External Water Sources Research Articles

Modeling Anthropogenic Impacts and Hydrological Processes on a Wetland in China

In many rapidly urbanizing countries like China, wetlands are constantly affected by anthropogenic factors such as landscaping, additional abstractions, reduction in catchment perviousness, etc. Thus, modeling of such anthropogenic factors should be explicitly considered when simulating wetlands. In this paper, the wetland module in a distributed hydrological model, SWAT, is modified to simulate the artificial water input to the designated wetlands. Local river runoff is used as the water sources to study the wetland restoration potential and to analyze the effects on local hydrological cycle and sea outflow. The QingDianWa depression, near Tianjin city, China, is used as a case study to study the restoration impact. Results showed that after restoration, the QinDianWa depression can reduce the potential impact of flooding by an average of 61 million m3 per year, increase the annual surface runoff by about 32 million m3 in non-flood seasons, and increase groundwater recharge by 9.4 million m3. This illustrated the importance of wetland restoration on flood control, river flow increase, groundwater recharge, and flood reclamation. But with local water resources is far from meeting the demands of wetland restoration, the actual restored water surface area is only 21.5 and 40.9 km2 for the designated surface areas of 60 and 150 km2 respectively. Compared with nature wetlands without human disturbance, the anthropogenic effect of reduction of runoff and groundwater recharge may be attributed to large amount of human consumption of local water. However, the results showed that the aims of restoring Tianjin wetlands cannot be achieved fully by relying solely on local water resources. It is necessary to consider a combination of external sources of water and using artificial recharge from reclaimed water.

Water availability, demand and reliability of in situ water harvesting in smallholder rain-fed agriculture in the Thukela River Basin, South Africa

Abstract. Water productivity in smallholder rain-fed agriculture is of key interest for improved food and livelihood security. A frequently advocated approach to enhance water productivity is to adopt water harvesting and conservation technologies (WH). This study estimates water availability for potential in situ WH, and supplemental water demand (SWD) in smallholder agriculture in South Africa's Thukela River Basin (29 000 km2, mean annual precipitation 550–2000 mm yr−1). The study includes process dynamics governing runoff generation and crop water demands, quantification of prediction uncertainty, and an analysis of the reliability of in situ WH. The agro-hydrological model SWAT (Soil and Water Assessment Tool) was calibrated and evaluated with the Sequential Uncertainty Fitting algorithm against observed discharge (at ten stations) and maize yield (the dominant crop type) for the period 1997–2006. The water availability was based on the generated surface runoff in smallholder areas. The SWD was derived from a scenario where crop water deficits were met from an unlimited external water source. The reliability was calculated as the percentage of years in which water availability ≥SWD. This reflects the risks of failure induced by the temporal variability in the water availability and the SWD. The calibration reduced the predictive uncertainty and resulted in a satisfactory model performance. For smallholder maize yield, the Root Mean Squared Error was 0.02 t ha−1 during both the calibration and the evaluation periods. The width of the uncertainty band was reduced by 23% due to the calibration. For discharge during the calibration (evaluation) period, the ten-station range in the weighted coefficient of determination (Φ) was 0.16–0.85 (0.18–0.73), and in the coefficient of determination (R2) 0.42–0.83 (0.28–0.72). The calibration reduced the width of the uncertainty band by 25% on average. The results show that the smallholder crop water productivity is currently low in the basin (spatiotemporal median: 0.08–0.22 kg m−3, 95% prediction uncertainty band (95PPU)). Water is available for in situ WH (spatiotemporal median: 0–17 mm year−1, 95PPU) which may aid in enhancing the crop water productivity by meeting some of the SWD (spatiotemporal median: 0–113 mm year−1, 95PPU). However, the reliability of in situ WH is highly location specific and overall rather low. Of the 1850 km2 of smallholder lands, 20–28% display a reliability ≥25%, 13–16% a reliability ≥50%, and 4–5% a reliability ≥75% (95PPU). This suggests that the risk of failure of in situ WH is relatively high in many areas of the basin.

Shape controlled zeolite EU-1 (EUO) catalysts: Dry gel conversion type synthesis, characterization and formation mechanisms

Shape controlled zeolite EU-1 (EUO) catalysts were synthesized by a dry gel conversion (DGC) type synthesis. Silica extrudates as starting material were impregnated to the incipient wetness point and subjected to a hydrothermal treatment under water vapor. All-zeolite extrudates with the same size and shape as initial silica extrudates were obtained. Formation mechanisms were investigated by varying the initial extrudate water content. Hydrothermal treatment was carried out in the presence or in the absence of an external water source. Extrudate H 2O/SiO 2 molar ratios of at least 1.5 were required to obtain zeolite extrudates with high crystallinity. In the presence of an external water source, a non-uniform progression of zeolite crystallization from the outer to the inner extrudate surface was observed. On the contrary in the absence of an external water source, first crystals appeared throughout the extrudates. For both methods, mesopores of silica extrudates disappeared during impregnation processes and micro- and macropores were formed during crystallization. In a m-xylene isomerization reaction shape controlled zeolite EU-1 extrudates display an enhanced p-xylene selectivity.

Cement-Shrinkage Measurement in Oilwell Cementing—A Comparative Study of Laboratory Methods and Procedures

Summary Cement-sheath shrinkage after placing the cement slurry in the annulus has been of concern in oilwell cementing because of the potential detrimental effects that shrinkage can have on long-term zonal isolation. This paper compares several methods for measuring cement shrinkage under a variety of conditions including those downhole. Using the various methods, cement shrinkage in environments where the cement slurry is either closed to or open to external water sources was measured, and the values were compared. The methods used for measurement under atmospheric conditions were based on American Petroleum Institute (API) test apparatus (balloon and ring mold) and flask methods currently used in laboratories. Measurements under pressure were performed using the balloon and ring-mold methods, in-house designed equipment, and commercial equipment. The values obtained by the different methods are discussed. Procedure modifications are proposed for measuring volume changes to cement slurries on the basis of placement time, shear history, and gel-strength development. The results presented in this work can be used to test shrinkage characteristics of cement slurries in the laboratory under downhole conditions and to select a cement system that is fit for purpose.

Soil Moisture Feedbacks on Convection Triggers: The Role of Soil–Plant Hydrodynamics

Abstract The linkages between soil moisture dynamics and convection triggers, defined here as the first crossing between the boundary layer height (hBL) and lifting condensation level (hLCL), are complicated by a large number of interacting processes occurring over a wide range of space and time scales. To progress on this problem, a soil–plant hydrodynamics model was coupled to a simplified ABL budget to explore the feedback of soil moisture on convection triggers. The soil–plant hydraulics formulation accounted mechanistically for features such as root water uptake, root water redistribution, and midday stomatal closure, all known to affect diurnal cycles of surface fluxes and, consequently, ABL growth. The ABL model considered the convective boundary layer as a slab with a discontinuity at the inversion layer. The model was parameterized using the wealth of data already collected for a maturing Loblolly pine plantation situated in the southeastern United States. A 30-day dry-down simulation was used to investigate the possible feedback mechanisms between soil moisture and convective rainfall triggers. Previous studies, which made use of surface flux measurements to drive an ABL model, have postulated that a negative feedback was possible, which could award the ecosystem with some degree of self-regulation of its water status. According to model simulation results here, this negative feedback is unlikely. However, drastic changes in external water sources to the ABL are needed for triggering convection when soil moisture is depleted. The apparent negative feedback originated from a decoupling between the water vapor sources needed to produce convection triggers and surface water vapor fluxes.

Water as the Key to Expansion of Ettringite in Cementitious Materials

Damage in sulfate-bearing soils and aggregate systems stabilized with additives containing lime, including lime and portland cement, has drawn considerable attention over the past two decades. Researchers and practitioners have made considerable contributions to the understanding of the problem, including the mechanisms involved in the formation of the two minerals, ettringite and thaumasite, that are most often associated with this damage. This paper provides a case history analysis of the expansion history compared with the ettringite growth history of three controlled low-strength mixtures containing fly ash with relatively high sulfate contents. Samples were subjected to three curing conditions: a dry cure, in which only mixing water was available for curing; a moist cure, in which an external source of water was available for curing; and a sulfate cure, in which an external source of sulfate-bearing water was made available for the duration of cure. Ettringite was quantified by using both differential scanning calorimetry and X-ray diffraction; the resulting volume changes in the samples were measured. Results suggested that sorption of water by the ettringite molecule was at least part of the reason for expansion. The importance of sorption was based on the fact that although expansion increased in moist cure compared to dry cure systems, the quantities of ettringite formed under each regime were about the same and remained about the same throughout the experiments. As expected, samples exposed to sulfate cure responded with the greatest expansion, which was concomitant with continued ettringite crystal growth due to a supply of the limiting reagent, sulfate.

Freezing-induced water migration in compacted base-course materials

Ramped-freezing tests were conducted on three base-course materials with fines contents of less than 7% and compacted at different initial states but always at degrees of saturation near or well below 60%. Three different quarries were studied. The natural fines from crushed gneiss with biotite, limestone, and monzonite were all frost susceptible. Frost heave was relatively small, but significant water intake occurred in all samples during freezing with access to an external water source, regardless of initial saturation level. The frost susceptibility of coarse-grained soils cannot be solely evaluated with respect to frost heave but needs also to consider the amount of water drawn to the freezing front during the freezing process and the consequences of this water during thaw. The normalized heave of the base-course layer of pavements is a practical and efficient indicator of the frost susceptibility of the base-course aggregates. If it is larger than 1%, the base-course material can be considered as frost susceptible leading to a significant increase in the degree of saturation once frozen. Current base-course material specifications based solely on grain-size distribution are not adequate to differentiate materials that are nonfrost susceptible from those that are frost susceptible. Hence, an additional criterion based on the fines frost susceptibility should be considered.

Hydrothermal Evolution of the Porphyry Copper Deposit at La Caridad, Sonora, Mexico, and the Relationship with a Neighboring High-Sulfidation Epithermal Deposit

Four main stages of alteration and mineralization are present at La Caridad, Sonora, Mexico. In stage I, quartz veins are associated with orthoclase-anhydrite-biotite hydrothermal alteration within the La Caridad intrusive complex and with pervasive biotitization in andesites and diorites. A zone of propylitic alteration surrounds this biotitic zone. Weak molybdenite ± chalcopyrite ± magnetite ± pyrite ± sphalerite mineralization formed in stage I. stage II, the main hydrothermal mineralization event, is represented by quartz veins with pyrite-sericite + chlorite and is associated with chalcopyrite + pyrite ± molybdenite. In stage III, lead-zinc-silver mineralization formed in veins that were emplaced peripherally to the main system, locally overprinting the earlier events. In stage IV, intermediate-sulfidation mineralization, represented by quartz-tennantite-chalcopyrite-pyrite-sericite veinlets, formed in the core of the porphyry deposit system. The La Caridad Antigua mine, located ~3 km east of the La Caridad orebody, has a high-sulfidation epithermal assemblage that includes pyrophillite, kaolinite, alunite, quartz, and barite as gangue, and a variety of sulfides including, chalcopyrite, pyrite, enargite, tetrahedrite-tennantite, and chalcocite. A new 206Pb/238U age of zircons from an associated porphyry stock at La Caridad Antigua is 55.0 ± 1.7 Ma, identical to previous U/Pb ages determined in the porphyry Cu deposit and consistent with a spatial and temporal link between La Caridad and La Caridad Antigua deposits. Fluid inclusion data indicate that stages I and II at La Caridad were deposited by saline hydrothermal fluid at 360° to 460° and 330° to 410°C, respectively. stages III and IV were deposited by low-salinity hydrothermal fluid at 330° to 370° and 260° to 320°C, respectively. Oxygen and hydrogen isotopes indicate deposition of stages I, II, and IV either from magmatic vapor or from mixtures of magmatic and highly evaporated (lacustrine?) formation water and mixing with meteoric water in stages II and III. Sulfur and oxygen isotopes in stage 1 anhydrite indicate external derivation of sulfate and mixing of igneous and external water sources.

New isotopic evidence for the origin of groundwater from the Nubian Sandstone Aquifer in the Negev, Israel

The geochemistry and isotopic composition (H, O, S, O sulfate, C, Sr) of groundwater from the Nubian Sandstone (Kurnub Group) aquifer in the Negev, Israel, were investigated in an attempt to reconstruct the origin of the water and solutes, evaluate modes of water–rock interactions, and determine mean residence times of the water. The results indicate multiple recharge events into the Nubian sandstone aquifer characterized by distinctive isotope signatures and deuterium excess values. In the northeastern Negev, groundwater was identified with deuterium excess values of ∼16‰, which suggests local recharge via unconfined areas of the aquifer in the Negev anticline systems. The δ 18O H 2O and δ 2H values (−6.5‰ and −35.4‰) of this groundwater are higher than those of groundwater in the Sinai Peninsula and southern Arava valley (−7.5‰ and −48.3‰) that likewise have lower deuterium excess values of ∼10‰. Based on the geochemical differences between groundwater in the unconfined and confined zones of the aquifer, a conceptual geochemical model for the evolution of the groundwater in the Nubian sandstone aquifer has been reconstructed. The isotopic composition of shallow groundwater from the unconfined zone indicates that during recharge oxidation of pyrite to SO 4 ( δ 34S SO 4 ∼−13‰; δ 18O SO 4 ∼+7.7‰) and dissolution of CaCO 3 ( 87Sr/ 86Sr ∼0.70787; δ 13C DIC = −3.7‰) occur. In the confined zone of the aquifer, bacterial SO 4 reduction removes a significant part of dissolved SO 4 2 - , thereby modifying its isotopic composition ( δ 34S SO 4 ∼−2‰; δ 18O SO 4 ∼+8.5‰) and liberating dissolved inorganic C that contains little or no radiocarbon ( 14C-free) with low δ 13C DIC values (<−12‰). In addition to local recharge, the Sr and S isotopic data revealed contribution of external groundwater sources to the Nubian Sandstone aquifer, resulting in further modifications of the groundwater chemical and isotopic signatures. In the northeastern Negev, it is shown that SO 4-rich groundwater from the underlying Jurassic aquifer contributes significantly to the salt budget of the Nubian Sandstone aquifer. The unique chemical and isotopic composition of the Jurassic groundwater ( δ 34S SO 4 ∼ +14‰; δ 18O SO 4 ∼ 14‰; 87Sr/ 86Sr ∼0.70764) is interpreted as reflecting dissolution of Late Triassic marine gypsum deposits. In the southern Arava Valley the authors postulate that SO 4-rich groundwater with distinctively high Br/Cl (3 × 10 −3) low 87Sr/ 86Sr (0.70734), and high δ 34S SO 4 values (+15‰) is derived from mixing with underlying brines from the Paleozoic units. The radiocarbon measurements reveal low 14C activities (0.2–5.8 pmc) in both the northeastern Negev and southern Arava Valley. Taking into account dissolution of carbonate rocks and bacterial SO 4 reduction in the unconfined area, estimated mean residence times of groundwater in the confined zone in the northeastern Negev are on the order of 21–38 ka, which suggests recharge predominantly during the last glacial period. The 14C signal in groundwater from the southern Arava Valley is equally low but due to evidence for mixing with external water sources the residence time estimates are questionable.

AquoMin: A software tool for Mass-Exchange Networks targeting and design

This study addresses the work developed at Process Integration Group of CPQ to build a software tool in the area of Pollution Prevention, regarding the Mass-Exchange Networks (MEN) targeting and design. Given a set of mass transfer operations and their limiting data, AquoMin can analyse different options to reduce fresh water consumption and minimise wastewater production. The available strategies to optimise these targets are (i) process without waste water reutilisation, (ii) process with waste reutilisation, and (iii) process with waste regeneration reuse. These options are analysed in two stages: first the targeting and then the design. The former stage uses algorithms based on pinch analysis to obtain the minimum consumption of external water and also the minimum wastewater production. These targets are then used in the second stage, the MEN design. The regeneration reuse strategy needs an extra step in the MEN design stage, which uses pinch analysis concepts to generate a mass balance model to obtain a set of optimal design parameters, the Split Operation Parameters (SOP). Furthermore, work on regeneration with recycle and reuse has been also developed. It was created a general algorithm to obtain the targets for this strategy: the external water source flowrate and the regeneration and recycle flowrate. A small AquoMin tutorial is presented using an Example Problem. The regeneration reuse strategy is focused and the methodology employed in both targeting and design stages were improved. Different scenarios evaluating the MEN structure's complexity and equipment cost were also developed and compared in the regeneration reuse strategy. In the regeneration recycle and reuse strategy, the Example Problem was used to emphasise a new option for the regeneration concentration value and the results of all the strategies are compared. Finally, a five operations’ Case Study was solved to obtain the targets and designs to accomplish the three Process Integration Strategies with a final reduction of 60% in the fresh water consumption.

Design procedure for water/wastewater minimization: single contaminant

Development of methodologies to minimize water use in industries has become an important area in Process Integration due to an increase in environmental awareness, at the cost of fresh water, of effluent treatment and disposal. In this context, this paper presents a heuristic algorithmic procedure, water source diagram procedure (WSD), to synthesize water mass-exchange networks for the following different situations: (i) water re-use; (ii) availability of multiple external water sources in the industrial plant; (iii) water losses along the process; (iv) flowrate constraints; (v) regeneration before re-use in other operations; and (vi) regeneration followed by recycling. WSD is a new tool presented as part of the approach and allow that example networks are also created. It was applied in a single contaminant example case, and it can easily be extended to multiple contaminant processes. This new procedure has advantageous features when compared to other available methodologies. Among them the procedure uses hand calculations, a very useful feature for process engineers.

Eggshell Composition of Squamate Reptiles: Relationship between Eggshell Permeability and Amino Acid Distribution

Most snakes and lizards produce eggs with flexible shells that interact with the environment to maintain water balance. Geckos produce rigid eggshells that are independent of an external source of water and can be oviposited in more open, dryer locations. In this study, we analyzed and compared the amino acid composition of 24 lizard species, six snake species, and four outgroups (including avian and reptilian elastin and chicken eggshell). Rigid Gecko eggshells had significantly lower levels of seven of the 17 amino acids evaluated. Multivariate analysis showed that proline was the most important amino acid in distinguishing between these two groups of eggshells, occurring at significantly higher levels in flexible eggshells. High levels of proline have also been observed in the eggshells of other species. Proline and other amino acids are associated with the alleviation of water and salt stress in plants.

The economic consequences of an international water market in the Paso del Norte region

This research estimates the economic benefit of a water market within the Rio Grande Project (Project). Currently, water is released from Elephant Butte Dam for the purpose of agricultural production. Cities including Ciudad Juárez obtain municipal water from groundwater sources. Ciudad Juárez and El Paso are significantly depleting the underlying Hueco Bolsom and will need to find more expensive external sources of water if the Project water remains unavailable. This research constructed a large optimization model of water users in the Rio Grande Project. For agriculture, multiple water irrigation activities including deficit and trickle irrigation were defined. For municipalities, desalination, groundwater recharge and import of distant groundwater activities were also defined. The model incorporates concepts of consumptive and priority water rights. The net income from water use in agriculture (net income remaining after payment to other factors) and willingness to pay based on urban demand formed the objective function. Environmental benefits of water allocated to riparian restoration are considered in an additional scenario. The model simulates allocation and net benefits of water use that would occur with exchange of consumptive use water rights assuming trade is free of transaction costs. An international water market would have substantial benefits in aggregate to water users in the Project. In particular, the City of El Paso and Ciudad Juárez would have present value benefits of $490 million dollars. The water market would not kill off agriculture. By 2043, there would still be very active production in the Mesilla Valley. Net income to farmers in both Elephant Butte Irrigation District and El Paso Water Conservation District #1 would increase between 200% and 250% respectively from current levels when water sales are factored into net income. Under full trade, restoration of the riparian Bosque cottonwoods along the river banks (4,000 acres) is economically viable.

Ground‐ice stratigraphy and formation at North Head, Tuktoyaktuk Coastlands, western Arctic Canada: a product of glacier–permafrost interactions

AbstractGlacially deformed permafrost at North Head, in the Tuktoyaktuk Coastlands of western Arctic Canada, contains a complex ground‐ice stratigraphy that formed during the course of the last glacial–interglacial cycle. Two generations of ground ice are distinguished within a single stratigraphic sequence. Pre‐deformation ice has been glacially deformed or eroded beneath the cold‐based margin of the Laurentide Ice Sheet (LIS) during Marine Isotope Stage 2. Such ice comprises (1) buried basal ice, (2) massive segregated ice and (3) ice clasts subglacially eroded from pre‐existing ground ice. In contrast, post‐deformation ice has not been glacially disturbed because it formed during or after deglaciation; it includes (4) dykes and sills of intrusive ice, (5) massive segregated‐intrusive ice, (6) ice wedges and composite wedges, (7) segregated ice and (8) pool ice.The superimposition of post‐deformation intrusive ice and massive segregated‐intrusive ice into permafrost containing pre‐deformation ground ice indicates that substantial quantities of overpressurized water were injected into ice‐marginal permafrost during or after deglaciation. The required external water source for the post‐deformation intrusive ice was probably overpressurized subpermafrost groundwater in front of the retreating margin of the LIS. Injection of this water into proglacial permafrost hydraulically fractured the permafrost and formed ice dykes, ice sills and massive segregated‐intrusive ice.A two‐stage model of massive‐ice development can be reconciled with known permafrost and glacial conditions in the ice‐marginal context of the Late Wisconsinan LIS. The model probably applies to some other glaciated terrains of the western Canadian Arctic. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Sources and transformations of nitrogen compounds along the Lower Jordan River.

The Lower Jordan River is located in the semiarid area of the Jordan Valley, along the border between Israel and Jordan. The implementation of the water sections of the peace treaty between Israel and Jordan and the countries' commitment to improve the ecological sustainability of the river system require a better understanding of the riverine environment. This paper investigates the sources and transformations of nitrogen compounds in the Lower Jordan River by applying a combination of physical, chemical, isotopic, and mathematical techniques. The source waters of the Lower Jordan River contain sewage, which contributes high ammonium loads to the river. Ammonium concentrations decrease from 20 to 0-5 mg N L(-1) along the first 20 km of the Lower Jordan River, while nitrate concentrations increase from nearly zero to 10-15 mg N L(-1), and delta(15)N (NO(3)) values increase from less than 5 per thousand to 15-20 per thousand. Our data analysis indicates that intensive nitrification occurs along the river, between 5 and 12 km from the Sea of Galilee, while further downstream nitrate concentration increases mostly due to an external subsurface water source that enters the river.

Investigations of the epidemiology of infections with hepatitis A virus in Jamaica.

Between January 1995 and August 1998, a study was conducted to elucidate the epidemiology of hepatitis A virus (HAV) in Jamaica. Participants were recruited from six sites across the island. The potential risk factors for transmission which were studied included age of the individual, gender, residence (urban v. rural area), sanitary facilities (flush toilet v. pit) and source of domestic water (indoor plumbing v. other). There were 128 male subjects and 211 female, aged 3-90 years. The mean ages of the males and females were 24.9 and 25.6 years, respectively. The seroprevalence of HAV in the study population, estimated by ELISA, was 59.9%. Logistic regression indicated that age (P < 0.001) and source of domestic water (P = 0.006) were the major contributors to exposure to HAV. The rate of exposure to the virus was seen to increase with age. By the age of 10 years, 30% of children had been exposed, and almost 100% of the oldest subjects were seropositive. Rates of exposure to HAV were higher among households which had external sources of water, including standpipes, rivers and tanks, than those with indoor plumbing. Although the seroprevalence of HAV in Jamaica is similar to that seen in developing countries, the age-related pattern of exposure mirrors the pattern seen in developed countries.

External Sources of Water for Mercury's Putative Ice Deposits

Radar images have revealed the possible presence of ice deposits in Mercury's polar regions. Although thermal models indicate that water ice can be stable in permanently shaded regions near Mercury's poles, the ultimate source of the water remains unclear. We use stochastic models and other theoretical methods to investigate the role of external sources in supplying Mercury with the requisite amount of water. By extrapolating the current terrestrial influx of interplanetary dust particles to that at Mercury, we find that continual micrometeoritic bombardment of Mercury over the last 3.5 byr could have resulted in the delivery of (3–60)×1016g of water ice to the permanently shaded regions at Mercury's poles (equivalent to an average ice thickness of 0.8–20 m). Erosion by micrometeoritic impact on exposed ice deposits could reduce the above value by about a half. For comparison, the current ice deposits on Mercury are believed to be somewhere between ∼2 and 20 m thick. Using a Monte Carlo model to simulate the impact history of Mercury, we find that asteroids and comets can also deliver an amount of water consistent with the observations. Impacts from Jupiter-family comets over the last 3.5 billion years can supply (0.1–200)×1016g of water to Mercury's polar regions (corresponding to ice deposits 0.05–60 m thick), Halley-type comets can supply (0.2–20)×1016g of water to the poles (0.07–7 m of ice), and asteroids can provide (0.4–20)×1016g of water to the poles (0.1–8 m of ice). Although all these external sources are nominally sufficient to explain the estimated amount of ice currently at Mercury's poles, impacts by a few large comets and/or asteroids seem to provide the best explanation for both the amount and cleanliness of the ice deposits on Mercury. Despite their low population estimates in the inner solar system, Jupiter-family comets are particularly promising candidates for delivering water to Mercury because they have a larger volatile content than asteroids and more favorable orbital and impact characteristics than Halley-type comets.

Water-stressed maize, barley and rice seedlings show species diversity in mechanisms of leaf growth inhibition

The possible occurrence of species diversity in mechanisms underlying leaf-growth inhibition by water stress, was investigated in related cereal plants. Water stress was generated by additions of the osmoticum polyethylene glycol 6000 to the root medium. Effects of external water potentials ranging from 0 to -0.6 MPa, on early growth parameters of emerging leaves were measured under controlled environment conditions, using pairs of maize, barley or rice genotypes with differing resistance to water stress under field conditions. Water potentials of -0.4 MPa for 24 h, similarly reduced leaf growth, comparative production rates of leaf epidermal cells and cell size in all genotypes. These reductions did not appear to be caused by reductions in the osmotic potential gradients between the expanding leaf cells and their external water source. However, growth inhibition in maize and barley, was accompanied by significant reductions in comparative leaf and cell wall extensibility. Moreover, regression plots revealed good linear correlations (r=0.83 ** for maize and r=0.77 ** for barley) between the reductions in leaf growth induced by a series of water potentials and associated reductions in leaf extensibility. In contrast, the reduction in growth of rice leaves, was not accompanied by any significant changes in leaf or cell wall extensibility. Similarly, regression plots revealed poor correlations between leaf growth and leaf extensibility in both paddy and upland rice (r=0.17 and r=0.07, respectively). Thus, despite numerous inter-species similarities, biophysical changes associated with stress-induced leaf growth inhibition in maize and barley, differed from those in rice.

Vertical distribution of Titan's atmospheric neutral constituents

The vertical distribution of Titan's neutral atmosphere compounds is calculated from a new photochemical model extending from 40 to 1432 km. This model makes use of many updated reaction rates, and of the new scheme for methane photolysis proposed by Mordaunt et al. [1993]. The model also includes a realistic treatment of the dissociation of N2, of the deposition of water in the atmosphere from meteoritic ablation, and of condensation processes. The sensitivity of the results to the eddy diffusion coefficient profile is investigated. Fitting the methane thermospheric profile and the stratospheric abundance of the major hydrocarbons requires a methane stratospheric mixing ratio of 1.5–2% rather than 3%. Fitting the HCN stratospheric profile requires an eddy diffusion coefficient at 100–300 km that is 5–20 times larger than that necessary for the hydrocarbons. Most species are reasonably well reproduced, with the exception of CH3C2H and HC3N. The formation of CH3CN may involve the reaction of CN with either CH4 or (preferably) C2H6. The observed CO2 profile can be modeled by assuming an external source of water of ∼6 × 106 cm−2 s−1. For a nominal CO mixing ratio of 5 × 10−5, the chemical loss of CO exceeds its production by ∼15%, and equilibrium is achieved for CO = 1 × 10−5.

Hydrogen and deuterium in the thermosphere of Venus: Solar cycle variations and escape

A strong solar cycle variation in hydrogen and deuterium densities is observed in the nightside thermosphere of Venus when Pioneer Venus Orbiter (PVO) measurements made during the first three Venus years of the mission are compared with those made during the preentry phase of the mission. Solar maximum conditions prevailed during the former period, while near solar minimum conditions occurred during the latter. Pronounced density enhancements in H of 6.5 times and D of 4 times are observed in the nightside bulge region as solar activity decreased from maximum to near‐minimum values. We attribute the buildup of H and D to a reduction in the escape fluxes as solar activity decreased, a behavior that is consistent with the known properties of the dominant escape processes due to the charge separation electric field (E) and charge exchange (CE). Application of this simple concept leads to expressions for the H and D escape fluxes which relate solar cycle variations of the respective bulge densities and escape fluxes to the dayside source fluxes. Planet averaged escape fluxes in the ranges and are obtained by this method without specifying any particular escape mechanism. Considering the uncertainties in the measured parameters, these flux ranges are in reasonable agreement with the escape fluxes derived from the specific processes, E and CE. When fractionation without or with an external source of water (e.g., comets) is applied to the range of possible escape fluxes, a fractionation factor f = 0.44 is obtained and an ancient reservoir of water equivalent to the range 125 m to 570 m of liquid uniformly distributed on the surface or 3.7% to 17% of a full terrestrial ocean is derived. When the specific escape processes E and E + CE are considered, the f values are 0.15 and 0.1 and the magnitudes of the reservoirs are lower, having a range of equivalent depths of 4.1 m to 36.8 m, respectively.