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1.
Keirith A. Snyder Robert Monnar Simon R. Poulson Peter Hartsough Franco Biondi 《Trees - Structure and Function》2010,24(3):585-595
Diurnal fluctuations of leaf water isotope ratios (δ18O and δD) were measured for Jeffrey (Pinus jeffreyi Balf.) and lodgepole (Pinus contorta Douglas ex Louden) pine. Two trees per species were sampled every few hours on 15–16 October 2005 and 19–20 June 2006. Diurnal
gas exchange was measured during the summer sampling. In fall 2005, leaf water δ18O ranged from 0.7 to 9.0‰, and leaf water δD ranged from −70 to −50‰. In summer 2006, leaf water δ18O ranged from 7.7 to 20.7‰, and leaf water δD ranged from −61 to −24‰. Diurnal variation of leaf water isotope values typically
reached a maximum in early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both periods
showed a high degree of enrichment relative to source water, with leaf water–source water enrichments ranging up to 37.8‰
for δ18O, and up to 95‰ for δD. Leaf water enrichment varied by season with summer enrichment being greater than fall enrichment.
A steady-state model (i.e., modified Craig–Gordon modeling) for leaf water isotope compositions did not provide a good fit
to measured values of leaf water. In summer, a non-steady state model provided a better fit to the measured data than the
steady-state model. Our findings demonstrate substantial leaf water enrichment above source water and diurnal variations in
the isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases
(water vapor, CO2, O2), climate studies based on the isotopic composition of tree rings, and ecosystem water fluxes. 相似文献
2.
The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees
at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low
humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of
–125/–15‰ (δD/δ18O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements
of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water
(both δD and δ18O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially
between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences
in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ18O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental
and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ18O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information
to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange
with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating
that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions.
Received: 7 April 1999 / Accepted: 8 December 1999 相似文献
3.
Modeled Effects of Dissolved Organic Carbon and Solar Spectra on Photobleaching in Lake Ecosystems 总被引:1,自引:0,他引:1
Dissolved organic matter (DOM) contains molecules that absorb light at various wavelengths. This chromophoric DOM (CDOM) influences
the transmission of both visible and ultraviolet energy through water. The absorption of light by CDOM often causes structural
changes that reduce its capacity to further absorb light, a process termed ‘photobleaching‘. A model was designed to assess
photobleaching through the entire water column of lake ecosystems. The model uses lake morphometry and dissolved organic carbon
(DOC) concentration in conjunction with a defined solar spectrum and experimentally measured photobleaching rates to compute
the total water columm photobleaching. The model was initially applied to a theoretical ‘average‘ lake using solar spectra
for both the north (N) and south (S) temperate western hemispheres and variable DOC from 0.3 to 30 mg L−1. The consequences of varying waveband-specific photobleaching coefficients and lake morphometry were explored in a second
set of simulations. Finally, the model was also applied to four temperate northern lakes for which we had prior measurements
of CDOM photobleaching rates. The model demonstrates that all three wavebands of solar radiation (UVB, UVA, and PAR) contribute
significantly to total water column photobleaching, with UVA being most important. The relative contributions of the three
wavebands were invariant for DOC more than 3 mg L−1. Total water column photobleaching at 440 nm was three to five times faster under the UV-enriched solar spectrum of the southern
hemisphere. Increasing the lake’s mean depth (from 0.37 to 9.39 m) resulted in five- or 15-fold slower rates of total water
column photobleaching for DOC concentrations of 1 or 10 mg L−1, respectively. Varying the waveband-specific photobleaching coefficients by 10-fold resulted in a similar change in total
water column photobleaching rates. Applying the model to four specific lakes revealed that photobleaching for the entire water
column would reduce CDOM light absorption by 50% in 18–44 days under summer conditions.
Received 17 November 1998; accepted 27 June 2000. 相似文献
4.
S. Even B. Thouvenin N. Bacq G. Billen J. Garnier L. Guezennec S. Blanc A. Ficht P. Le Hir 《Hydrobiologia》2007,588(1):13-29
Within the framework of the European Water Framework Directive, the Seine-Normandie Water Agency has defined prospective scenarios
describing possible trends of evolution of the pressures on water resources. In order to evaluate the resulting water quality
improvement or degradation of water bodies in the Seine river basin, an integrated modelling was proposed. The approach consisted
in coupling three models, the seneque model for upstream sub-basins, the ProSe model for the Seine river and main tributaries and finally the s
i
am1d model for the downstream estuary. After consistency verification, the integrated model was applied to scenarios proposed
by the Seine-Normandie Water Agency. As a result of improvement in the nitrogen treatment by waste water treatment plants,
the annual load of ammonia at the basin scale will be reduced by 65%. The oxygen and ammonia criteria in the estuary will
improve from “bad” to “good”. However the nitrate criteria will remain “poor”, given the strong influence of non-point sources.
Despite a 70–75% drop of the point orthophosphate loads, the criteria for this variable will also remain “poor”. The nutrient
levels will be high enough to maintain eutrophication in the system; a general trend to a shift from N-limitation to P-limitation
will be accentuated. 相似文献
5.
Use of the root contact concept,an empirical leaf conductance model and pressure-volume curves in simulating crop water relations 总被引:5,自引:0,他引:5
A simulation model “DanStress” was developed for studying the integrated effects of soil, crop and climatic conditions on
water relations and water use of field grown cereal crops. The root zone was separated into 0.1 m deep layers of topsoil and
subsoil. For each layer the water potential at the root surface was calculated by a single root model, and the uptake of water
across the root was calculated by a root contact model. Crop transpiration was calculated by Monteith's combination equation
for vapour flow. Crop conductance to water vapour transfer for use in Monteith's combination equation was scaled up from an
empirical stomatal conductance model used on sunlit and shaded crop surfaces of different crop layers. In the model, transpirational
water loss originates from root water uptake and changes in crop water storage. Crop water capacitance, used for describing
the water storage, was derived from the slope of pressure-volume (PV) curves of the leaves. PV curves were also used for deriving
crop water potential, osmotic potential, and turgor pressure. The model could simulate detailed diurnal soil-crop water relations
during a 23-day-drying cycle with time steps of one hour.
During the grain filling period in spring barley (Hordeum distichum L.), grown in a sandy soil in the field, measured and predicted values of leaf water and osmotic potential, RWC, and leaf
stomatal conductance were compared. Good agreement was obtained between measured and predicted values at different soil water
deficits and climatic conditions. In the field, measured and predicted volumetric soil water contents (θ) of topsoil and subsoil
layers were also compared during a drying cycle. Predicted and measured θ-values as a function of soil water deficits were
similar suggesting that the root contact model approach was valid.
From the investigation we concluded: (I) a model, which takes the degree of contact between root surface and soil water into
account, can be used in sandy soil for calculation of root water uptake, so that the root conductance during soil water depletion
only varies by the degree of contact; (II) crop conductance, used for calculation of crop transpiration, can be scaled up
from an empirical single leaf stomatal conductance model controlled by the level of leaf water potential and micrometeorological
conditions; (III) PV curves are usable for describing crop water status including crop water storage. 相似文献
6.
Marcelo S. Mielke M. A. Oliva Nairam F. de Barros Ricardo M. Penchel Carlos A. Martinez Auro C. de Almeida 《Trees - Structure and Function》1999,13(3):152-160
Predawn leaf water potential, stomatal conductance and microclimatic variables were measured on 13 sampling days from November
1995 through August 1996 to determine how environmental and physiological factors affect water use at the canopy scale in
a plantation of mature clonal Eucalyptus grandis Hill ex-Maiden hybrids in the State of Espirito Santo, Brazil. The simple ”big leaf” Penman-Monteith model was used to estimate
canopy transpiration. During the study period the predawn leaf water potential varied from –0.4 to –1.3 MPa, with the minimum
values observed in the winter months (June and August 1996), while the average estimated values for canopy conductance and
canopy transpiration fell from 17.3 to 5.8 mm s–1 and from 0.54 to 0.18 mm h–1, respectively. On the basis of all measurements, the average value of the decoupling coefficient was 0.25. During continuous
soil water shortage a proportional reduction was observed in predawn leaf water potential and in daily maximum values of stomatal
conductance, canopy transpiration and decoupling coefficient. The results showed that water vapour exchange in this canopy
is strongly dominated by the regional vapour pressure deficit and that canopy transpiration is controlled mainly by stomatal
conductance. On a seasonal basis, stomatal conductance and canopy transpiration were mainly related to predawn leaf water
potential and, thus, to soil moisture and rainfall. Good results were obtained with a multiplicative empirical model that
uses values of photosynthetically active radiation, vapour pressure deficit and predawn leaf water potential to estimate stomatal
conductance.
Received: 10 June 1998 / Accepted: 20 July 1998 相似文献
7.
This article reports on quantified soil water gains and their possible effects on summer water relationships in a semiarid
Stipa tenacissima L. grasslands located in SE Spain. We believe that the net soil water gains detected using minilysimeters could be from soil
water vapour adsorption (WVA). Our study of high water-stress showed stomatal conductance (21.8–43.1 mmol H2O m−2 s−1) in S. tenacissima leaves unusual for the summer season, and the evapotranspiration from S. tenacissima grassland, estimated by a multi-source sparse evapotranspiration model, closely corresponding to total WVA. This highlights
the importance of summer soil WVA to stomatal conductance and vital transpiration in S. tenacissima. This study measured pre-dawn leaf water potential (ψ) response to sporadic light rainfall, finding that a light summer rainfall
(1.59 mm day−1) was sufficient to vary ψ in S. tenacissima from −3.8 (close to the turgour loss point) to −2.7 MPa. We hypothesize that soil WVA can supply vegetation with water vital
to its survival in seasons with a severe water deficit, giving rise to a close relationship between soil water dynamics and
plant water response. 相似文献
8.
Binder H 《European biophysics journal : EBJ》2007,36(4-5):265-279
The ordering and H-bonding characteristics of the hydration water of the lipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC)
were studied using polarized infrared spectroscopy by varying either the temperature or the relative humidity of the ambient
atmosphere of multibilayer samples. The OH-stretching band of lipid-bound water was interpreted by a simplified two-state
model of well-structured, low density “network” water and of less-structured dense “multimer” water. The IR-spectroscopic
data reflect a rather continuous change of the water properties with increasing distance from the membrane and with changing
temperature. Network and multimer water distribute across the whole polar interphase with changing composition and orientation.
Upon dehydration the fraction of network water increases from about 30 to 60%, a value which is similar to that in supercooled
water at −25°C. The highly ordered gel phase gives rise to an increased fraction of structured network water compared with
the liquid crystalline phase. The IR order parameter shows that the water dipoles rearrange from a more parallel towards a
more perpendicular orientation with respect to the membrane normal with progressive hydration.
Dedicated to Prof. K. Arnold on the occasion of his 65th birthday. 相似文献
9.
Summer water use by California coastal prairie grasses: fog,drought, and community composition 总被引:2,自引:0,他引:2
Plants in the Mediterranean climate region of California typically experience summer drought conditions, but correlations
between zones of frequent coastal fog inundation and certain species’ distributions suggest that water inputs from fog may
influence species composition in coastal habitats. We sampled the stable H and O isotope ratios of water in non-photosynthetic
plant tissue from a variety of perennial grass species and soil in four sites in northern California in order to determine
the proportion of water deriving from winter rains and fog during the summer. The relationship between H and O stable isotopes
from our sample sites fell to the right of the local meteoric water line (LMWL) during the summer drought, providing evidence
that evaporation of water from the soil had taken place prior to the uptake of water by vegetation. We developed a novel method
to infer the isotope values of water before it was subjected to evaporation in which we used experimental data to calculate
the slope of the δH versus δO line versus the LMWL. After accounting for evaporation, we then used a two-source mixing model
to evaluate plant usage of fog water. The model indicated that 28–66% of the water taken up by plants via roots during the
summer drought came from fog rather than residual soil water from winter rain. Fog use decreased as distance from the coast
increased, and there were significant differences among species in the use of fog. Rather than consistent differences in fog
use by species whose distributions are limited to the coast versus those with broader distributions, species responded individualistically
to summer fog. We conclude that fogwater inputs can mitigate the summer drought in coastal California for many species, likely
giving an advantage to species that can use it over species that cannot. 相似文献
10.
Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community 总被引:11,自引:0,他引:11
We conducted a study to test the predictions of Walter's two-layer model in the shortgrass steppe of northeastern Colorado.
The model suggests that grasses and woody plants use water resources from different layers of the soil profile. Four plant
removal treatments were applied in the spring of 1996 within a plant community codominated by Atriplex canescens (a C4 shrub) and Bouteloua gracilis (a C4 grass). During the subsequent growing season, soil water content was monitored to a depth of 180 cm. In addition, stem and
leaf tissue of Atriplex, Bouteloua and the streamside tree Populus sargentii were collected monthly during the growing seasons of 1995 and 1996 for analysis of the δ18O value of plant stem water (for comparison with potential water sources) and the δ13C value of leaves (as an indicator of plant water status). Selective removal of shrubs did not significantly increase water
storage at any depth in the measured soil profile. Selective removal of the herbaceous understory (mainly grasses) increased
water storage in the top 60 cm of the soil. Some of this water gradually percolated to lower layers, where it was utilized
by the shrubs. Based on stem water δ18O values, grasses were exclusively using spring and summer rain extracted from the uppermost soil layers. In contrast, trees
were exclusively using groundwater, and the consistent δ13C values of tree leaves over the course of the summer indicated no seasonal changes in gas exchange and therefore minimal
water stress in this life-form. Based on anecdotal rooting-depth information and initial measurements of stem water δ18O, shrubs may have also had access to groundwater. However, their overall δ18O values indicated that they mainly used water from spring and summer precipitation events, extracted from subsurface soil
layers. These findings indicate that the diversity of life-forms found in this shortgrass steppe community may be a function
of the spatial partitioning of soil water resources, and their differential use by grasses, shrubs, and trees. Consequently,
our findings support the two-layer model in a broad sense, but indicate a relatively flexible strategy of water acquisition
by shrubs.
Received: 23 December 1997 / Accepted: 16 September 1998 相似文献
11.
Experimental measurement of sediment nitrification and denitrification in Hamilton Harbour,Canada 总被引:1,自引:0,他引:1
This research examines the role of sediment nitrification and denitrification in the nitrogen cycle of Hamilton Harbour. The
Harbour is subject to large ammonia and carbon loadings from a waste-water treatment plant and from steel industries. Spring
ammonia concentrations rapidly decrease from 4.5 to 0.5 mg 1−1, while spring nitrate concentrations increase from 1 to 2 mg l−1, by mid-summer. A three-layer sediment model was developed. The first layer is aerobic; in it, oxidation of organics and
nitrification occurs. The second layer is for denitrification, and the third layer is for anaerobic processes. Ammonia sources
for nitrification include diffusion from the water column, sources associated with the oxidation of organics, sources from
denitrification and from anaerobic processes. Diffusion of oxygen, ammonia and nitrate across the sediment-water interface
occurs. Temperature effects are modelled using the Arrhenius concept. A combination of zero-order kinetics for nitrate or
ammonia consumption with diffusion results in a half-order reaction, with respect to the water column loss rate to sediments.
From experimental measurement, the rate of nitrification is 200 mg N 1−1 sediment per day, while that of denitrification is 85 mg N 1–1 sediment per day at 20 °C. The Arrhenius activation energy
is estimated as 15 000 cal/ mole-K and 17 000 cal/ mole-K for nitrification and denitrification, respectively, between 10
°C and 20 °C. Calculations of the flux of ammonia with the sediments, using the biofilm model, compare favourably with experimental
observations. The ammonia flux from the water column is estimated to account for 20% of the observed decrease in water column
stocks of ammonia, while the nitrate flux from the water column is estimated to account for 25% of the total nitrogen produced
by the sediments. 相似文献
12.
A simple model was developed to describe the transport of water vapour from subjects working in hot environments while wearing
chemical-protective clothing. The goal of the modelling was to obtain a better estimate of evaporative cooling of the subjects,
as it was hypothesised that calculations of evaporative heat loss based on changes in dressed weight over-estimate the actual
benefit experienced by the subjects. The model employed measured values of vapour pressure within the clothing ensemble to
estimate the skin vapour pressure. The resistance of the clothing ensemble to water vapour transport was calculated from measurements
of the physical properties of the materials in conjunction with estimates of the resistance of air layers between the clothing
layers. The model predicts mean evaporation rates from the skin that are approximately 60% of those calculated from measured
changes in dressed weight. Error analysis failed to account for the magnitude of this difference and possible explanations
for the difference are advanced. A brief examination of the effect of wicking suggests that some of the difference results
from a reduction of the resistance of the garment to water vapour due to wicking of liquid sweat through fabric layers.
Received: 4 June 1997 / Accepted: 21 October 1997 相似文献
13.
A simple, physiologically based model was devised and used for estimating the respiration rate and the overall conversion
efficiency of photosynthate into the grain dry-matter in dehydrated plants relative to well watered controls. The model described
mathematically the partitioning of assimilate produced by current photosynthesis and of assimilate stored previously between
the grain and those plants parts other than grain (the “straw”). Using data obtained from the dry-matter analysis and CO2 gas exchange measurements, the model gave us two independent estimates of the respiration rate and the overall conversion
efficiency; one for the plants given a prolonged dark period and the other for those grown in a normal light and dark cycle.
The rate of dark respiration increased with mild water stress: 4.3 mg g−1 day−1 in control plants with leaf water potential of around −0.4 MPa and 11.3 mg g−1 day−1 in dehydrated plants with leaf water potential of around −1 MPa, when both the control and dehydrated plants were left in
the dark for ten days. Similar values were obtained for plants in a normal light and dark cycle: 5.6 in well watered and 8.1
mg g−1 day−1 in the stressed plants. Accordingly, the overall conversion efficiency (the ratio of grain dry-matter against the gross carbohydrate
input to the construction and maintenance processes) was 0.7 to 0.8 for the well hydrated control and 0.4 to 0.5 for moderately
dehydrated plants. With increasing water deficits, however, the respiration rate decreased: 4.8 mg g−1 day−1 when plants were severely stressed (below −3 MPa in midday leaf water potential). The decrease in straw dry weight alone
overestimates dry-matter partition of the stored assimilate in the straw into grain by 20 to 30% in well watered plants and
the error increases to 50 to 60% in more dehydrated plants. 相似文献
14.
Hydrogen and oxygen isotope ratios of tree-ring cellulose for riparian trees grown long-term under hydroponically controlled environments 总被引:1,自引:0,他引:1
Saplings of three riparian tree species (alder, birch and cottonwood) were grown for over 5 months in a hydroponics system
that maintained the isotopic composition of source water in six treatments, ranging from –120 to +180‰δD and –15 to +10‰δ18O. The trees were grown in two greenhouses maintained at 25°C and at either 40 or 75% relative humidity, creating differences
in transpiration rates and leaf water isotopic evaporative enrichment. The cellulose produced in the annual growth ring was
linearly related to source water with differences in both slope and offset associated with greenhouse humidity. The slope
of the isotopic composition of source water versus tree-ring cellulose was less than 1 for both δD and δ18O indicating incomplete isotopic exchange of carbohydrate substrate with xylem water during cellulose synthesis. Tests using
the outer portion of the tree-ring and new roots were similar and showed that the tree-ring values were representative of
the cellulose laid down under the imposed environmental conditions. The fraction of H and O in carbohydrate substrate that
isotopically exchange with medium water was calculated to be 0.36 and 0.42 respectively, and biochemical mechanisms for these
observed fractions are discussed. A mechanistic model of the biochemical fractionation events for both δD and δ18O leading to cellulose synthesis was robust over the wide range of cellulose stable isotope ratios. The experimental results
indicate that both water source and humidity information are indeed recorded in tree-ring cellulose. These results help to
resolve some of the disparate observations regarding the interpretation of stable isotope ratios in tree-rings found in the
literature.
Received: 4 January 1999 / Accepted: 12 August 1999 相似文献
15.
A simulation model for nitrogen retention in a papyrus wetland near Lake Victoria,Uganda (East Africa) 总被引:1,自引:0,他引:1
A. A. van Dam A. Dardona P. Kelderman F. Kansiime 《Wetlands Ecology and Management》2007,15(6):469-480
Papyrus wetlands around Lake Victoria, East Africa play an important role in the nutrient flows from the catchment to the
lake. A dynamic model for nitrogen cycling was constructed to understand the processes contributing to nitrogen retention
in the wetland and to evaluate the effects of papyrus harvesting on the nitrogen absorption capacity of the wetlands. The
model had four layers: papyrus mat, water, sludge and sediment. Papyrus growth was modelled as the difference between nitrogen
uptake and loss. Nitrogen uptake was modelled with a logistic equation combined with a Monod-type nitrogen limitation. Nitrogen
compartments were papyrus plants, organic material in the floating mat; and total ammonia, nitrate and organic nitrogen in
the water, sludge and sediment. Apart from the uptake and decay rates of the papyrus, the model included sloughing and settling
of mat material into the water, mineralization of organic matter, and nitrification and diffusion of dissolved inorganic nitrogen.
Literature data and field measurements were used for parameterization. The model was calibrated with data from Kirinya wetland
in Jinja, Uganda which receives effluent from a municipal wastewater treatment plant. The model simulated realistic concentrations
of dissolved nitrogen with a stable biomass density of papyrus and predicted accumulation of organic sludge in the wetland.
Assuming that this sludge is not washed out of the wetland, the overall nitrogen retention of the wetland over a three-year
period was 21.5 g N m−2 year−1 or about 25% of input. Harvesting 10, 20 and 30% of the papyrus biomass per year increased nitrogen retention capacity of
the wetland to 32.3, 36.8 and 38.1 g m−2 year−1, respectively. Although the nutrient flows estimated by the model are within the ranges found in other papyrus wetlands,
the model could be improved with regard to the dynamics of detrital nitrogen. Actual net retention of nitrogen in the sludge
is likely to be lower than 21.5 g N m−2 year−1 because of flushing out of the sludge to the lake during the rainy season. 相似文献
16.
The results of application of 2D finite element model SHYFEM to the Curonian lagoon (Baltic Sea) are considered. SHYFEM consist
of a physical processes module and an eutrophication module EUTRO adapted for the SHYFEM code from well known modelling system
WASP. The SHYFEM/EUTRO model calibration results were compared with the performance of various biogeochemical models analysed
in other studies (153 studies published from 1990 to 2002). The performance of all corresponding state variables—dissolved
oxygen, NO3, NH4, PO4, phyto- and zooplankton—was slightly lower than median model performance which could be considered satisfactory given the
initial state of model formulation and calibration. Model underestimates phytoplankton autumn blooms, especially for the southern
part of the lagoon, where fine sediments dominate and water residence time is high. It can be concluded that, in order to
increase model performance, the eutrophication module should be improved to account for the dominance of different phytoplankton
groups as well as for the exchanges between the sediments and the water column. The amount and quality of the data available
for the model setup and calibration are unsatisfactory and should be improved for the development of the next enhanced model
version.
Guest editors: A. Razinkovas, Z. R. Gasiūnaitė, J. M. Zaldivar & P. Viaroli
European Lagoons and their Watersheds: Function and Biodiversity 相似文献
17.
Viruses must be removed from the ultrapure water environment, as they have the potential to deposit on microelectronic devices
and generate killer defects. Controlled and well-defined challenges by MS-2 and PRD-1 bacteriophages were treated in a pilot-scale
ultrapure water system using ultraviolet radiation (UV), ozone, mixed bed ion exchange adsorption, and reverse osmosis filtration
technologies typical of those used in industrial systems. Applying a first order kinetic model to the data generated rate
constants for MS-2 removal by UV-185, 50 mg L−1 ozone, mixed bed ion exchange or reverse osmosis filtration of 15.5, 12.9, 3.9, and 10.4 min−1, respectively, and PRD-1 removal of 13.8, 15.5, 8.2, and 11.9 min−1, respectively. In all cases, removal of viruses by oxidative mechanisms such as ozone and UV were far superior to adsorption
and filtration mechanisms. A theoretical viral population balance was generated to model the removal of the bacteriophages
by these unit operations. This model relates the inlet time-dependent profile of viruses to the output, destruction, and accumulation
profiles; it also relates these profiles to the unit operation’s treatment mechanisms including oxidation, adsorption, and
filtration. This model is the first step in generating a site-independent theoretical model to project the persistence of
viruses in ultrapure water systems.
Received 19 October 1998/ Accepted in revised form 29 May 1999 相似文献
18.
19.
Root clumping may affect the root water potential and the resistance to soil-root water transport 总被引:6,自引:0,他引:6
We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots (‘soil
resistance’) is low under most field conditions, so that root water potential would closely follow the mean soil water potential.
Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees
of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes
a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting
soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference
between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater
effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to
plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases.
In the regular case, calculated ‘soil resistances’ were similar to those calculated using Newman's (1969) method. Conversely
they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality
of the view that ‘soil resistance’ is low under common field conditions. They are consistent with the results of field experiments,
especially with recent data dealing with root-to-shoot communication. 相似文献
20.
The purpose of the present study was to investigate the potential use of two PEGylated derivatives of rosin (PD) as sustained
release film forming materials. The derivatives differed chemically by their acid numbers—PD-1 with 120.93 and PD-2 with 88.19.
The derivative films were characterized for surface morphology, water uptake-weight loss, angle of contact, water vapor transmission
rate, mechanical properties and permeability study. Dissolution of diclofenac sodium (DS) and propranolol hydrochloride (PHL)
as model drugs was studied from coated pellets. The films of derivatives with and without plasticizers were smooth and continuous.
PD-2 films developed greater numbers of pores when in contact with phosphate buffer pH 6.8. The low weight loss, low angles
of contact and high water vapor transmission rate of PD-2 films were related to presence of higher concentration of PEG esters.
Higher tensile strength and percent elongation of PD-2 films was due to greater degree of internal plasticization of the derivative.
The permeability of films to model drugs propranolol hydrochloride and diclofenac sodium was inversely proportional to the
film thickness and dibutyl phthalate concentration in them; the permeability being greatest in PD-2 films containing 10% PEG
200. Dissolution rate of propranolol hydrochloride was higher from the coated pellets. The dissolution data followed zero
order, Baker–Lonsdale equation and Hixon–Crowell equation of release kinetics with high correlation coefficients. The mechanism
of drug release from these coated systems however followed class II transport (n > 1.0). The derivatives investigated could successfully retard release of the model drugs and offers an alternative to the
conventionally used polymers. 相似文献