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Pressure Head (pressure + head)
Selected AbstractsSteady Flow into a Tunnel with a Constant Pressure HeadGROUND WATER, Issue 5 2000ShiZhong Lei No abstract is available for this article. [source] An Analytical Method of Analyzing Oscillatory Slug Test Pressure Heads from Shallow or Deep Pressure TransducersGROUND WATER, Issue 1 2007Article first published online: 28 JUL 200 No abstract is available for this article. [source] Non-equilibrium water flow characterized by means of upward infiltration experimentsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2001Summary Upward infiltration experiments under tension were used to demonstrate the presence of non-equilibrium flow in soils, the phenomenon that has important implications for the accelerated movement of fertilizers, pesticides, non-aqueous liquids, and other pollutants. Data obtained from these experiments were analysed using the single-porosity Richards equation, as well as a variably saturated, dual-porosity model and a dual-permeability model for characterizing non-equilibrium water flow. The laboratory experiments were carried out on 0.10-m-long soil cores having an internal diameter of 0.10 m. Constant pressure heads of ,0.10 and ,0.01 m were used as the lower boundary condition. Each infiltration was followed by a single-rate evaporation experiment to re-establish initial conditions, and to obtain the drying soil hydraulic properties. Pressure heads inside the cores were measured using five tensiometers, while evaporative water loss from the top was determined by weighing the soil samples. The data were analysed to estimate parameters using a technique that combined a numerical solution of the governing flow equation (as implemented in a modified version of the Hydrus-1D software) with a Marquardt,Levenberg optimization. The objective function for the parameter estimation was defined in terms of pressure head readings, the cumulative infiltration rate, and the final total water volume in the core during upward infiltration. The final total water volume was used, as well as the pressure head readings during the evaporation part. Analysis of flow responses obtained during the infiltration experiment demonstrated significant non-equilibrium flow. This behaviour could be well characterized using a model of physical non-equilibrium that divides the medium into inter- and intra-aggregate pores with first-order transfer of water between the two systems. The analysis also demonstrated the importance of hysteresis. [source] Sphagnum under pressure: towards an ecohydrological approach to examining Sphagnum productivityECOHYDROLOGY, Issue 4 2008D. K. Thompson Abstract The genus Sphagnum is the key peat-forming bryophyte in boreal ecosystems. Relying entirely on passive capillary action for water transport, soil moisture is often the limiting factor in Sphagnum production, and hence peat accumulation. While several hydrological models of peat physics and peatland water movement exist, these models do not readily interface with observations and models of peatland carbon accumulation. A conflict of approaches exists, where hydrological studies primarily utilize variables such as hydraulic head, while ecological models of Sphagnum growth adopt the coarse hydrological variables of water table (WT), volumetric water content (VWC) or gravimetric water content (WC). This review examines the potential of soil pressure head as a measurement to link the hydrological and ecological functioning of Sphagnum in peatlands. The non-vascular structure of Sphagnum mosses and the reliance on external capillary transport of water in the mosses make them an ideal candidate for this approach. The main advantage of pressure head is the ability to mechanistically link plot-scale hydrology to cellular-scale water requirements and carbon exchange. Measurement of pressure head may improve photosynthetic process representation in the next generation of peatland models. Copyright © 2008 John Wiley & Sons, Ltd. [source] Occurrence, prediction and hydrological effects of water repellency amongst major soil and land-use types in a humid temperate climateEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 5 2006S. H. Doerr Summary Knowledge of soil water repellency distribution, of factors affecting its occurrence and of its hydrological effects stems primarily from regions with a distinct dry season, whereas comparatively little is known about its occurrence in humid temperate regions such as typified by the UK. To address this research gap, we have examined: (i) water repellency persistence (determined by the water drop penetration time method, WDPT) and degree (determined by the critical surface tension method, CST) for soil samples (0,5, 10,15 and 20,25 cm depth) taken from 41 common soil and land-use types in the humid temperate climate of the UK; (ii) the supposed relationship of soil moisture, textural composition and organic matter content with sample repellency; and (iii) the bulk wetting behaviour of undisturbed surface core samples (0,5 cm depth) over a period of up to 1 week. Repellency was found in surface samples of all major soil textural types amongst most permanently vegetated sites, whereas tilled sites were virtually unaffected. Repellency levels reached those of the most severely affected areas elsewhere in the world, decreased in persistence and degree with depth and showed no consistent relationship with soil textural characteristics, organic matter or soil moisture contents, except that above a water content of c. 28% by volume, repellency was absent. Wetting rate assessments of 100 cm3 intact soil cores using continuous water contact (,20 mm pressure head) over a period of up to 7 days showed that across the whole sample range and irrespective of texture, severe to extreme repellency persistence consistently reduced the maximum water content at any given time to well below that of wettable soils. For slightly to moderately repellent soils the results were more variable and thus hydrological effects of such repellency levels are more difficult to predict. The results imply that: (i) repellency is common for many land-use types with permanent vegetation cover in humid temperate climates irrespective of soil texture; (ii) supposedly influential parameters (texture, organic matter, specific water content) are poor general predictors of water repellency, whereas land use and the moisture content below which repellency can occur seem more reliable; and (iii) infiltration and water storage capacity of very repellent soils are considerably less than for comparable wettable soils. [source] The Lisse Effect RevisitedGROUND WATER, Issue 6 2002Edwin P. Weeks The Lisse effect is a rarely noted phenomenon occurring when infiltration caused by intense rain seals the surface soil layer to airflow, trapping air in the unsaturated zone. Compression of air by the advancing front results in a pressure increase that produces a water-level rise in an observation well screened below the water table that is several times as large as the distance penetrated by the wetting front. The effect is triggered by intense rains and results in a very rapid water-level rise, followed by a recession lasting a few days. The Lisse effect was first noted and explained by Thal Larsen in 1932 from water-level observations obtained in a shallow well in the village of Lisse, Holland. The original explanation does not account for the increased air pressure pushing up on the bottom of the wetting front. Analysis of the effect of this upward pressure indicates that a negative pressure head at the base of the wetting front, ,f, analogous to that postulated by Green and Ampt (1911) to explain initially rapid infiltration rates into unsaturated soils, is involved in producing the Lisse effect. Analysis of recorded observations of the Lisse effect by Larsen and others indicates that the water-level rise, which typically ranges from 0.10 to 0.55 m, should be only slightly larger than |,f| and that the depth of penetration of the wetting front is no more than several millimeters. [source] Influence of pore size and geometry on peat unsaturated hydraulic conductivity computed from 3D computed tomography image analysisHYDROLOGICAL PROCESSES, Issue 21 2010F. Rezanezhad Abstract In organic soils, hydraulic conductivity is related to the degree of decomposition and soil compression, which reduce the effective pore diameter and consequently restrict water flow. This study investigates how the size distribution and geometry of air-filled pores control the unsaturated hydraulic conductivity of peat soils using high-resolution (45 µm) three-dimensional (3D) X-ray computed tomography (CT) and digital image processing of four peat sub-samples from varying depths under a constant soil water pressure head. Pore structure and configuration in peat were found to be irregular, with volume and cross-sectional area showing fractal behaviour that suggests pores having smaller values of the fractal dimension in deeper, more decomposed peat, have higher tortuosity and lower connectivity, which influences hydraulic conductivity. The image analysis showed that the large reduction of unsaturated hydraulic conductivity with depth is essentially controlled by air-filled pore hydraulic radius, tortuosity, air-filled pore density and the fractal dimension due to degree of decomposition and compression of the organic matter. The comparisons between unsaturated hydraulic conductivity computed from the air-filled pore size and geometric distribution showed satisfactory agreement with direct measurements using the permeameter method. This understanding is important in characterizing peat properties and its heterogeneity for monitoring the progress of complex flow processes at the field scale in peatlands. Copyright © 2010 John Wiley & Sons, Ltd. [source] Rapid simulated hydrologic response within the variably saturated near surfaceHYDROLOGICAL PROCESSES, Issue 3 2008Brian A. Ebel Abstract Column and field experiments have shown that the hydrologic response to increases in rainfall rates can be more rapid than expected from simple estimates. Physics-based hydrologic response simulation, with the Integrated Hydrology Model (InHM), is used here to investigate rapid hydrologic response, within the variably saturated near surface, to temporal variations in applied flux at the surface boundary. The factors controlling the speed of wetting front propagation are discussed within the Darcy,Buckingham conceptual framework, including kinematic wave approximations. The Coos Bay boundary-value problem is employed to examine simulated discharge, pressure head, and saturation responses to a large increase in applied surface flux. The results presented here suggest that physics-based simulations are capable of representing rapid hydrologic response within the variably saturated near surface. The new InHM simulations indicate that the temporal discretization and measurement precision needed to capture the rapid subsurface response to a spike increase in surface flux, necessary for both data-based analyses and evaluation of physics-based models, are smaller than the capabilities of the instrumentation deployed at the Coos Bay experimental catchment. Copyright © 2007 John Wiley & Sons, Ltd. [source] Field and laboratory estimates of pore size properties and hydraulic characteristics for subarctic organic soilsHYDROLOGICAL PROCESSES, Issue 19 2007Sean K. Carey Abstract Characterizing active and water-conducting porosity in organic soils in both saturated and unsaturated zones is required for models of water and solute transport. There is a limitation, largely due to lack of data, on the hydraulic properties of unsaturated organic soils in permafrost regions, and in particular, the relationship between hydraulic conductivity and pressure head. Additionally, there is uncertainty as to what fraction of the matrix and what pores conduct water at different pressure heads, as closed and dead-end pores are common features in organic soil. The objectives of this study were to determine the water-conducting porosity of organic soils for different pore radii ranges using the method proposed by Bodhinayake et al. (2004) [Soil Sci. Soc. Am. J. 68:760,769] and compare these values to active pore size distributions from resin-impregnated laboratory thin sections and pressure plate analysis. Field experiments and soil samples were completed in the Wolf Creek Research Basin, Yukon. Water infiltration rates were measured 16 times using a tension infiltrometer (TI) at 5 different pressure heads from , 150 to 0 mm. This data was combined with Gardiner's (1958) exponential unsaturated hydraulic conductivity function to provide water-conducting porosity for different pore-size ranges. Total water-conducting porosity was 1·1 × 10,4, which accounted for only 0·01% of the total soil volume. Active pore areas obtained from 2-D image analysis ranged from 0·45 to 0·60, declining with depth. Macropores accounted for approximately 65% of the water flux at saturation, yet all methods suggest macropores account for only a small fraction of the total porosity. Results among the methods are highly equivocal, and more research is required to reconcile field and laboratory methods of pore and hydraulic characteristics. However, this information is of significant value as organic soils in permafrost regions are poorly characterized in the literature. Copyright © 2007 John Wiley & Sons, Ltd. [source] Evaluation of pedotransfer functions predicting hydraulic properties of soils and deeper sedimentsJOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 2 2004Bernhard Wagner Abstract Eight pedotransfer functions (PTF) originally calibrated to soil data are used for evaluation of hydraulic properties of soils and deeper sediments. Only PTFs are considered which had shown good results in previous investigations. Two data sets were used for this purpose: a data set of measured pressure heads vs. water contents of 347 soil horizons (802 measured pairs) from Bavaria (Southern Germany) and a data set of 39 undisturbed samples of tertiary sediments from deeper ground (down to 100 m depth) in the molasse basin north of the Alps, containing 840 measured water contents vs. pressure head and unsaturated hydraulic conductivity. A statistical analysis of the PTFs shows that their performance is quite similar with respect to predicting soil water contents. Less satisfactory results were obtained when the PTFs were applied to prediction of water content of sediments from deeper ground. The predicted unsaturated hydraulic conductivities show about the same uncertainty as for soils in a previous study. Systematic deviations of predicted values indicate that an adaptation of the PTFs to the specific conditions of deeper ground should be possible in order to improve predictions. Bewertung von Pedotransferfunktionen zur Prognose der hydraulischen Kennwerte von Böden und tieferen Sedimenten Acht Pedotransferfunktionen (PTF), die ursprünglich anhand von Bodendaten kalibriert wurden, werden für die Prognose der hydraulischen Kennwerte sowohl von Böden als auch tieferen Sedimenten eingesetzt. Es wurden nur PTFs untersucht, die in anderen Untersuchungen gute Ergebnisse geliefert hatten. Zwei Datensätze standen für die Bewertung der PTFs zur Verfügung: ein Datensatz mit gemessenen Saugspannungen vs. Wassergehalten von 347 über ganz Bayern verteilten Bodenhorizonten (802 Messpaare) und ein Datensatz von 39 ungestörten Sedimentproben der miozänen Oberen Süßwassermolasse (OSM) des voralpinen Molassebeckens aus Tiefen von bis zu 100 m mit insgesamt 840 gemessenen Wassergehalten vs. Saugspannungen und ungesättigten Wasserleitfähigkeiten. Die statistische Analyse der acht PTFs zeigt, dass die meisten untersuchten PTFs die gemessenen Wassergehalte der Böden ungefähr gleich gut abschätzen. Alle PTFs ergaben bei der Vorhersage der Wassergehalte der tieferen Sedimente deutlich weniger gute Ergebnisse. Dennoch konnten mit den PTFs die ungesättigten Wasserleitfähigkeiten mit etwa der gleichen Genauigkeit wie bei Böden in einer früheren Studie prognostiziert werden. Systematische Abweichungen der Prognosewerte zeigen, dass eine spezifische Anpassung der PTFs auf die Bedingungen des tieferen Untergrundes zur Verbesserung der Vorhersagegenauigkeit möglich sein müsste. [source] Evaluation of the Impeller Shroud Performance of an Axial Flow Ventricular Assist Device Using Computational Fluid DynamicsARTIFICIAL ORGANS, Issue 9 2010Boyang Su Abstract Generally, there are two types of impeller design used in the axial flow blood pumps. For the first type, which can be found in most of the axial flow blood pumps, the magnet is embedded inside the impeller hub or blades. For the second type, the magnet is embedded inside the cylindrical impeller shroud, and this design has not only increased the rotating stability of the impeller but has also avoided the flow interaction between the impeller blade tip and the pump casing. Although the axial flow blood pumps with either impeller design have been studied individually, the comparisons between these two designs have not been conducted in the literature. Therefore, in this study, two axial flow blood pumps with and without impeller shrouds were numerically simulated with computational fluid dynamics and compared with each other in terms of hydraulic and hematologic performances. For the ease of comparison, these two models have the same inner components, which include a three-blade straightener, a two-blade impeller, and a three-blade diffuser. The simulation results showed that the model with impeller shroud had a lower static pressure head with a lower hydraulic efficiency than its counterpart. It was also found that the blood had a high possibility to deposit on the impeller shroud inner surface, which greatly enhanced the possibility of thrombus formation. The blood damage indices in both models were around 1%, which was much lower than the 13.1% of the axial flow blood pump of Yano et al. with the corresponding experimental hemolysis of 0.033 g/100 L. [source] Numerical Study of a Bio-Centrifugal Blood Pump With Straight Impeller Blade ProfilesARTIFICIAL ORGANS, Issue 2 2010Guoliang Song Abstract Computational fluid dynamic simulations of the flow in the Kyoto-NTN (Kyoto University, Kyoto, Japan) magnetically suspended centrifugal blood pump with a 16-straight-bladed impeller were performed in the present study. The flow in the pump was assumed as unsteady and turbulent, and blood was treated as a Newtonian fluid. At the impeller rotating speed of 2000 rpm and flow rate of 5 L/min, the pump produces a pressure head of 113.5 mm Hg according to the simulation. It was found that the double volute of the pump has caused symmetrical pressure distribution in the volute passages and subsequently caused symmetrical flow patterns in the blade channels. Due to the tangentially increasing pressure in the volute passages, the flow through the blade channels initially increases at the low-pressure region and then decreases due to the increased pressure. The reverse flow and vortices have been identified in the impeller blade channels. The high shear stress of the flow in the pump mainly occurred at the inlet and outlet of the blade channels, the beginning of the volute passages and the regions around the tips of the cutwater and splitter plate. Higher shear stress is obtained when the tips of the cutwater and splitter plate are located at the impeller blade trailing edges than when they are located at the middle of the impeller blade channel. It was found that the blood damage index assessed based on the blood corpuscle path tracing of the present pump was about 0.94%, which has the same order of magnitude as those of the clinical centrifugal pumps reported in the literature. [source] Estimation of Pump Flow Rate and Abnormal Condition of Implantable Rotary Blood Pumps During Long-Term In Vivo StudyARTIFICIAL ORGANS, Issue 4 2000K. Nakata Abstract: The control system for an implantable rotary blood pump is not clearly defined. A detection system is considered to be necessary for pump flow monitoring and abnormal conditions such as back flow or a sucking phenomenon where the septum or left ventricle wall is sucked into the cannula, etc. The ultrasound flowmeter is durable and reliable but the control system should not be totally dependent on the flowmeter. If the flowmeter breaks, the rotary blood pumps have no control mechanism. Therefore, the authors suggest controlling the pumps by an intrinsic parameter. One left ventricular assist device (LVAD) calf model was studied where the flow rate and waveform of the pump flow proved to identify the sucking phenomenon. Thus, the pump flow rate was calculated from the required power, motor speed, and heart rate. The value of the coefficient of determination (R2) between the measured and estimated pump flow rate was 0.796. To estimate this abnormal phenomenon, 2 methods were evaluated. One method was the total pressure head in which the pump flow rate and motor speed were estimated. During normal conditions the total pressure head is 79.5 ± 7.0 mm Hg whereas in the abnormal condition, it is 180.0 ± 2.8 mm Hg. There was a statistical difference (p < 0.01). Another method is using a current waveform. There is an association between the current and pump flow waves. The current was differentiated and squared to calculate the power of the differentiated current. The normal range of this value was 0.025 ± 0.029; the abnormal condition was 11.25 ± 15.13. There was a statistical difference (p < 0.01). The predicted flow estimation method and a sucking detection method were available from intrinsic parameters of the pump and need no sensors. These 2 methods are simple, yet effective and reliable control methods for a rotary blood pump. [source] Meltwater discharge through the subglacial bed and its land-forming consequences from numerical experiments in the Polish lowland during the last glaciationEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2009Jan A. Piotrowski Abstract Numerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in NW Poland: primarily its flow pattern, velocities and fluxes. We have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady-state and transient conditions. The results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin. All model runs show water discharge at the ice forefield driven by ice-sheet-thickness-modulated, down-ice-decreasing hydraulic heads. In relation to non-glacial times, the transient 3D model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca. 90 m below the ice,bed interface and up to 40 km in front of the glacier. Comparison with empirical data suggests that, depending on the model run, only between 5 and 24% of the meltwater formed at the ice sole drained through the bed as groundwater. This is consistent with field observations documenting abundant occurrence of tunnel valleys, indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system. Groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely. In these areas the relief shows distinct palaeo-ice lobes, indicating fast ice flow, possibly triggered by the undrained water at the ice,bed interface. Owing to the abundance of low-permeability strata in the bed, the simulated groundwater flow depth is less than ca. 200 m. Copyright © 2009 John Wiley & Sons, Ltd. [source] Rainfall thresholds for shallow landsliding derived from pressure-head monitoring: cases with permeable and impermeable bedrocks in Boso Peninsula, JapanEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2007Yuki Matsushi Abstract Rainfall thresholds for shallow landslide initiation were determined for hillslopes with two types of bedrock, permeable sandstone and impermeable mudstone, in the Boso Peninsula, Japan. The pressure-head response to rainfall was monitored above a slip scarp due to earlier landslides. Multiple regression analysis estimated the rainfall thresholds for landsliding from the relation between the magnitude of the rainfall event and slope instability caused by the increased pressure heads. The thresholds were expressed as critical combinations of rainfall intensity and duration, incorporating the geotechnical properties of the hillslope materials and also the slope hydrological processes. The permeable sandstone hillslope has a greater critical rainfall and hence a longer recurrence interval than the impermeable mudstone hillslope. This implies a lower potential for landsliding in sandstone hillslopes, corresponding to lower landslide activity. Copyright © 2007 John Wiley & Sons, Ltd. [source] Non-equilibrium water flow characterized by means of upward infiltration experimentsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 1 2001Summary Upward infiltration experiments under tension were used to demonstrate the presence of non-equilibrium flow in soils, the phenomenon that has important implications for the accelerated movement of fertilizers, pesticides, non-aqueous liquids, and other pollutants. Data obtained from these experiments were analysed using the single-porosity Richards equation, as well as a variably saturated, dual-porosity model and a dual-permeability model for characterizing non-equilibrium water flow. The laboratory experiments were carried out on 0.10-m-long soil cores having an internal diameter of 0.10 m. Constant pressure heads of ,0.10 and ,0.01 m were used as the lower boundary condition. Each infiltration was followed by a single-rate evaporation experiment to re-establish initial conditions, and to obtain the drying soil hydraulic properties. Pressure heads inside the cores were measured using five tensiometers, while evaporative water loss from the top was determined by weighing the soil samples. The data were analysed to estimate parameters using a technique that combined a numerical solution of the governing flow equation (as implemented in a modified version of the Hydrus-1D software) with a Marquardt,Levenberg optimization. The objective function for the parameter estimation was defined in terms of pressure head readings, the cumulative infiltration rate, and the final total water volume in the core during upward infiltration. The final total water volume was used, as well as the pressure head readings during the evaporation part. Analysis of flow responses obtained during the infiltration experiment demonstrated significant non-equilibrium flow. This behaviour could be well characterized using a model of physical non-equilibrium that divides the medium into inter- and intra-aggregate pores with first-order transfer of water between the two systems. The analysis also demonstrated the importance of hysteresis. [source] Field and laboratory estimates of pore size properties and hydraulic characteristics for subarctic organic soilsHYDROLOGICAL PROCESSES, Issue 19 2007Sean K. Carey Abstract Characterizing active and water-conducting porosity in organic soils in both saturated and unsaturated zones is required for models of water and solute transport. There is a limitation, largely due to lack of data, on the hydraulic properties of unsaturated organic soils in permafrost regions, and in particular, the relationship between hydraulic conductivity and pressure head. Additionally, there is uncertainty as to what fraction of the matrix and what pores conduct water at different pressure heads, as closed and dead-end pores are common features in organic soil. The objectives of this study were to determine the water-conducting porosity of organic soils for different pore radii ranges using the method proposed by Bodhinayake et al. (2004) [Soil Sci. Soc. Am. J. 68:760,769] and compare these values to active pore size distributions from resin-impregnated laboratory thin sections and pressure plate analysis. Field experiments and soil samples were completed in the Wolf Creek Research Basin, Yukon. Water infiltration rates were measured 16 times using a tension infiltrometer (TI) at 5 different pressure heads from , 150 to 0 mm. This data was combined with Gardiner's (1958) exponential unsaturated hydraulic conductivity function to provide water-conducting porosity for different pore-size ranges. Total water-conducting porosity was 1·1 × 10,4, which accounted for only 0·01% of the total soil volume. Active pore areas obtained from 2-D image analysis ranged from 0·45 to 0·60, declining with depth. Macropores accounted for approximately 65% of the water flux at saturation, yet all methods suggest macropores account for only a small fraction of the total porosity. Results among the methods are highly equivocal, and more research is required to reconcile field and laboratory methods of pore and hydraulic characteristics. However, this information is of significant value as organic soils in permafrost regions are poorly characterized in the literature. Copyright © 2007 John Wiley & Sons, Ltd. [source] | |||||||||||||