cores showed clay accumulation within and below the mobile layer of the bedforms, with the peak concentration occurring at the most frequent bedform scour depth. These results demonstrate the tight coupling between bed sediment morphodynamics, fine particle (clay) deposition, and hyporheic exchange. Suspended and bed load transport rates are diminished by the transfer of suspended load to the sediment via hyporheic exchange. This coupling should be considered when estimating sediment transport rates. Key Points: Initially suspended fine clay particles within the water column rapidly accumulate within the sediment bed due to hyporheic exchangeFine clay particle storage occurs beneath the mobile layer of the sediment bed defined by the extent of bedform scourFormation of the fine particle layer results in reductions of bedform celerity, height, and sediment flux while length is unchanged [ABSTRACT FROM AUTHOR]

Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO2) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO2. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32‐fold to 66‐fold upon sediment rewetting. Structural equation modeling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use, and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO2 emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2–0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting‐drying cycles on respiration and CO2 emissions in stream networks. Key Points: Sediment respiration in intermittent rivers and ephemeral streams increases substantially in response to rewettingRespiration pulses are driven by sediment properties, which, in turn, are influenced by climate and catchment characteristicsEffects of wetting‐drying cycles on respiration and CO2 emissions in stream networks need consideration in upscaling and modeling efforts [ABSTRACT FROM AUTHOR]

Abstract: The lack of a complete understanding of the complex reciprocal interactions between hydrological processes and the structure and function of microbial communities limits our ability to improve the predictions of microbial metabolism in streams. We report here on how overlying water velocity and losing and gaining flux interact with bacterial community structure and its activity to control oxygen consumption in a sandy streambed under high nutrient levels. We used an experimental flume packed with natural sediment and measured the bacterial biomass distribution and oxygen profiles in the streambed and across bed forms. Local oxygen consumption rates were calculated with a 1‐D numerical model (GRADIENT). Bacterial abundance and production rates varied across the bed form within 1 order of magnitude, while their taxonomic classes were similar across bed forms despite variations in flow conditions and sediment disturbance events. However, bacterial production rates were not directly correlated with bacterial abundance. On the other hand, oxygen consumption rates ranged over 4 orders of magnitude across the bed forms and were highly correlated with the vertical exchange flux between the water and the streambed. The results strongly suggest that under high nutrient levels, the system is, in general, transport limited and that predicting oxygen consumption rates depends on local vertical exchange fluxes. [ABSTRACT FROM AUTHOR]

Bed form-induced hyporheic exchange flux ( qH) is increasingly viewed as a key process controlling water fluxes and biogeochemical processes in river networks. Despite the fact that streambeds are inherently heterogeneous, the majority of bed form flume-scale studies were done on homogeneous systems. We conducted salt and dye tracer experiments to study the effects of losing and gaining flow conditions on qH using a laboratory recirculating flume system packed with a heterogeneous streambed, and equipped with a drainage system that enabled us to apply losing or gaining fluxes. We found that when either losing or gaining fluxes increased (regardless of whether the flux was upward or downward), qH followed an exponential decline, the volume of the hyporheic flow cell drastically reduced, and the mean residence times declined moderately. A numerical flow model for the heterogeneous streambed was set up and fitted against the experimental data in order to test whether an equivalent homogeneous case exists. The measured qH were accurately predicted with the heterogeneous model, while it was underestimated using a homogeneous model characterized by the geometric mean of the hydraulic conductivity. It was also shown that in order to produce the results of the heterogeneous model with an equivalent hydraulic conductivity, the latter had to be increased as the losing or gaining fluxes increase. The results strongly suggest that it is critical to adequately account for the heterogeneous streambed structure in order to accurately predict the effect of vertical exchange fluxes between the stream and groundwater on hyporheic exchange. [ABSTRACT FROM AUTHOR]

Free-space optical interconnection (FSOI) technology enables high interconnection speed in next-generation servers. FSOIs use laser links between server components and provide a lower bound on propagation delay due to the shortest line-of-sight path and the low index of refraction of air, when compared with the indices common in waveguide technologies. One of the main problems of FSOI systems is the inevitable turbulence effect that results from the air cooling of server components. Cooling fans produce a flow of hot air, due to excessive thermal emission from electronic components inside the chassis; this produces turbulence-induced fading and may influence the communication performance. The objective of this paper is to experimentally examine the influence of the turbulence effect on FSOI channels inside a server chassis. This includes studying scintillation statistics and characterizing temporal and spatial in-chassis turbulence. The results of the analysis reveal that the in-chassis optical channel preserves the well-known log-normal scintillation distribution. However, some temporal and spatial characteristics do not fit the commonly accepted turbulence theory predictions. [ABSTRACT FROM PUBLISHER]

The article explores the fate of nitrate in the clay vadose zone underlying dairy earthen waste lagoons and their margins, by carrying out an array of vadose zone monitoring systems (VMSs). Specifically, it examines the effect of infiltration mechanism and sediment water content on ammonium oxidation and denitrification in the subsurface. Results show that the vadose zone below the waste sources remained unsaturated throughout the monitoring period. All infiltrating ammonium was oxidized in the upper 0.5 m. The ammonium oxidation was coupled with nitrate reduction and relied on the sediment water content. Coupled nitrification-denitrification (CND) was reportedly the key factor regulating nitrogen's fate in the vadose zone underlying earthen waste lagoons.

Data centers have become the major technology for collecting and processing information. The data-rates required for transferring the information through the server's backplane has been increasing from year to year at an almost exponential pace. Therefore a technology-shift from the electronic to the optic domain has already been initiated in order to meet the required data-rate demands for next-generation servers. The future technology that is analyzed in the paper is free space optics (FSO). FSO technology uses laser links between the server cards and the backplane and provides a lower bound on propagation delay due to the low index of refraction of air, when compared with waveguide or fiber technologies. However, turbulence occurs due to the high temperature of the chips and to ventilation inside the server, which results in deterioration of the communication performance. In this paper, we examine the feasibility of wavelength diversity as a means to mitigate the effect of turbulence on the server communication link performance. It is shown that using the diversity scheme cuts the outage probability by half for the same total transmitter power by comparison with the single wavelength scheme. Therefore, the power consumption (and operational costs) of the data center could be reduced dramatically if a wavelength diversity scheme were adopted. The authors believe that this is the first time that a laboratory experiment has been performed modeling a server backplane with FSO communication using wavelength diversity. [ABSTRACT FROM PUBLISHER]

Over recent decades, there have been increasing demands for high data-rate backplanes for enhanced performance in computers. The present technology of electronic backplanes cannot meet the required data-rate demands for next-generation computers, indicating a need for a technology shift from the electronic to the optic domain. In this paper, we develop a mathematical model to describe the effect of air turbulence on a short range free space optical communication link, such as would be found in computer backplane communication, and conduct experiments to validate the model. The air turbulence under discussion resembles that which would result from the high temperature of chips on the board together with the ventilation of the air by the chip fan. In our experiment, the communication performance, expressed as bit error rate (BER), is presented as a function of the location of the turbulence source and the log amplitude variance. The log amplitude variance was evaluated by two independent methods. One of the main results of this study is the indication that the performance of the backplane deteriorates even for very small values of air turbulence strength. This is exacerbated by the fact that extremely low BER performance is required at the backplane (e.g., from 10-14 to 10- 18 ). We also demonstrate that increasing the distance between the turbulence source and the optical link reduces the influence of the air turbulence in an exponential manner. This fact is important for the design of future-generation optical backplanes. It is noted that operation at wavelengths around 1550 nm yields communication performance that is less severely degraded by air turbulence effects than at 670 nm. [ABSTRACT FROM PUBLISHER]

Research examined the relative importance of peer groups for young adolescents as compared with diverse adult socialization agents--family, school, and community. The factors involved were teenagers' activities, preferences, feelings, and thoughts as to how they spend their leisure time, their preferences for help providers, and their sense of attachment to their community. These comparisons were made with religious and non-religious youngsters, in both rural and urban communities, and in gender subgroups. Questionnaires were administered to teenagers at secondary schools in a northern peripheral region of Israel. Findings showed the primary importance of peer groups and family in leisure activities and support, and the secondary importance of school and community. No evidence was found of a sharp generation gap. Community could also be significant if its organizations accepted youth as a peer group, and not only individually, on an equal and cooperating basis. [ABSTRACT FROM AUTHOR]