Short-term Temporal Variations Research Articles

Potential of turbidity monitoring for measuring the transport of suspended solids in streams

AbstractInvestigating the transport of suspended solids by water sampling usually leads to an underestimation of loads and an unrealistically high sampling frequency is required to properly characterize temporal trends. An alternative method is to use in situ optical turbidimeters to estimate the suspended solids concentration; however, the relationship between turbidity and suspended solids concentration is potentially confounded by variations in particle size, particle composition and water colour. Field measurements, and laboratory measurements using the type of natural material suspended in streamwater, were made to quantify the influences of these factors on nephelometric turbidity (Hach 2100A) and attenuance turbidity (Partech 7000 3RP MKII). The attenuance turbidity was approximately 2.5 times higher than nephelometric turbidity. The turbidity instruments were most sensitive to dispersions with a median diameter of 1.2‐1.4γm. Particle size variation can cause the turbidity to vary by a factor of four for the same concentration of suspended solids. However, the numerous close correlations between turbidity and suspended solids concentration reported previously suggests that either the particle size variations are not usually great, or that particle size variations are often associated with variations in suspended solids concentration. For the same concentration and particle size, organic particles gave attenuance turbidity values two to three times higher than mineral particles. However, shortterm temporal variations from purely organic to purely mineral particle loads are rare in nature, so variations in the percentage of organic matter in the paniculate load will not confound turbidity to this extent. Coloured dissolved organic matter is unlikely to alter the turbidity reading by more than 10%. An adequate relationship between turbidity measured in the field and suspended solids concentration should be expected in most situations. Some variance can be tolerated because a continuous estimate of suspended solids concentration overcomes the problem of infrequent sampling, which is the greatest source of error in the estimation of stream sediment loads.

Deducing turbulence parameters from transionospheric scintillation measurements

The theoretical framework and experimental methodology used to interpret observations of ionospheric scintillations in terms of geophysical processes are reviewed and recent experimental observations of ionospheric scintillations are discussed in this paper. During the past 15 years significant progress has been made in several areas. In particular, significant advances have been made in theoretical understanding of the strong scintillation regime and the effects of short-term temporal variations of the scintillation producing irregularities on observations made with spaced-receiver geometries in both weak and strong scintillations. This improved understanding of the scintillation process has significantly increased the utility of the technique particularly in the equatorial latitudes where geometrical effects are least important.

Evidence of highly localized auroral scatterers from 50-MHz CW radar interferometry

High resolution cross spectral analysis of CW radar interferometer data is used to examine short-term temporal and spatial variations of VHF 50-MHz radio aurora. At times, extremely localized scatterers with mean transverse size less than 1 km are resolved. In such localized regions plasma irregularities appear to propagate with well defined phase velocities (identified as narrow peaks in Doppler spectra). Other transient scattering events show large variability in Doppler velocity within an extremely localized region (less than a few hundred meters across) and within a short time interval (<1 s). It is suggested that if many such localized sources of scatter are found within the scattering volume of the radar at one time, interference may significantly alter the spectra received at widely spaced interferometric antenna arrays.

A phytoplankton bloom and associated processes observed with a long-term moored system in antarctic waters

A moored system was designed for monitoring both longand short-term temporal variations in phytoplankton communities. It consisted of a chlorophyll buoy, a current meter and a sequential, multiple-sampling sedment trap, set at depths of 52. 57 and 120 m. respectively. The system was deployed at 7Oo11.54'S, 2418.68'E in Breid Bay, Antarctica, from 28 December 1985 to 13 February 1986. A total of 1127 hourly data sets and 12 trap samples were obtained. Equivalent chlorophyll a concentration fluctuated mainly in the range 0.69 to 5.60 yg 1-l; values tended to decrease toward early February. Tides caused the depth of the buoy to vary between 51 0 and 52.9 m. A strong positive correlation was found between chlorophyll a level and water temperature and indicates a possible advective water movement in the Bay. Vertical flux of particulate pigments showed a marked increase in mid-January, 3.5 to 10.5 d after an increase was observed In surface chlorophyll. Sediment samples were dominated by intact cells of Thalassiosira antarctica throughout the period of observation.

Upwelling by internal tides and kelvin waves at the continental shelf break on the Great Barrier Reef

A time series of 50 days duration was obtained of sea levels and winds and of temperature and currents at six depths from 27 to 104 m at 18�19'S.,147�21'E. on the continental shelf break between the Great Barrier Reef and the Coral Sea. The sea-level signal had a predominantly mixed solar and lunar semidiurnal tidal period. The currents consisted of a semidiurnal tidal component oriented primarily cross-shelf, except near the sea floor, superimposed on a low-frequency, predominantly longshore, southward component, coherent with depth, in geostrophic balance, and modulated by the longshore wind component Large fluctuations in temperature were observed, consisting of a low-frequency component, possibly generated by internal Kelvin waves, and iiucruarions of predominantiy solar semidiurnai iidai period. The latter fiiictuations are interpreted as evidence of internal tides of amplitude up to 110 m that may be generated by the interaction of the longshore currents with topographic irregularities in the shelf. It is suggested that, during any long-term studies of water properties near the shelf break, some additional monitoring of short-term temporal variations should be carried out to avoid data aliasing by internal tides. The bottom boundary layer appears to be very active in vertical mixing. Internal tides may be very important in introducing other water components, e.g. nutrients, to the outer Great Barrier Reef.

A review of data on production and import of organic carbon in the western Wadden Sea

Annual phytoplankton primary production in the tidal channels of the western Wadden Sea cannot be estimated more precisely than 150±50 g C.m−2a−1, due to large spatial and short-term temporal variations. This implies that year-to-year variations and eventually long-term trends have to be very pronounced to be measurable even with a weekly sampling program.

Observation of Dust Generated Hydrogen in the Solar Vicinity

Solar wind protons impinging on interplanetary dust grains are trapped, deionized, and subsequently desorbed. The steady state distribution of desorbed neutral hydrogen inside of 0.4 AU can be deduced by observation of resonantly scattered solar 121.6 nm radiation. Calculated integral intensities and spectral profiles are given, showing the clear spectral separation of the different radiation components. Given a specific solar wind proton flux, the interplanetary dust distribution can be determined. Conversely, dust density profiles from zodiacal light measurements can be used to deduce solar wind proton fluxes at heliocentric distances of 0.4 to 0.15 AU. Observations of latitudinal and short-term temporal proton flux variations seem feasible.

Short-term temporal variations of X-ray bright points

Skylab S-054 data have been used to examine the flux from X-ray bright points with ∼ 90 s time resolution. There is evidence of a steady heating input, similar to one reported for active region loops. Also observed are impulsive brightenings of bright points and rapid decays which are consistent with a sudden turn-off of the steady heating.

Solute Variations in Small Catchment Streams: Some Comments

The study of temporal variations in solute concentrations in streams has attracted the attention of many workers during the last twenty years. They have demonstrated that solute concentration can exhibit marked seasonal and stormbased variation and several important controls on solute levels have been proposed. In order to document variations in solute concentrations over short periods of time, in response to hydrometeorological controls, an intensive sampling programme is required. Results obtained from continuous monitoring and frequent sampling in some Devon streams are presented to illustrate the nature and extent of short-term temporal variations in solute levels. IN a recent paper, Imeson (1973) has pointed to the increasing interest in the solute loads of rivers shown by geomorphologists and ecologists and to the particular need to focus attention upon temporal variations. However, if it is suggested that this particular field has received little attention, then this ignores a considerable volume of literature representing studies by workers in various disciplines and which can be only briefly sampled in this communication. The variation in solute levels associated with fluctuations in discharge has been documented by many workers, particularly in the United States. In 1948, Hem (1948) pointed to interesting temporal variations in specific conductance and chloride concentration in rivers of the southwestern United States and these were attributed primarily to the dilution of baseflow constituents during periods of high flow, although diurnal fluctuations were also described. This dilution effect was subsequently substantiated, elaborated and quantified by many other studies (e.g. Durum, I953; Korven and Wilcox, 1964; Ward, I963). Goto (1961) described detailed stormperiod variations in several chemical components within a Japanese stream and Hendrickson and Krieger (I960) and Toler (1965) reported clockwise and anticlockwise hysteretic effects in the relationship between solute concentrations and discharge during individual storm events in American rivers. Examples have also been cited where certain solute concentrations exhibit either a positive relationship with discharge or no significant relationship at all (e.g. Edwards, I973). The frequent use of the dilution effect to explain temporal variations in solute levels has prompted the application of simple mixing models based on the mass balance equation and involving individual run-off components (e.g. Hart et al., 1964; Hem, I970; Steele, 1968; Walling, I974) or theoretical storage reservoirs (Hall, I970, I97I; Johnson et al., 1969). Other studies have highlighted control by additional factors. Seasonal effects on the relationship between solute concentrations and discharge have been described by Glancy et al. (1972), Gunnerson (1967), Ledbetter and Gloyna (1964) and Steele (1968). Work in Russia (e.g. Skakalskiy, 1966) has focused attention on the flow paths of individual run-off components. Temporarily increased concentrations during the initial periods of storm run-off events have been related to the 'flushing' of accumulated salts by Edwards (i973), Steele (1968) and Walling (1974), and Gburek and Heald (1970) have attempted to distinguish between those ions which show smooth changes during storm events and those which fluctuate more rapidly as a result of control

The High Altitude Observatory White Light Coronagraph Experiment

The coronagraph has obtained observations, both by astronaut and ground command, spaced periodically throughout the mission on a time center of approximately six to eight hours. Programs of more frequent operation – to examine short-term temporal variations and transient activity in the corona – have also been run at various times during the mission. The initial coronagraph observations of structures near limb passage experimentally verify that there are several time scales on which visual changes in these structures occur: (a) approximately one-half rotation, presumably accompanying major reorientations of coronal magnetic fields governing large scale coronal structures, (b) hours to days wherein changes to smaller coronal features are due either to structural changes of particular coronal features or to perspective effects and (c) less than hours – during coronal transients – which caused major reorientation of coronal structures by their passage through the coronal medium. Observations of the latter phenomena have provided some of the more spectacular results from the coronagraph (see Figure 1). The particular case illustrated was one stage of a complex series of events associated with activity in region 137, National Oceanic and Atmospheric Administration numbering, near east limb passage on 10 June 1973. The limb exhibited a major system of active prominences extended from the equator to north 25° and produced surges and sprays through the early hours of the day. From approximately 0700 to 0900 UT several Hα and small X-ray flares occurred and 0815 an eruptive prominence was observed by ground-based telescopes to ascend to 1.4 R⊙. This latter event is presumably the source of the transient observed by the coronagraph between 0929 and 1001 by ground command. During this period the material front moved with the apparent projected velocity of 450 km s−1 from 3.6 to 4.8 R⊙, while the arch-like structure expanded to a diameter greater than 2 R⊙. Figure 1 shows the event at 0943 GMT; it is one photograph of the 144 obtained. Within the structure are numerous bright areas and extended structures apparently mapping the distended magnetic field configuration. The general appearance of this transient is indicative of a large magnetic loop, or bottle, expanded outward from the Sun, its leading edge compressed by interaction with the ambient corona. Although there apparently was not a metric radio burst associated with this event, other transients observed later in the mission have associated metric wavelength activity.

Temporal variations in night-time hydroxyl rotational temperature

Airborne and groundbased OH measurements showing short-term temporal variations in OH rotational temperature are presented. The variations are interpreted on the basis of a dynamic model of the OH emitting region; they are also employed to clarify the apparent contradiction in latitude dependence of OH intensity reported in the literature.

Fine Structure and Stability of the Sound Channel in the Ocean

The sound-channel region in the ocean is traditionally assumed to be stable and uniform over large areas, although indications of rapid and appreciable variations have been observed. To improve understanding of the true situation, a study of the fine structure and short-term temporal and spatial variations in this region has been undertaken. The instrumentation and method developed for this purpose are described briefly. An underwater package containing two independent velocimeters, an FM pressure gauge, and auxiliary circuitry is repeatedly lowered and raised on a single-conductor armored cable through the channel region. Sensor information is telemetered to the ship and recorded. Results from three stations south of Bermuda in the Fall of 1963 are described and analyzed. A surprising amount of activity is found, proving that the sound-channel region, at the base of the main thermocline, is subject to much larger rapid changes than generally recognized. Considerable fine structure, evidence of internal waves, layers, and cells of “strange” waters have been observed. Sound-velocity changes of several meters per second in a few hours, at the same depth, are common. From August to October, the average sound velocity on the axis has decreased from 1494.5 to 1491 m/sec, the axis depth from 1300 to 1150 m.