Water-filled Conduits Research Articles

Bubbles rising through a layer of Carbopol capped with water

Air bubbles are injected into a Carbopol column capped with water to mimic ebullition from lake sediments. The first bubble sets an invisible path as it rises through the Carbopol. Subsequent bubbles rise along this path and, unlike the first bubble, break up into two separate bubbles. The leading bubble rises through the water column, while the smaller breakup bubble separates and remains trapped within the Carbopol. A conduit that forms above this breakup bubble gradually fills with water from the upper layer. After a period of growth this conduit reaches an equilibrium depth, and then starts to shrink. Inside the conduit, the rising bubbles resemble Taylor bubbles, but have velocities approximately 5–7 times the maximum velocity of standard Taylor bubbles. The variation in rise velocity depends on the interaction of the rising bubble with previous breakup bubbles. In addition, rising bubbles expel water from the conduit and water flows back after bubbles escape. This process can enhance the transport of heat and contaminants to and from aquatic sediments during ebullition events. • A damaged pathway forms inside Carbopol due to the passage of bubbles. • Bubbles break up as they cross the Carbopol–water interface. • Upper portion of the damaged pathway evolves into a water-filled conduit. • Bubbles rise much faster inside the conduit than standard Taylor bubbles. • Water flows in and out of the conduit, enhancing mixing in aquatic sediments.

Monitoring the seasonal changes of an englacial conduit network using repeated ground-penetrating radar measurements

Abstract. Englacial conduits act as water pathways to feed surface meltwater into the subglacial drainage system. A change of meltwater into the subglacial drainage system can alter the glacier's dynamics. Between 2012 and 2019, repeated 25 MHz ground-penetrating radar (GPR) surveys were carried out over an active englacial conduit network within the ablation area of the temperate Rhonegletscher, Switzerland. In 2012, 2016, and 2017 GPR measurements were carried out only once a year, and an englacial conduit was detected in 2017. In 2018 and 2019 the repetition survey rate was increased to monitor seasonal variations in the detected englacial conduit. The resulting GPR data were processed using an impedance inversion workflow to compute GPR reflection coefficients and layer impedances, which are indicative of the conduit's infill material. The spatial and temporal evolution of the reflection coefficients also provided insights into the morphology of the Rhonegletscher's englacial conduit network. During the summer melt seasons, we observed an active, water-filled, sediment-transporting englacial conduit network that yielded large negative GPR reflection coefficients (<-0.2). The GPR surveys conducted during the summer provided evidence that the englacial conduit was 15–20 m±6 m wide, ∼0.4m±0.35m thick, ∼250m±6m long with a shallow inclination (2∘), and having a sinusoidal shape from the GPR data. We speculate that extensional hydraulic fracturing is responsible for the formation of the conduit as a result of the conduit network geometry observed and from borehole observations. Synthetic GPR waveform modelling using a thin water-filled conduit showed that a conduit thickness larger than 0.4 m (0.3× minimum wavelength) thick can be correctly identified using 25 MHz GPR data. During the winter periods, the englacial conduit no longer transports water and either physically closed or became very thin (<0.1 m), thereby producing small negative reflection coefficients that are caused by either sediments lying within the closed conduit or water within the very thin conduit. Furthermore, the englacial conduit reactivated during the following melt season at an identical position as in the previous year.

Factors controlling drought resistance in grapevine (Vitis vinifera, chardonnay): application of a new microCT method to assess functional embolism resistance.

Quantifying resistance to embolism in woody plants is important for understanding their drought response. Methods to accurately quantify resistance to embolism continue to be debated. We used a new microCT-based approach that quantifies embolized conduits and also analyzes conductive conduits by using an x-ray-dense, iodine-rich tracer that moves though the vascular system and can easily be observed in microCT images. Many previous microCT studies assumed that all conduits were initially conductive, which may not be the case if there are developing or occluded conduits. We compared microCT results to a standard benchtop dehydration method and a centrifuge method. During dehydration, we measured gas exchange and quantified water potential at mortality. Our microCT curves agreed with previously published microCT curves from the same greenhouse-grown cultivar. We found a significant difference in embolism estimates if we assumed that all water-filled conduits were functional rather than only those containing tracer. Embolism estimates from microCT differed from both the benchtop and centrifuge methods. The benchtop and centrifuge methods did not differ from one another. The new microCT method presented here is valuable in sampling species that may contain nonconductive conduits. Disagreement between microCT and two other methods was likely due to differences in the ways they quantify embolism. MicroCT assess the theoretical effect of embolism, whereas benchtop and centrifuge methods directly measure hydraulic conductivity. The theoretical approach does not fully account for the resistances of flow through a complex 3D vascular network.

On the dissipation mechanism of lattice Boltzmann method when modeling 1-d and 2-d water hammer flows

This paper uses the multiple relaxation times lattice Boltzmann method (LBM) to model acoustic wave propagation in water-filled conduits (i.e. water hammer applications). The LBM scheme is validated by solving some classical water hammer (WH) numerical tests in one-dimensional and two-dimensional cases. In addition, this work discusses the accuracy, stability and robustness of LBM when solving high frequency (i.e. radial modes are excited) acoustic waves in water-filled pipes. The results are compared with a high order finite volume scheme based on Riemann Solver. The results show that LBM is capable to model WH applications with high accuracy and performance at low frequency cases; however, it loses stability and performance for high frequency (HF) cases. It is discussed that this is due to the neglected high order terms of the equivalent macroscopic equations considered for LB numerical formulation.

Integrated surface geophysical approach to locate a karst conduit: a case study from Royal Spring Basin, Kentucky, USA

Groundwater flow in karst terrains is difficult to map because it can be concentrated through conduits that do not necessarily coincide with the surface features. We applied electrical resistivity (ER) and self-potential (SP) techniques at three sites to locate an inferred trunk conduit feeding a major spring in the Inner Bluegrass region of Kentucky (USA). Royal Spring is the primary water supply for the city of Georgetown; the upper part of its basin coincides with the Cane Run watershed. ER profiles (972 m total length) were measured using a dipole–dipole electrode configuration with 2- to 3-m spacing. SP measurements were taken along those ER lines and an additional test profile (230 m) using one stationary reference electrode and another roving electrode at a fixed interval. The SP technique has been used by many researchers to detect the electro kinetic potential generated by groundwater flow. The low resistivity of water in the conduit, as compared to the high background resistivity of limestone bedrock, was the ER exploration target. A negative SP anomaly corresponded to a low ER anomaly for most of the profiles, but a few are not comparable. Although SP data collected over multiple days along the test profile differed significantly, they showed similar trends. Field drift in SP data was found to be highly sensitive to temperature changes during the time of measurement. Although the overall trends of the final SP profiles for different dates were similar, the SP magnitudes varied with the amount of precipitation and the average soil temperature. The low-resistivity anomalies in the 2D inverted sections and corresponding negative SP anomalies at two sites (Berea Road and Kentucky Horse Park) encountered water-filled conduits, although mudfilled voids encountered during drilling at University of Kentucky Agricultural Research Farm sites suggest that these may be tributary conduits rather than the trunk conduit.

X-ray microtomography observations of xylem embolism in stems of Laurus nobilis are consistent with hydraulic measurements of percentage loss of conductance.

Drought-induced xylem embolism is a serious threat to plant survival under future climate scenarios. Hence, accurate quantification of species-specific vulnerability to xylem embolism is a key to predict the impact of climate change on vegetation. Low-cost hydraulic measurements of embolism rate have been suggested to be prone to artefacts, thus requiring validation by direct visualization of the functional status of xylem conduits using nondestructive imaging techniques, such as X-ray microtomography (microCT). We measured the percentage loss of conductance (PLC) of excised stems of Laurus nobilis (laurel) dehydrated to different xylem pressures, and compared results with direct observation of gas-filled vs water-filled conduits at a synchrotron-based microCT facility using a phase contrast imaging modality. Theoretical PLC calculated on the basis of microCT observations in stems of laurel dehydrated to different xylem pressures overall were in agreement with hydraulic measurements, revealing that this species suffers a 50% loss of xylem hydraulic conductance at xylem pressures averaging -3.5MPa. Our data support the validity of estimates of xylem vulnerability to embolism based on classical hydraulic techniques. We discuss possible causes of discrepancies between data gathered in this study and those of recent independent reports on laurel hydraulics.

An approach for collection of nearfield groundwater samples in submerged limestone caverns

Walls of submerged caves feeding Florida springs are often lined with a heavy mat of filamentous bacteria, many of which are able to oxidize reduced sulfur in groundwater migrating from the porous bedrock into the cave conduit. To determine changes in water chemistry as water passes through the microbial mat, a simple device made from standard well screen and sealed with a rubber stopper and controllable vents was installed in a hole drilled in the wall of the cave passage. The sampler was sealed in place with marine epoxy. We measured anions in water from the sampler and from the water-filled conduit taken just outside the sampler. Most anions measured viz., Cl−, NO3−, and PO43−, increased slightly between the matrix and conduit waters. However, traces of sulfide were measured in the water from the rock matrix, but not in the conduit. SO42− concentrations in the conduit were about twice that measured in the water from the sampler, about 22 and 11 mg SO42− L−1, respectively, providing further evidence that sulfur oxidation is an important process in the bacterial mats attached to the limestone surfaces in these caves. An additional use of the sampling device is to measure discharge from the local bedrock into the cave conduit.Keywords: sulfur oxidation, bacteria, acid dissolution, groundwater chemistry.DOI:10.3986/ac.v42i2-3.664

Electrical resistivity tomography (ERT) of a coastal carbonate aquifer (Port-Miou, SE France)

As part of a larger regional research program “KarstEAU”, the authors have applied electrical resistivity tomography (ERT) techniques to characterize heterogeneities in the Port-Miou coastal karst aquifer (Cassis, SE France). Field surveys were carried out on intensely fractured and karstified Urgonian carbonates. Extensive research has characterized macro- and micro-scale geology of the Port-Miou area and particularly underground water-filled conduits and fault/fracture and karst systems within a former quarry. The authors applied 2D ERT along two surface profiles of length 420 and 595 m to test capability for delineating subsurface conduits and possibly relationship between conduit and fault/fracture/karst orientation; and 3D ERT with a dense 120 electrode array at 1 m spacing (11 × 10 m) was applied over an area of the quarry that had been profiled using 3D georadar and which has had intensive nearby structural geological interpretation. The 2D profiling imaged several underground conduits at depths to >50 m below ground surface and below sea level, including possibly the main Port Miou submarine spring and smaller springs. The 2D profiling within the quarry provided a better understanding of the connectivity between major fractures and faults on the quarry walls and secondary springs along the coast supporting flow of the secondary springs along interpreted fracture orientations. In addition, 2D inversion-derived conductivity models indicate that high resistivity zones above sea-level are associated with non-saturated zones and low resistivity anomalies in the non-saturated zone are associated with residual clays in paleokarsts. A partitioned lower resistivity zone below sea-level can be associated with a higher porosity/permeability zone with fractures and karstic features. Inversion models of the dense 3D ERT data indicate a higher resistivity volume within the larger surveyed block. The survey characterized the non-saturated zone and the ERT resistivities are correlated with karst features interpreted by 3D georadar and visible in the inferior wall of the quarry.

Video observations inside conduits of erupting geysers in Kamchatka, Russia, and their geological framework: Implications for the geyser mechanism

Research Article| April 01, 2013 Video observations inside conduits of erupting geysers in Kamchatka, Russia, and their geological framework: Implications for the geyser mechanism A. Belousov; A. Belousov 1Institute of Volcanology and Seismology, Piip Boulevard 9, Petropavlovsk 683006, Russia Search for other works by this author on: GSW Google Scholar M. Belousova; M. Belousova 1Institute of Volcanology and Seismology, Piip Boulevard 9, Petropavlovsk 683006, Russia Search for other works by this author on: GSW Google Scholar A. Nechayev A. Nechayev 2Moscow State University, Vorobiovy Gory 1, Moscow 11992, Russia Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (4): 387–390. https://doi.org/10.1130/G33366.1 Article history received: 02 Mar 2012 rev-recd: 19 Jun 2012 accepted: 16 Oct 2012 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. Belousov, M. Belousova, A. Nechayev; Video observations inside conduits of erupting geysers in Kamchatka, Russia, and their geological framework: Implications for the geyser mechanism. Geology 2013;; 41 (4): 387–390. doi: https://doi.org/10.1130/G33366.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Several models have been proposed to explain periodic eruptions of geysers. In essence, the models all use two principally different types of geyser plumbing configurations, dealing with two different physical mechanisms. Here we present data on direct video observations of interior conduit systems for four erupting geysers in Geyser Valley, Kamchatka, Russia. The video footage reveals highly contorted water-filled conduits that periodically discharge voluminous parcels of steam bubbles during eruptions. These observations do not favor the models that use the most popular long vertical conduit type of plumbing, where eruptions are caused by sudden flashing of superheated water into steam. In contrast, our data fit the models using the less-explored type of plumbing, where pressurized steam gradually accumulates in an underground cavity (bubble trap) and periodically erupts through a water-filled, highly contorted conduit with the configuration of an inverted siphon. Hydrodynamic calculations show that such a plumbing configuration produces periodic eruptions when the volume of the bubble trap exceeds the volume of the conduit connecting it to the ground surface. Conduits of the studied geysers were developed from erosion by ascending geothermal water in landslide deposits; chaotic internal structures of the deposits facilitated the formation of conduit systems with highly contorted configurations of the bubble trap type. We suggest that geyser fields are rare on Earth because they require the combination of hydrothermal discharge and geological formations having specific mechanical properties and structures (that facilitate the generation of highly contorted conduits). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geophysical signatures of Barton Springs (Parthenia, Zenobia and Eliza) of the Edwards Aquifer, Austin, Texas

Barton Springs is a major discharge site for the Barton Springs Segment of the Edwards Aquifer and is located in Zilker Park in Austin, Texas. Barton Springs actually consists of four springs: (1) The Main Barton Springs discharges into the Barton Springs pool from the Barton Springs Fault and several outlets along a fault and from a cave, several fissures, and gravel-filled solution cavities on the floor of the pool west of the fault. The thin-bedded unit on the southwest side of the fault is the regional dense member, and the lower Georgetown Formation of the Edwards Group is exposed on the northeast side of the fault. The offset of the fault is between 40 and 70 ft (12–21 m). (2) Old Mill Springs is located in the sunken gardens southeast of the Barton Springs Pool and is primarily fed by relatively mineralized groundwater from the Saline-Line Flow Route. (3) Eliza Springs is also located along the Barton Springs Fault north of Barton Springs pool. (4) The Upper Barton Springs is located upstream of the Barton Springs pool on the south bank. Surface geophysical surveys [resistivity imaging and natural potential (NP)] were performed over the first three springs (Main Barton, Old Mill and Eliza Springs). Conductivity (EM) surveys were conducted in some areas to distinguish utility lines. The purpose of the surveys was to: (1) locate the precise location of submerged conduits carrying flow to Main Barton Springs on the north and south banks of the Barton Springs pool; (2) characterize the hydraulic relation between the Main Barton, Old Mill and Eliza Springs; (3) determine the potential location of caves and active flow paths beneath the three springs; and (4) characterize the geophysical signatures of the fault crossing the Barton Springs pool. The geophysical surveys revealed three general types of anomalies. Resistivity results from the south of the Barton Springs swimming pool indicate presence of a thick, laterally extensive high conductivity layer above the pool elevation. This high conductivity layer is interpreted to be lateral clay deposits, either associated with the Del Rio clay or clay-rich alluvial deposits associated with Barton Creek. These clay layers appear to overlie the Edwards Aquifer south of the pool. Also south and east of the pool are cylindrical high conductivity anomalies that extend deeper than the elevation of the submerged cave observed in Barton Springs pool. These cylindrical high conductivity anomalies are also associated with NP anomalies, suggesting groundwater flow. One hypothesis is that the alignment of the high conductivity and NP anomalies corresponds to the Saline-Line Flow Route that is known to discharge primarily at Old Mill Springs, and is hydraulically connected to Main Barton Springs and Eliza Springs. This hypothesis is favored because the Saline-Line Flow Route carries relatively mineralized groundwater and is known to connect to Old Mill Springs and at some times Main Barton and Eliza Springs. There are likely several conduit paths to the pool from the southern part of Zilker Park. Flow paths to Barton Springs from the east may be localized within the uppermost leached and collapsed members of the Edwards Group, which is known for its extensive horizontal cave development. A third type of anomaly, generally found west and immediately adjacent to the Barton Springs Fault, southwest of Barton Springs pool are circular low conductive features associated with NP anomalies. These circular anomalies are interpreted to be groundwater flow conduits bearing less mineralized groundwater associated with the Sunset Valley and possibly Manchaca Flow Routes that are expected to cross that area. The surveys allow an opportunity to compare geophysical responses to directly observed features. The Barton Springs Fault appears to be associated with circular high- and low-resistivity anomalies. Eliza and Old Mill Springs also indicate significant resistivity and NP anomalies suggesting presence of submerged conduits and faults in the vicinity. Although it is obvious such conduits are present adjacent to major springs, the surveys allow examination of how such water-filled conduits appear using various geophysical methods. Known underground infrastructure was also included in the surveys to see how air and water-filled pipes responded. An air-filled train tunnel corresponded to a high-resistivity anomaly. Local utility lines crossing the site showed no significant resistivity anomaly but metal pipes were detectable with EM conductivity surveys.

Differenzialindikationen für verschiedene Harnableitungsformen

When urinary diversion is indicated, patient information concerning the advantages and disadvantages of different types of urinary diversion and their choices is of utmost importance for the functional outcome and patient satisfaction. There is a variety of choices for incontinent urinary diversion (ureterocutaneostomy, ileal conduit, colonic conduit) and continent urinary diversion (continent anal urinary diversion, continent cutaneous urinary diversion and urethral bladder substitution). In the individual case, the choices may be limited by patient criteria and/or medical criteria. Important patient criteria are preference, age and comorbidity, BMI, motivation, underlying disease and indication for cystectomy. Medical criteria which possibly limit choices of type of urinary diversion are kidney function/upper urinary tract status and limitations concerning the gastrointestinal tract, concerning urethra/sphincter as well as the ability and motivation to perform intermittent self-catheterization. Preoperative information may use simulation of certain postoperative scenarios (urethral self-catheterization, fixation of water-filled conduit bags, holding test for anal liquids) to allow the individual patient to choose the optimal type of urinary diversion for his/her given situation from the mosaic of choices and possible individual limitations.

Intracellular Proton Access Mechanism of the CLC-ec1 Cl−/H+ Exchanger

CLC-ec1 is a bacterial CLC Cl-/H+ exchanger that catalyzes 2:1 exchange of Cl- and H+. E148 is the pH-dependent external gate of both Cl- and H+ transport and E203 mediates proton transport from intracellular solution to the protein interior. Substitution of non-protonatable residues on either glutamate uncouples the exchanger by completely abolishing H+ transport. However, it is not clear how protons in the intracellular solution gain access to E203, which is buried under a cytoplasmic “lid” formed by the protein's N-terminal helix. Truncation of the N-terminal 29 residues removes this lid and preserves 2:1 coupling of Cl- and H+ and absolute transport rate. This result implies that proton transport from intracellular solution to E203 is not rate-limiting in the transport cycle, and that protons are somehow facilitated in their movement to the buried E203. In order to discover proton facilitators, we examined polar residues near E203. Nonpolar mutants of Q207, S446 and R403 only slightly inhibit H+-transport rate. However, mutations on the proximate glutamate, E113, and disruption of a nearby salt-bridge (E117I and R209I) decrease H+-transport rate ∼5-fold. These residues appear to form a water-filled conduit for proton access to E203. Interestingly, nonpolar mutants of E202, located near the dimer interface and near to intracellular solution, show 30-100-fold reduced H+-transport rates compared to wildtype. All mutations here preserve H+-coupled Cl- transport, although those with severely reduced rates display somewhat higher Cl-/H+ stoichiometry, indicative of Cl- slippage. One or two crystallographic water molecules were found between E203 and E202. We propose that these water molecules form an access pathway for H+ from bulk water, which is blocked by bulky, hydrophobic amino acids at E202.

Challenges of using electrical resistivity method to locate karst conduits—A field case in the Inner Bluegrass Region, Kentucky

Abstract Conduits serve as major pathways for groundwater flow in karst aquifers. Locating them from the surface, however, is one of the most challenging tasks in karst research. Geophysical methods are often deployed to help locate voids by mapping variations of physical properties of the subsurface. Conduits can cause significant contrasts of some physical properties that can be detected; other subsurface features such as water-bearing fractures often yield similar contrasts, which are difficult to distinguish from the effects of the conduits. This study used electrical resistivity method to search for an unmapped karst conduit that recharges Royal Spring in the Inner Bluegrass karst region, Kentucky, USA. Three types of resistivity techniques (surface 2D survey, quasi-3D survey, and time-lapse survey) were used to map and characterize resistivity anomalies. Some of the major anomalies were selected as drilling targets to verify the existence of the conduits. Drilling near an anomaly identified by an electrical resistivity profile resulted in successful penetration of a major water-filled conduit. The drilling results also suggest that, in this study area, low resistivity anomalies in general are associated with water-bearing features. However, differences in the anomaly signals between the water-filled conduit and other water-bearing features such as water-filled fracture zones were undistinguishable. The electrical resistivity method is useful in conduit detection by providing potential drilling targets. Knowledge of geology and hydrogeology about the site and professional judgment also played important roles in locating the major conduit.

An englacial image and water pathways of the fourcade glacier on King George Island, Antarctic Peninsula, inferred from ground-penetrating radar

The distribution of small fractures and water content of the Fourcade glacier on King George Island, Antarctica, was investigated in November 2006 and December 2007 by two ground-based (470- and 490-m-long profiles) and one helicopter-borne (470-m-long profile) ground-penetrating radar (GPR) surveys using 50-, 100-, and 500-MHz antennas. Radar images in the pre-migrated GPR sections are characterized by a smooth ice surface and irregular bed topography, numerous diffraction hyperbolas in the ice and at the glacier bed, strong scattering noise, and near-surface folded layers. Scattering noise above a mound in the center of the profiles is associated with an area of dense fractures extending down from the ice surface that has relatively low reflection strength. Near the northeast ends of the profiles where few englacial fractures occur, scattering noise may result from the presence of warmer ice. A water-filled conduit and an air-filled cavity are interpreted as the source of two distinct hyperbolas in sub-glacial valleys based on the polarity of the reflections. Through migration velocity analysis on 106 hyperbolas, radar velocities were obtained for the 100-MHz ground-based profile. Using the velocities and Paren’s mixture formula, we calculated the water content of the ice to have been in the range of 0.00–0.09. High water content occurs near the glacier margin, in sub-glacial valleys, and in zones of scattering noise.

Geophysical characterisation of karstic networks – Application to the Ouysse system (Poumeyssen, France)

In the framework of the management of karstic aquifers, geophysical reconnaissance can be used to locate conduits and caves, and to characterise the surrounding limestone matrix. Suitable characterisation of heterogeneities in the karstic environment is, however, challenging for ground-based geophysical methods. The present article describes the results, and evaluates the response and accuracy of combined geophysical measurements carried out at the Poumeyssen test site in France, involving electrical resistivity imaging (ERI), magnetic resonance sounding (MRS), “ mise-à-la-masse” electrical mapping, and seismic tomography. This site provides the opportunity to study a relatively wide, shallow, water-filled conduit whose location and shape are known from topographic work carried out by cave divers. Seismic and MRS provided the exact location and width of the conduit, to within a few meters. The seismic and electrical data suggest that the limestone medium surrounding the conduit is not homogeneous.

Locating the Zone of Saline Intrusion in a Coastal Karst Aquifer Using Springflow Data

Coastal fresh water aquifers are an increasingly desirable resource. In a karstic aquifer, sea water intrusion occurs as a salt water wedge, like in porous media. However, preferential flow conduits may alter the spatial and temporal distribution of the salt water. This is typically the case when the outlet of the aquifer is a brackish spring. This paper shows that salinity and flow rate variations at a spring, where salinity is inversely proportional to discharge, can help to understand the hydrodynamic functioning of the aquifer and to locate the fresh water-sea water mixing zone deep inside the aquifer. The volume of water-filled conduit between the sea water intrusion zone and the spring outlet is calculated by the integral over time of the flow rate during the time lag between the flow rate increase and the salinity decrease as measured at the spring. In the example of the spring at Almyros of Heraklio (Crete, Greece), this time lag is variable, depending on the discharge, but the volume of water-filled conduit appears to be constant, which shows that the processes of salt water intrusion and mixing in the conduit are constant throughout the year. The distance between the spring and the zone where sea water enters the conduit is estimated and provides an indication of the position where only fresh water is present in the conduit.

Anisotropic radio-wave scattering from englacial water regimes, Mýrdalsjökull, Iceland

AbstractColinear-polarized 5 MHz radar profiling data were obtained on Mýrdalsjökull, a temperate glacier in Iceland. Radar transects, and therefore polarization planes, were aligned approximately parallel, transverse and oblique to the ice flow direction. Echoes from the shallower half to two-thirds of the ice were 10–20dB stronger on the oblique and longitudinal transects than those on the transverse transects. Anisotropy as a function of depth is clearly seen at the sites where the transects cross. Strong scattering on longitudinal transects apparently caused extinction of a radar-reflecting layer that was continuously profiled on the transverse transects. A radio-wave scattering model shows that scattering from a longitudinal water-filled conduit parallel to the glacier surface can explain the observed azimuthal variations of the echo. We conclude that low-frequency (~MHz) radio waves can help to characterize englacial water regimes.

Magnetic Resonance Micro-Imaging of Xylem Sap Distribution and Necrotic Lesions in tree Stems

To get high quality magnetic resonance (MR) micro-images for the physiological and anatomical investigation of tree stems, the imaging with various combinations of parameters was tested on a Signa VH/i 3.0 T MR imaging system (General Electric Medical Systems). Proton density and T2 weighted spin echo (SE) sequences are commonly used to detect areas with abundant water. Cambial zone and the area consisting of water-filled conduits looked whitish in the MR images of healthy Pinus densiflora and Quercus serrata stems that were obtained by these sequences. In the MR images of diseased P. densiflora that had been inoculated with a wilt pathogen, Bursaphelenchus xylophilus, the area dehydrated as a result of embolism appeared pitch-black. In the images of diseased Quercus crispula saplings, that had been inoculated with the wilt pathogen Raffaelea quercivora, the dehydrated xylem was shown as a dark area. The T1 weighted SE or gradient echo (FSPGR) sequence, that is used to detect fat and protein distribution, provided high intensity MR signals for the pathogenic heartwood and necrotic areas resulting from fungal activity. The resolution of MR images obtained by SE sequences was estimated 100–150 μm from the FOV and image size. The data sets obtained by the FSPGR sequence that consist of 0.8 mm thick serial sections enables the three-dimensional analysis of the affected area. This technique is helpful for detecting the water distribution and areas affected by pathogenic microorganisms in living tree stems.

Use of Positive Pressures to Establish Vulnerability Curves

Loss of hydraulic conductivity occurs in stems when the water in xylem conduits is subjected to sufficiently negative pressure. According to the air-seeding hypothesis, this loss of conductivity occurs when air bubbles are sucked into water-filled conduits through micropores adjacent to air spaces in the stem. Results in this study showed that loss of hydraulic conductivity occurred in stem segments pressurized in a pressure chamber while the xylem water was under positive pressure. Vulnerability curves can be defined as a plot of percentage loss of hydraulic conductivity versus the pressure difference between xylem water and the outside air inducing the loss of conductivity. Vulnerability curves were similar whether loss of conductivity was induced by lowering the xylem water pressure or by raising the external air pressure. These results are consistent with the air-seeding hypothesis of how embolisms are nucleated, but not with the nucleation of embolisms at hydrophobic cracks because the latter requires negative xylem water pressure. The results also call into question some basic underlying assumptions used in the determination of components of tissue water potential using "pressure-volume" analysis.