Transit Time Estimation Research Articles

Quasi-two-dimensional properties of a single shallow-water vortex with high initial Reynolds numbers

The evolution and dynamics of a shallow-water vortex system with high initial Reynolds numbers are investigated experimentally without background rotation. A single vortex is generated by rotating a water mass at the centre of an experimental tank using a bottomless cylinder with internal sectors. The surface velocity field is observed via particle image velocimetry. The experimentally observed vorticity fields indicate that strong shallowness (the ratio of the cylinder diameter to the water depth) and high Reynolds number contribute to the formation of large-scale coherent structures in the form of a tripolar vortex system. The shallow-water vortices with high initial Reynolds numbers experience the transition from turbulent to laminar regimes in their decay process. The proposed first-order vortex decay model predicts that a shallow-water vortex decays as t−1 in the initial turbulent stage and as e−t in the later laminar stage due to horizontal diffusion and bottom friction. The estimated transition time scale from the turbulent to laminar stage increases with initial vortex Reynolds number and with shallowness. By taking the vortex expansion into consideration, the second-order vortex decay model is also presented. The azimuthally ensemble-averaged data elucidate effects of the vortex instabilities and of turbulent energy transfer on the formation of large-scale coherent flow structures. Normal mode analysis of the vortex systems is conducted to study the effect of shallowness and Reynolds number on the generation of two-dimensional large-scale coherent structures. The results show that the perturbation wavenumber of mode 2 is the fastest-growing instability in shallow-water conditions, and its effect depends on initial Reynolds number and shallowness.

Gamma distribution models for transit time estimation in catchments: Physical interpretation of parameters and implications for time‐variant transit time assessment

In hydrological tracer studies, the gamma distribution can serve as an appropriate transit time distribution (TTD) as it allows more flexibility to account for nonlinearities in the behavior of catchment systems than the more commonly used exponential distribution. However, it is unclear which physical interpretation can be ascribed to its two parameters (α, β). In this study, long‐term tracer data from three contrasting catchments in the Scottish Highlands were used for a comparative assessment of interannual variability in TTDs and resulting mean transit times (MTT = αβ) inferred by the gamma distribution model. In addition, spatial variation in the long‐term average TTDs from these and six additional catchments was also assessed. The temporal analysis showed that the β parameter was controlled by precipitation intensities above catchment‐specific thresholds. In contrast, the α parameter, which showed little temporal variability and no relationship with precipitation intensity, was found to be closely related to catchment landscape organization, notably the hydrological characteristics of the dominant soils and the drainage density. The relationship between β and precipitation intensity was used to express β as a time‐varying function within the framework of lumped convolution integrals to examine the nonstationarity of TTDs. The resulting time‐variant TTDs provided more detailed and potentially useful information about the temporal dynamics and the timing of solute fluxes. It was shown that in the wet, cool climatic conditions of the Scottish Highlands, the transit times from the time‐variant TTD were roughly consistent with the variations of MTTs revealed by interannual analysis.

Estimation of perfusion and arterial transit time in myocardium using free‐breathing myocardial arterial spin labeling with navigator‐echo

Arterial spin labeling (ASL) provides noninvasive measurement of tissue blood flow, but sensitivity to motion has limited its application to imaging of myocardial blood flow. Although different cardiac phases can be synchronized using electrocardiography triggering, breath holding is generally required to minimize effects of respiratory motion during ASL scanning, which may be challenging in clinical populations. Here a free-breathing myocardial ASL technique with the potential for reliable clinical application is presented, by combining ASL with a navigator-gated, electrocardiography-triggered TrueFISP readout sequence. Dynamic myocardial perfusion signals were measured at multiple delay times that allowed simultaneous fitting of myocardial blood flow and arterial transit time. With the assist of a nonrigid motion correction program, the estimated mean myocardial blood flow was 1.00 ± 0.55 mL/g/min with a mean transit time of ∼ 400 msec. The intraclass correlation coefficient of repeated scans was 0.89 with a mean within subject coefficient of variation of 22%. Perfusion response during mild to moderate stress was further measured. The capability for noninvasive, free-breathing assessment of myocardial blood flow using ASL may offer an alternative approach to first-pass perfusion MRI for clinical evaluation of patients with coronary artery disease.

Reduction of errors in ASL cerebral perfusion and arterial transit time maps using image de‐noising

In this work, the performance of image de-noising techniques for reducing errors in arterial spin labeling cerebral blood flow and arterial transit time estimates is investigated. Simulations were used to show that the established arterial spin labeling cerebral blood flow quantification method exhibits the bias behavior common to nonlinear model estimates, and as a result, the reduction of random errors using image de-noising can improve accuracy. To assess the effect on precision, multiple arterial spin labeling data sets acquired from the rat brain were processed using a variety of common de-noising methods (Wiener filter, anisotropic diffusion filter, gaussian filter, wavelet decomposition, and independent component analyses). The various de-noising schemes were also applied to human arterial spin labeling data to assess the possible extent of structure degradation due to excessive spatial smoothing. The animal experiments and simulated data show that noise reduction methods can suppress both random and systematic errors, improving both the precision and accuracy of cerebral blood flow measurements and the precision of transit time maps. A number of these methods (and particularly independent component analysis) were shown to achieve this aim without compromising image contrast.

Fallout deposition in the Marshall Islands from Bikini and Enewetak nuclear weapons tests.

Deposition densities (Bq m(-2)) of all important dose-contributing radionuclides occurring in nuclear weapons testing fallout from tests conducted at Bikini and Enewetak Atolls (1946-1958) have been estimated on a test-specific basis for 32 atolls and separate reef islands of the Marshall Islands. A complete review of various historical and contemporary data, as well as meteorological analysis, was used to make judgments regarding which tests deposited fallout in the Marshall Islands and to estimate fallout deposition density. Our analysis suggested that only 20 of the 66 nuclear tests conducted in or near the Marshall Islands resulted in substantial fallout deposition on any of the 23 inhabited atolls. This analysis was confirmed by the fact that the sum of our estimates of 137Cs deposition from these 20 tests at each atoll is in good agreement with the total 137Cs deposited as estimated from contemporary soil sample analyses. The monitoring data and meteorological analyses were used to quantitatively estimate the deposition density of 63 activation and fission products for each nuclear test, plus the cumulative deposition of 239+240Pu at each atoll. Estimates of the degree of fractionation of fallout from each test at each atoll, as well as of the fallout transit times from the test sites to the atolls were used in this analysis. The estimates of radionuclide deposition density, fractionation, and transit times reported here are the most complete available anywhere and are suitable for estimations of both external and internal dose to representative persons as described in companion papers.

Runoff generating processes in adjacent tussock grassland and pine plantation catchments as indicated by mean transit time estimation using tritium

Abstract. The east Otago uplands of New Zealand's South Island have long been studied because of the environmental consequences of converting native tussock grasslands to other land covers, notably forestry and pasture for stock grazing. Early studies showed that afforestation substantially reduced annual water yield, stream peak flows, and 7-day low flows, mainly as a consequence of increased interception. Tritium measurements have indicated that surprisingly old water is present in catchments GH1 and GH2, and the small headwater wetland and catchment (GH5), and contributes strongly to baseflow. The data have been simulated assuming the presence of two types of water in the baseflow, young water from shallow aquifers connecting hillside regolith with the stream, and old water from deep bedrock aquifers, respectively. The mean transit time of the young water is approximately one month, while that of the old water is 25–26 years as revealed by the presence of tritium originating from the bomb-peak in NZ rainfall in late 1960s and early 1970s. Such a long transit time indicates slow release from groundwater reservoirs within the bedrock, which constitute by far the larger of the water stores. Comparison of the results from catchments GH1 (tussock) and GH2 (pine forest) suggests that about equal quantities of water (85 mm/a) are contributed from the deep aquifers in the two catchments, although runoff from the shallow aquifers has been strongly reduced by afforestation in GH2. This study has revealed the presence of a long transit time component of water in runoff in a catchment with crystalline metamorphic bedrock.

Wireless Capsule Endoscopy and Endoscopic Imaging: A Survey on Various Methodologies Presented

Wireless capsule endoscopy (WCE) is a recently established methodology that offers to medical doctors (gastroenterologists) the capability to examine the interior of the small intestine with a noninvasive procedure. Before the introduction of WCE, it was impossible for a physician to examine tissues of the small intestine without performing a surgical operation. Although WCE has the advantage of investigating the whole digestive system, the viewing and evaluation of each WCE video is a time-consuming process (2-3 h) for MD gastroenterologists. This makes the WCE methodology not widely efficient and acceptable by MDs.

On the role of aspect to quantify water transit times in small mountainous catchments

In the current empirical study, we provide evidence about how the hydrologic responses of small mountain catchments are related to aspect (slope direction and exposure) at Redondo Peak, located in the Valles Caldera, New Mexico, United States. Specifically, we test the hypothesis that the transit time of water is related to the catchment aspect. Aspect is an easily measurable and transferable topographic characteristic that is related to the amount of direct solar radiation a particular catchment receives, and therefore, different catchments with different aspects have different rates of snow ablation, evapotranspiration, and water cycling in general. Transit times, which describe the time between when water enters the catchment as precipitation and when it leaves as stream flow, captures many hydrologic features such as flow path variability and the combined effects of water storage and water fluxes. We have designed an experiment that involves field data collection, isotopic analysis of stream and precipitation samples, and the estimation of transit times using lumped‐parameter convolution for 15 sites in small (1–15 km2) catchments that drain different aspects of Redondo Peak. Our data suggest that isotopic variability and estimated transit times are both related to aspect. Other potential relationships between topographical features (such as flow path length, slope gradient, and elevation) and isotopic measurements of stream water suggest that landscape and hydrological features are interconnected at Redondo Peak, but these links are not conclusive, suggesting that these topographic indicators do not fully explain the variability of water cycling in these small mountain catchments.

Transit time evaluation using a chloride concentration input step shift after forest cutting in a Japanese headwater catchment

AbstractMean transit times were estimated for a small headwater catchment in Japan (the Fukuroyamasawa Experimental Watershed) using the step shift in input chloride (Cl−) concentrations that occurred immediately after an episode of forest clear‐cutting. Measured Cl− concentrations in stream water began to decrease immediately after clear‐cutting, and this trend continued for 6 years. Before clear‐cutting, the input Cl− concentrations were controlled by wet and dry deposition processes, and most of the dry Cl− deposition was collected by the forest canopy and reached the ground as throughfall and stemflow. After clear‐cutting, dry deposition was no longer collected by the canopy in this way, thus causing a sharp decrease in input Cl− concentrations. By comparing measured Cl− concentrations in stream water with estimates based on the input and evaporative Cl− concentrations, it was shown that the decrease in stream water Cl− concentrations was caused mainly by this step shift in the Cl− input. It was proposed that the change in Cl− concentrations after forest cutting could be used to represent the replacement of ‘old’ water that existed before cutting by ‘new’ water that was supplied after cutting. The breakthrough curve for the new water fraction gave an approximately exponential distribution of transit times in flow‐corrected time. The mean flow‐corrected transit time was estimated as 1068 days (runoff: 3497 mm). It was therefore concluded that the step change in input Cl− concentrations immediately following forest clear‐cutting could be successfully used to estimate transit times for the entire catchment. Copyright © 2009 John Wiley & Sons, Ltd.

Regionalization of transit time estimates in montane catchments by integrating landscape controls

Mean transit time (MTT) is being increasingly used as a metric of hydrological function in intercatchment comparisons. Estimating MTT usually involves relating the temporally varying input concentration of a conservative tracer to the signal in the stream using various transfer functions as transit time distributions (TTDs). Most studies have been confined to data collection periods of 1–2 years at single sites, often limiting the transferability of the findings as such short periods usually only capture a narrow range of climatic variability within a spatially restricted area. In this study, we use longer‐term (up to 17 years) weekly input‐output relationships of Cl− to estimate MTTs using a range of TTD models in 20 headwater catchments (ranging from <1 to 35 km2) in seven geomorphologically and climatically distinct parts of the Scottish Highlands. The MTTs obtained from a Gamma distribution model were the best identified and ranged from about 50 to 1700 days for individual catchments. The MTTs, in conjunction with GIS analysis of landscape characteristics and climatic indices, allowed the development of a robust multiple‐regression model to establish the relative importance of different landscape and climate controls on MTTs. The best model combines the prediction variables percent responsive soil cover, drainage density, precipitation intensity, and topographic wetness index and yields R2adj = 0.88. Cross validation shows small absolute error, suggesting that the model can be used to estimate MTTs in ungauged headwater catchments throughout the Scottish Highlands and potentially in similar regions where comparable information is available.

Spatiotemporal Visualizations for the Measurement of Oropharyngeal Transit Time From Videofluoroscopy

Videofluoroscopy remains one of the mainstay methods for clinical swallowing assessment, yet its interpretation is both complex and subjective. This, in part, reflects the difficulties associated with estimation of bolus transit time through the oral and pharyngeal regions by visual inspection, and problems with consistent repeatability. This paper introduces a software-only framework that automatically determines the time taken for the bolus to cross 1-D anatomical landmarks representing the oral and pharyngeal region boundaries ( Fig. 1). The user-steered delineation algorithm live-wire and straight-line annotators are used to demarcate the landmark on a frame prior to the swallow action. The rate of change of intensity of the pixels in each landmark is used as the detection feature for bolus presence that can be visualized on a spatiotemporal plot. Artifacts introduced by head and neck movement are removed by updating the landmark coordinates using affine parameters optimized by a genetic-algorithm-based registration method. Heuristics are applied to the spatiotemporal plot to identify the frames during which the bolus passes the landmark. Correlation coefficients between three observers visually inspecting twenty-four 5-mL single swallow clips did not exceed 0.42. Yet the same measurements taken using this framework on the same clips had correlation coefficients exceeding 0.87.

A model for large glacial–interglacial climate-induced changes in dust and sea salt concentrations in deep ice cores (central Antarctica): palaeoclimatic implications and prospects for refining ice core chronologies

A semi-empirical model has been developed to reproduce glacial–interglacial changes of continental dust and marine sodium concentrations (factor of ∼50 and ∼5, respectively) observed in inland Antarctic ice cores. The model uses conceptual pathways of aerosols within the high troposphere; assumes the dry deposition of impurities on the Antarctic surface; uses estimates of aerosol transit times taken independent of climate; assumes a temperature-dependent removal process during aerosol pathways from the mid-latitudes. The model is fitted to the data over the last four climate cycles from Vostok and EPICA Dome C Antarctic sites. As temperature is cooling, the aerosol response suggests different modes of climate couplings between latitudes, which can be continuous or below temperature thresholds for sodium and dust, respectively. The model estimates a southern South America dust source activity two to three times higher for glacial periods than for the Holocene and a glacial temperature over the Southern Ocean 3–5 ◦C cooler. Both estimates appear consistent with independent observations. After removal of temperature effects, dust and sodium residuals for both sites show orbital frequencies in opposite phase at the precession timescale. Such long-term insolation-related modulation of terrestrial and marine aerosol input, could provide a chemical pacemaker useful for refining ice core chronologies.

Multiple isotope approach to the determination of the natural attenuation potential of a high-alpine karst system

Summary 3 H, δ 18 O and δ 2 H measurements on groundwater and chemical and isotope analyses on groundwater sulfate were used to determine the origin and mean transit times of groundwater and the sources affecting groundwater sulfate in a high-alpine karstic catchment area in southern Germany. Modelling results using a lumped parameter approach yielded mean transit times of water between 4 and 12 years for different karst springs. However, results obtained from δ 18 O and δ 2 H measurements on groundwater showed that a calculated mean transit time of 12 years for groundwater flow in one karst system can only be explained by mixing of young and old tritium-free ice and snowmelt water. Groundwater sulfate characterized by δ 34 S values of around 20‰ and δ 18 O values of ca. 12‰ in concert with sulfate concentrations of approximately 11 mg/L are probably affected by evaporites. In contrast, δ 34 S values of around 6‰ and δ 18 O values up to 9.4‰ in concert with sulfate concentrations between 1 and 7 mg/L mainly derive from atmospheric deposition. However, a δ 34 S value of −7.3‰ identified for one of the karst springs demonstrates that the oxidation of sulfide-containing minerals must be considered as an additional source, affecting sulfate concentrations in groundwater. The estimated transit time distribution of groundwater shows relatively high mean transit times between 2 and 5 years with a low contribution of very short transit times of less than 1 year. The high contribution of long mean transit times of groundwater in the alpine karst system, which represents an important drinking water resource, is an important function for drinking water supplies.

Multiphasic contrast injection for improved precision of parameter estimates in functional CT

Contrast injection protocol is known to affect the estimation of kinetic parameters in functional CT. A novel method is proposed to maximize the precision of parameter estimates by modulating the contrast injection scheme. The method models the intravenous contrast bolus to be dispersed by a "patient function" to give rise to the arterial input function, which, in turn, carries the contrast agent to tissue leading to contrast enhancement. The covariance matrix analysis was applied to calculate the uncertainty of parameter estimates as the coefficients of variation (CV) in the adiabatic tissue homogeneity (ATH), two-compartment, and the modified Kety model in which tumor pathophysiology is modeled. An optimization scheme was used to determine the optimal injection protocol which would minimize the CV of a particular kinetic parameter. For clinical utility, a recommended injection protocol was suggested from a statistical analysis with the optimal injection protocols obtained from the first group of 12 patients with cervix cancer. The efficacy of the recommended injection protocol was tested with a second group of 12 patients. In addition, the robustness of the recommended injection protocol to longitudinal study has been investigated in the presence of variations in arterial input function and tumor pathophysiology. Based on the data of the second group of patients, and using the ATH model, the recommended biphasic injection of two boluses improves the precision in the estimation of blood flow and mean transit time (MTT), by 36.9% and 38.4%, respectively, compared to the standard uniphasic injection protocol in the CV. However, measurement of the permeability surface area product and extravascular extracellular space volume favors a single fast bolus of the same contrast amount. The two-compartment model and the modified Kety model also benefited from the single fast bolus. The effect of variation in the arterial input function and tumor pathophysiology on the applicability of the recommended injection was also investigated. Based on computer simulation for a range of variations in the arterial input function and pathophysiology, the recommended biphasic injection was found to improve the precision in blood flow and MTT estimates by 31.4% and 36.5% on average, respectively, compared to the uniphasic injection.

Susceptibility to Enhanced Chemical Migration from Depression‐Focused Preferential Flow, High Plains Aquifer

Aquifer susceptibility to contamination is controlled in part by the inherent hydrogeologic properties of the vadose zone, which includes preferential‐flow pathways. The purpose of this study was to investigate the importance of seasonal ponding near leaky irrigation wells as a mechanism for depression‐focused preferential flow and enhanced chemical migration through the vadose zone of the High Plains aquifer. Such a mechanism may help explain the widespread presence of agrichemicals in recently recharged groundwater despite estimates of advective chemical transit times through the vadose zone from diffuse recharge that exceed the historical period of agriculture. Using a combination of field observations, vadose zone flow and transport simulations, and probabilistic neural network modeling, we demonstrated that vadose zone transit times near irrigation wells range from 7 to 50 yr, which are one to two orders of magnitude faster than previous estimates based on diffuse recharge. These findings support the concept of fast and slow transport zones and help to explain the previous discordant findings of long vadose zone transit times and the presence of agrichemicals at the water table. Using predictions of aquifer susceptibility from probabilistic neural network models, we delineated approximately 20% of the areal extent of the aquifer to have conditions that may promote advective chemical transit times to the water table of <50 yr if seasonal ponding and depression‐focused flow exist. This aquifer‐susceptibility map may help managers prioritize areas for groundwater monitoring or implementation of best management practices.

A system for route optimization in ice-covered waters

Information about ice is indispensable to navigation in ice-covered sea areas. For vessels traveling long distances in ice, it is worth planning routes that will reduce fuel consumption and travel time, as well as the risk of ending up in hazardous areas or getting stuck in the ice. In addition to observations on board, there is a multitude of data sources available for seafarers like satellite images, ice model data, weather observations and forecasts. However, it is difficult for a human to take into consideration all the time-varying data parameters when planning a route. In this paper, a prototype system for optimizing routes through the ice field is presented. The system integrates state-of-the-art ice modeling, ship transit modeling, and an end-user system as a route optimization tool for vessels navigating in ice-covered waters. The system has recently been validated on board merchant vessels in the Baltic Sea, and the system's performance has been analyzed statistically using AIS data. Based on the AIS data analysis the mean relative error of the estimated transit time was 0.144 [s/s] with a standard deviation of 0.147 [s/s] for long routes (90-650 km), and 0.018 [s/s] with standard deviation of 0.193 [s/s] for 50 km route segments.

Complete post-separation of overlapping ultrasonic signals by combining hard and soft modeling

In some ultrasonic measurement situations, an adequate signal separation is difficult to achieve. A typical situation is material characterization of thin media using pulse-echo or through-transmission techniques, when the time-of-flight in the media is shorter than the emitted signal’s time support. Separated signals are necessary to obtain accurate estimates of material properties and transit times. In this paper a new method is proposed that enables complete post-separation of measured coinciding signals. The method is based on a combination of hard physical and soft empirical models, which allows for a description of both known and unknown properties making a complete separation possible. The validity and limitations of the model and the separation results are thoroughly addressed. The proposed technique is verified using real measurements on thin dispersive samples and validated using residual analysis. The experimental results show a complete separation with uncorrelated and normally distributed residuals. The method enables characterization and/or flow analysis in difficult overlapping situations.

Age and the time lag method

The time lag method is one of the most straightforward methods used to estimate transit times from experimental data and is therefore widely used. The transit time between two points is estimated from the analysis of time series taken at these two points that suggest the propagation of a signal from one point to the other. To account for the distortion of the signal during its propagation between the two points an optimum time lag can be estimated by the analysis of the cross-correlation of the two time series. This study clarifies the relation between the transit time estimated by the time lag method and the well-defined concept of the age of a water mass. It is shown, through simplified process models, that the time lag method systematically underestimates the true mean age. The error can be quantified by means of a dimensionless parameter which is the inverse of a Peclet number based on a characteristic length given by the ratio of the velocity of the flow to the frequency of the signal.

Automated Topographic Segmentation and Transit Time Estimation in Endoscopic Capsule Exams

Endoscopic capsule is a recent medical technology with important clinical benefits but suffering from a practical handicap: long exam annotation times. This paper proposes and compares two approaches (Bayesian and support vector machines) that can be used to segment the gastrointestinal tract into its four major topographic areas, allowing the automatic estimation of the clinically relevant gastric and intestinal sections and corresponding transit times. According to medical specialists, this can reduce exam annotation times by up to 12% (15 min). This automatic tool has been integrated into our CapView annotation software that is currently being used by three medical institutions.

P-278 Estimation of the intestinal transit time by radiopaque markers(Poster Presentation Gastrointestinal function, constipation,Better Life for Sick Children, Better Future for Pediatric Surgery,the 45th Annual Meeting of Japanese Society of Pediatric Surgeons)

P-278 Estimation of the intestinal transit time by radiopaque markers(Poster Presentation Gastrointestinal function, constipation,Better Life for Sick Children, Better Future for Pediatric Surgery,the 45th Annual Meeting of Japanese Society of Pediatric Surgeons)