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At least 127 records · Page 7Linked to original sources
PubMed2002

Modeling diffuse pollution with a distributed approach.

The transferability of parameters for non-point source pollution models to other watersheds, especially those in remote areas without enough data for calibration, is a major problem in diffuse pollution modeling. A water quality component was developed for WATFLOOD (a flood forecast hydrological model) to deal with sediment and nutrient transport. The model uses a distributed group response unit approach for water quantity and quality modeling. Runoff, sediment yield and soluble nutrient concentrations are calculated separately for each land cover class, weighted by area and then routed downstream. The distributed approach for the water quality model for diffuse pollution in agricultural watersheds is described in this paper. Integrating the model with data extracted using GIS technology (Geographical Information Systems) for a local watershed, the model is calibrated for the hydrologic response and validated for the water quality component. With the connection to GIS and the group response unit approach used in this paper, model portability increases substantially, which will improve non-point source modeling at the watershed scale level.

Agriculture↗
PubMed1991

Ca2+ movement in smooth muscle cells studied with one- and two-dimensional diffusion models.

Although many of the processes involved in the regulation of Ca2+ in smooth muscle have been studied separately, it is still not well known how they are integrated into an overall regulatory system. To examine this question and to study the time course and spatial distribution of Ca2+ in cells after activation, one- and two-dimensional diffusion models of the cell that included the major processes thought to be involved in Ca regulation were developed. The models included terms describing Ca influx, buffering, plasma membrane extrusion, and release and reuptake by the sarcoplasmic reticulum. When possible these processes were described with known parameters. Simulations with the models indicated that the sarcoplasmic reticulum Ca pump is probably primarily responsible for the removal of cytoplasmic Ca2+ after cell activation. The plasma membrane Ca-ATPase and Na/Ca exchange appeared more likely to be involved in the long term regulation of Ca2+. Pumping processes in general had little influence on the rate of rise of Ca transients. The models also showed that spatial inhomogeneities in Ca2+ probably occur in cells during the spread of the Ca signal following activation and during the subsequent return of Ca2+ to its resting level.

Animals↗
PubMed2005

Sorption of cadmium, copper, and zinc ions onto bone char using Crank diffusion model.

The rates of adsorption of cadmium, copper and zinc ions onto bone char have been studied in three single component systems using an agitated batch adsorber. The effects of initial metal ion solution concentration and bone char mass have been studied. The capacities of the three metals on bone char are 0.477, 0.709 and 0.505 mmol g(-1) for cadmium, copper and zinc respectively which are much higher than most other adsorbents reported in the literature for these three metals. In order to model the experimental concentration decay curves, a film-surface diffusion equation proposed by has been solved. The model results correlate the experimental better than a previously applied film-pore diffusion model. However, the results indicate that the surface diffusion coefficient was a variable function of initial metal ion concentration. The surface diffusivities vary from 3.00 x 10(-9) to 8.80 x 10(-9) cm(2)s(-1), 2.11 x 10(-9) cm(2)s(-1) to 5.4 x 10(-9) cm(2)s(-1) and 3.10 x 10(-9) to 8.49 x 10(-9) cm(2)s(-1) for cadmium, copper and zinc respectively, for metal ion solution concentrations of 2.10-5.40 mmol dm(-3).

Adsorption↗
PubMed1996

Theoretical design of prodrug-enhancer combination based on a skin diffusion model: prediction of permeation of acyclovir prodrugs treated with 1-geranylazacycloheptan-2-one.

PURPOSE: A theoretical design of percutaneous penetration enhancement in which prodrug derivation and enhancer application are combined is proposed based on the skin diffusion model and it is experimentally verified. METHODS: Employing acyclovir as a model drug, the hypothesis was tested by synthesis of its prodrugs and evaluation of their in vitro permeation in the rat skin, with or without a penetration enhancer, 1-geranylazacycloheptan-2-one(GACH). RESULTS: Among five acyclovir prodrugs, those with higher lipophilicities (propionate, butyrate, valerate, and hexanoate prodrugs) showed greater skin penetration than those of hydrophilic prodrugs (acetate), when administered in combination with GACH. Furthermore, the observed enhancement ratios were in good agreement with those predicted by theoretical consideration. CONCLUSIONS: Thus, skin permeation of prodrugs applied with an enhancer can be predicted and optimized by model analysis.

Acyclovir↗
PubMed1996

A reaction-diffusion model of cancer invasion.

We present mathematical analyses, experimental data, and clinical observations which support our novel hypothesis that tumor-induced alteration of microenvironmental pH may provide a simple but complete mechanism for cancer invasion. A reaction-diffusion model describing the spatial distribution and temporal development of tumor tissue, normal tissue, and excess H+ ion concentration is presented. The model predicts a pH gradient extending from the tumor-host interface, which is confirmed by reanalysis of existing experimental data. Investigation of the structure and dynamics of the tumor-host interaction within the context of the model demonstrates a transition from benign to malignant growth analogous to the adenoma-carcinoma sequence. The effect of biological parameters critical to controlling this transition are supported by experimental and clinical observations. Tumor wave front velocities determined via a marginal stability analysis of the model equations are consistent with in vivo tumor growth rates. The model predicts a previously unrecognized hypocellular interstitial gap at the tumor-host interface which we demonstrate both in vivo and in vitro. A direct correlation between the interfacial morphology and tumor wave front velocity provides an explicit, testable, clinically important prediction.

Carcinoma, Squamous Cell↗
PubMed2001

Film-pore diffusion model for the fixed-bed sorption of copper and cadmium ions onto bone char.

The sorption of copper and cadmium ions onto bone char in single component systems has been studied using fixed-bed column adsorbers. The effects of solution flowrate, initial metal ion concentration and bone char particle size have been studied. A film-pore diffusion model has been developed to predict the fixed-bed breakthrough curves for the two metal ions. A sensitivity analysis has been carried out to investigate the influence of the external mass transfer coefficient (film resistance), the effective diffusion coefficient (pore diffusion) and the solid phase loading capacity. It is found that under the experimental conditions employed in the study, film diffusional resistance was low and the Biot numbers were relatively high. Furthermore, a constant effective pore diffusivity was not sufficient to correlate the breakthrough curves accurately and a variable dependent effective diffusivity was required; suggesting a possible contribution from surface diffusion. Since the metal ion-bone char systems take a long time to reach equilibrium, the solid phase loading capacity, as predicted by the "best-fit" equilibrium isotherm, was not suitable for use in the diffusional mass transport model and the mass balance solid phase loading was utilised instead.

Adsorption↗
PubMed2002

Comparison of holographic photopolymer materials by use of analytic nonlocal diffusion models.

The one-dimensional diffusion equation governing holographic grating formation in photopolymers, which includes both nonlocal material response and generalized dependence of the rate of polymerization on the illuminating intensity, has been previously solved under the two-harmonic expansion assumption. The resulting analytic expressions for the monomer and polymer concentrations have been derived and their ranges of validity tested in comparison with the more accurate numerical four-harmonic case. We used these analytic expressions to carry out a study of experimental results presented in the literature over a 30-year period. Automatic fitting of the data with these formulas allows material parameters, including the nonlocal chain-length variance sigma, to be estimated. In this way, (i) a quantitative comparison of different materials can be made, and (ii) a standard form of experimental result presentation is proposed to facilitate such a procedure.

Journal Article↗
PubMed2002

Modeling diffusion of innovations in a social network.

A simple model of diffusion of innovations in a social network with upgrading costs is introduced. Agents are characterized by a single real variable, their technological level. According to local information, agents decide whether to upgrade their level or not, balancing their possible benefit with the upgrading cost. A critical point where technological avalanches display a power-law behavior is also found. This critical point is characterized by a macroscopic observable that turns out to optimize technological growth in the stationary state. Analytical results supporting our findings are found for the globally coupled case.

Journal Article↗
PubMed2003

Fractional diffusion model for force distribution in static granular media.

We present the results of a numerical and an experimental investigation of the probability distribution of normal contact forces in static packs of particles with two different hardnesses. Force distributions are computed and compared with existing models and experimental data. It is found that the probability distribution function of normal contact forces P(f) is well described by a semiempirical model derived from a fractional diffusion equation. This model reproduces most of the features common to force distributions observed in experimental and numerical studies including the finite value for P(f) as the forces tend to zero. The results indicate that the fractional model fits well both the numerical and experimental data over a wide range of particle deformations in contrast to the existing models. These results provide an insight into the physics of granular media and complement previous findings.

Journal Article↗
PubMed1975

Convective diffusion model for a transport-controlled dissolution rate process.

A mathematical model based on convective diffusion was developed to describe the rate of dissolution form the surface of a compressed compact. Experimental studies were carried out to test the model. The basic experimental apparatus consisted of a modified rotating-filter-stationary basket dissolution test apparatus. Dissolution rates from rectangular and circular surfaces of an homologous series of p-aminobenzoate esters permitted testing the theory with respect to solubility, geometry, and agitation conditions. The correlation between experimental results and theory was reasonalby good considering that the test conditions were somewhat less than ideal.

Aminobenzoates↗
PubMed2001

Quantitative evaluation of capacity-limited hepatobiliary transport based on hepatocellular diffusion model by MULTI(FEM).

The dose-dependency of hepatic uptake and hepatobiliary transport of a drug was evaluated by means of a nonlinear least square program incorporating the finite element method, MULTI(FEM). A perfusion experiment using isolated rat livers following a pulse input (i.e., under nonsteady-state conditions) was performed at three dose levels of cefpiramide as a model drug. The hepatic extraction ratio (E(H)) of cefpiramide decreased with an increase in dose, which demonstrates that the hepatic uptake is capacity-limited. The outflow time-profiles from the liver were represented by a two-compartment dispersion model with central Michaelis-Menten elimination, and the maximal elimination rate per central compartment volume (Vmax) and the Michaelis constant (Km) were estimated to be 1420 microg/ml/min and 235 microg/ml, respectively. The biliary mean transit time (t(bile)) increased slightly with an increase in dose. The hepatocellular diffusion model under non-steady-state conditions considering nonlinear transport across the bile canalicular membrane was adopted to evaluate dose-dependency in the biliary excretion of cefpiramide. The maximal penetration velocity across the bile canalicular membrane per liver (V=(bcm)max) and the affinity constant of penetration across the bile canalicular membrane (k(bcm)m = K(bcm)m A(H)L) were estimated to be 40.1 microg/min and 123 microg, respectively. Considering that the volume of a rat liver (A(H)L) is approximately 10 ml, the Michaelis constant of penetration (K(bcm)m), which is an apparent parameter, was estimated to be approximately 12.3 microg/ml. In conclusion, MULTI(FEM) is useful for evaluation of capacity-limited local disposition.

Animals↗
PubMed2003

A diffusion model analysis of the effects of aging on brightness discrimination.

The effects of aging on decision time were examined in a brightness discrimination experiment with young and older subjects (ages, 60-75 years). Results showed that older subjects were slightly slower than young subjects but just as accurate. Ratcliff's (1978) diffusion model was fit to the data, and it provided a good account of response times, their distributions, and response accuracy. There was a 50-msec slowing of the nondecision components of response time for older subjects relative to young subjects, but response criteria settings and rates of accumulation of evidence from stimuli were roughly equal for the two groups. These results are contrasted with those obtained from letter discrimination and signal-detection-like tasks.

Aged↗
PubMed2003

A diffusion model analysis of the effects of aging on letter discrimination.

The effects of aging on accuracy and response time were examined in a letter discrimination experiment with young and older subjects. Results showed that older subjects (ages 60-75) were generally slower and less accurate than young subjects. R. Ratcliff's (1978) diffusion model was fit to the data, and it provided a good account of response times, their distributions, and response accuracy. The results produce similar age effects on the nondecision components of response time (about 50 ms slowing) and the response criteria (more conservative settings) to those from R. Ratcliff, A. Thapar, and G. McKoon (2001), but also show a reduced rate of accumulation of evidence for older subjects. The model-based approach has the advantage of allowing the separation of aging effects on different components of processing.

Aged↗
PubMed2004

A diffusion model analysis of the effects of aging in the lexical-decision task.

The effects of aging on response time (RT) are examined in 2 lexical-decision experiments with young and older subjects (age 60-75). The results show that the older subjects were slower than the young subjects, but more accurate. R. Ratcliff s (1978) diffusion model provided a good account of RTs, their distributions, and response accuracy. The fits show an 80-100-ms slowing of the nondecision components of RT for older subjects relative to young subjects and more conservative decision criterion settings for older subjects than for young subjects. The rates of accumulation of evidence were not significantly different for older compared with young subjects (less than 2% and 5% higher for older subjects relative to young subjects in the 2 experiments).

Adult↗
PubMed2006

A diffusion model analysis of adult age differences in episodic and semantic long-term memory retrieval.

Two experiments investigated adult age differences in episodic and semantic long-term memory tasks, as a test of the hypothesis of specific age-related decline in context memory. Older adults were slower and exhibited lower episodic accuracy than younger adults. Fits of the diffusion model (R. Ratcliff, 1978) revealed age-related increases in non-decisional reaction time for both episodic and semantic retrieval. In Experiment 2, an age difference in boundary separation also indicated an age-related increase in conservative criterion setting. For episodic old-new recognition (Experiment 1) and source memory (Experiment 2), there was an age-related decrease in the quality of decision-driving information (drift rate). As predicted by the context-memory deficit hypothesis, there was no corresponding age-related decline in semantic drift rate.

Adolescent↗
PubMed2004

A transient diffusion model yields unitary gap junctional permeabilities from images of cell-to-cell fluorescent dye transfer between Xenopus oocytes.

As ubiquitous conduits for intercellular transport and communication, gap junctional pores have been the subject of numerous investigations aimed at elucidating the molecular mechanisms underlying permeability and selectivity. Dye transfer studies provide a broadly useful means of detecting coupling and assessing these properties. However, given evidence for selective permeability of gap junctions and some anomalous correlations between junctional electrical conductance and dye permeability by passive diffusion, the need exists to give such studies a more quantitative basis. This article develops a detailed diffusion model describing experiments (reported separately) involving transport of fluorescent dye from a "donor" region to an "acceptor" region within a pair of Xenopus oocytes coupled by gap junctions. Analysis of transport within a single oocyte is used to determine the diffusion and binding characteristics of the cellular cytoplasm. Subsequent double-cell calculations then yield the intercellular junction permeability, which is translated into a single-channel permeability using concomitant measurements of intercellular conductance, and known single-channel conductances of gap junctions made up of specific connexins, to count channels. The preceding strategy, combined with use of a graded size series of Alexa dyes, permits a determination of absolute values of gap junctional permeability as a function of dye size and connexin type. Interpretation of the results in terms of pore theory suggests significant levels of dye-pore affinity consistent with the expected order of magnitude of typical (e.g., van der Waals) intermolecular attractions.

Animals↗
PubMed2002

The power integration diffusion model for production breaks.

The learning curve during repetitive production and the associated forgetting during production breaks are fundamental issues in the understanding of behavior. A model is suggested that combines 3 basic findings, namely, that single memory traces decay according to a power function of the retention interval, that aggregated memory traces can be combined by integration, and that the time to produce a unit can be described by a diffusion process on the memory trace. This power integration diffusion model is validated with empirical data, and the result fits better than 14 other published forgetting models.

Attention↗
PubMed2002

Monotone travelling fronts in an age-structured reaction-diffusion model of a single species.

We consider a partially coupled diffusive population model in which the state variables represent the densities of the immature and mature population of a single species. The equation for the mature population can be considered on its own, and is a delay differential equation with a delay-dependent coefficient. For the case when the immatures are immobile, we prove that travelling wavefront solutions exist connecting the zero solution of the equation for the matures with the delay-dependent positive equilibrium state. As a perturbation of this case we then consider the case of low immature diffusivity showing that the travelling front solutions continue to persist. Our findings are contrasted with recent studies of the delayed Fisher equation. Travelling fronts of the latter are known to lose monotonicity for sufficiently large delays. In contrast, travelling fronts of our equation appear to remain monotone for all values of the delay.

Animals↗