Showing papers on "Water flow published in 1993"

A dual-porosity model for simulating the preferential movement of water and solutes in structured porous media

Abstract: A one-dimensional dual-porosity model has been developed for the purpose of studying variably saturated water flow and solute transport in structured soils or fractured rocks. The model involves two overlaying continua at the macroscopic level: a macropore or fracture pore system and a less permeable matrix pore system. Water in both pore systems is assumed to be mobile. Variably saturated water flow in the matrix as well as in the fracture pore system is described with the Richards' equation, and solute transport is described with the convection-dispersion equation. Transfer of water and solutes between the two pore regions is simulated by means of first-order rate equations. The mass transfer term for solute transport includes both convective and diffusive components. The formulation leads to two coupled systems of nonlinear partial differential equations which were solved numerically using the Galerkin finite element method. Simulation results demonstrate the complicated nature of solute leaching in structured, unsaturated porous media during transient water flow. Sensitivity studies show the importance of having accurate estimates of the hydraulic conductivity near the surface of soil aggregates or rock matrix blocks. The proposed model is capable of simulating preferential flow situations using parameters which can be related to physical and chemical properties of the medium.

A model for the hydrologic and climatic behavior of water on Mars

Abstract: An analysis is carried out of the hydrologic response of a water-rich Mars to climate change and to the physical and thermal evolution of its crust, with particular attention given to the potential role of the subsurface transport, assuming that the current models of insolation-driven change describe reasonably the atmospheric leg of the planet's long-term hydrologic cycle. Among the items considered are the thermal and hydrologic properties of the crust, the potential distribution of ground ice and ground water, the stability and replenishment of equatorial ground ice, basal melting and the polar mass balance, the thermal evolution of the early cryosphere, the recharge of the valley networks and outflow, and several processes that are likely to drive the large-scale vertical and horizontal transport of H2O within the crust. The results lead to the conclusion that subsurface transport has likely played an important role in the geomorphic evolution of the Martian surface and the long-term cycling of H2O between the atmosphere, polar caps, and near-surface crust.

Aquaporin CHIP: the archetypal molecular water channel

Abstract: Despite longstanding interest by nephrologists and physiologists, the molecular identities of membrane water channels remained elusive until recognition of CHIP, a 28-kDa channel-forming integral membrane protein from human red blood cells originally referred to as "CHIP28." CHIP functions as an osmotically driven, water-selective pore; 1) expression of CHIP conferred Xenopus oocytes with markedly increased osmotic water permeability but did not allow transmembrane passage of ions or other small molecules; 2) reconstitution of highly purified CHIP into proteoliposomes permitted determination of the unit water permeability, i.e., 3.9 x 10(9) water molecules.channel subunit-1 x s-1. Although CHIP exists as a homotetramer in the native red blood cell membrane, site-directed mutagenesis studies suggested that each subunit contains an individually functional pore that may be reversibly occluded by mercurial inhibitors reacting with cysteine-189. CHIP is a major component of both apical and basolateral membranes of water-permeable segments of the nephron, where it facilitates transcellular water flow during reabsorption of glomerular filtrate. CHIP is also abundant in certain other absorptive or secretory epithelia, including choroid plexus, ciliary body of the eye, hepatobiliary ductules, gall bladder, and capillary endothelia. Distinct patterns of CHIP expression occur at these sites during fetal development and maturity. Similar proteins from other mammalian tissues and plants were later shown to transport water, and the group is now referred to as the "aquaporins." Recognition of CHIP has provided molecular insight into the biological phenomenon of osmotic water movement, and it is hoped that pharmacological modulation of CHIP function may provide novel treatments of renal failure and other clinical problems.

The interaction between channel geometry, water flow, sediment transport and deposition in braided rivers

Abstract: Abstract Models of braided-river deposition must be detailed, fully 3D, and preferably quantitative to be of use in understanding and predicting the nature of ancient deposits. In order to construct and validate adequate predictive models it is necessary to have information on: (1) variation and interaction of channel geometry, water flow and sediment transport in time and space in modern channel belts, as these control erosion and deposition, the formation and migration of channels and bars, and channel abandonment and filling; (2) 3D variation of bed geometry, texture, sedimentary structures and paleocurrents throughout modern channel-belt deposits, including the age and spatial arrangement of preserved parts of bars and channel fills; (3) long-term (more than hundreds of years) trends in channel and floodplain geometry, flow and sedimentary processes in order to understand channel-belt movements such as avulsions, and the spatial arrangement of channel-belt deposits relative to overbank deposits. Such information is rare because: (1) it is difficult to study modern braided-river geometry, flow and sedimentary processes throughout a range of the all-important high discharges; (2) detailed reconstructions of braided channel and bar geometry and movement are only available for the past half-century and cannot readily be linked to causative mechanisms; (3) 3D documentation of modern deposits below the water table (especially large scale features like lateral-accretion bedding) requires extensive coring and dating of the deposits, and geophysical profiling. As a result of this lack of information, and because of the quality of analysis and presentation of the information available, existing braided-river facies models are virtually useless as interpretive and predictive tools. The nature of the information available is critically reviewed. Using information from recent detailed field and laboratory studies of the geometry, flow and sedimentary processes in braided rivers of simple geometry, in single river bends, in channel confluences, and using some theoretical reasoning, it has been possible to construct fully 3D qualitative and quantitative models of braided river deposits. These models can be used to provide sophisticated quantitative interpretations of palaeochannel geometry, hydraulics and migration, as illustrated by comparison with some particularly well described examples of ancient braided river deposits.

Evaluation of a first-order water transfer term for variably saturated dual-porosity flow models

Abstract: Variably saturated water flow in a dual-porosity medium may be described using two separate flow equations which are coupled by means of a sink source term Γw, to account for the transfer of water between the macropore (or fracture) and soil (or rock) matrix pore systems. In this study we propose a first-order rate expression for Γw, which assumes that water transfer is proportional to the difference in pressure head between the two pore systems. A general expression for the transfer coefficient αw was derived using Laplace transforms of the linearized horizontal flow equation. The value of αw could be related to the size and shape of the matrix blocks (or soil aggregates) and to the hydraulic conductivity Ka of the matrix at the fracture/matrix interface. The transfer term Γw, was evaluated by comparing simulation results with those obtained with equivalent one- and two-dimensional single-porosity flow models. Accurate results were obtained when Ka was evaluated using a simple arithmetic average of the interface conductivities associated with the fracture and matrix pressure heads. Results improved when an empirical scaling coefficient γw was included in αw. A single value of 0.4 for γw was found to be applicable, irrespective of the hydraulic properties or the initial pressure head of the simulated system.

Timing of stagnation of Ice Stream C, West Antarctica, from short-pulse radar studies of buried surface crevasses

Abstract: Five short-pulse radar profiles were run across the edge of inactive Ice Stream C, one of the “Ross” ice streams that flows from the West Antarctic inland ice sheet into the Ross Ice Shelf Scatter from buried crevasses, which we presume were at the surface of the ice stream when it was active, creates hyperbolae on the radar records A density-depth curve and local accumulation rates were used to convert the picked travel times of the apices of the hyperbolae into stagnation ages for the ice stream Stagnation ages are 130 ± 25 year for the three profiles farthest downstream and marginally less (100 ± 30 year) for the fourth The profile farthest upstream shows a stagnation age of only ~30 year We believe that these results indicate a “wave” of stagnation propagating at a diminishing speed upstream from the mouth of the ice stream, and we suggest that the stagnation process involves a drop in water pressure at the bed due to a conversion from sheet flow to channelized water flow

Modeling of Carbon Dioxide Transport and Production in Soil 1. Model Development

Abstract: Knowledge of the CO2 concentration in the unsaturated zone is essential for prediction of solution chemistry in the vadose zone and groundwater recharge as well as for quantifying carbon source/sink terms as part of the global CO2 mass balance. In this paper we present a predictive simulation model, SOILCO2, based on process-oriented relationships. The model includes one-dimensional water flow and multiphase transport of CO2 utilizing the Richards and the convection-dispersion equations, respectively, as well as heat flow and a CO2 production model. The transport of CO2 in the unsaturated zone can occur in both the liquid and gas phases. The gas transport equation accounts for production of CO2 and uptake of CO2 by plant roots associated with root water uptake. The CO2 production model considers both microbial and root respiration which is dependent on water content, temperature, growth, salinity and plant and soil characteristics. Heat flow is included, since some gas transport parameters, partitioning coefficients and production parameters are strongly temperature dependent. The resulting set of partial differential equations is solved numerically using the finite element and finite difference methods.

Soil-borne mobile colloids as influenced by water flow and organic carbon

Abstract: Paucity of understanding mechanisms relevant to the generation of subsurface mobile colloids is a major limitation to our current knowledge of colloid-facilitated contaminant transport. To evaluate the roles of natural organic materials and pore water velocity on mobile colloid generation, colloids generated from 14-m{sup 3} lysimeters containing reconstructed soil profiles were collected and characterized. Colloids generated during low flow rates were 1030% less abundant, contained at least 65% more iron oxides and gibbsite, were 80% smaller, and had 40% greater electrophoretic mobility than colloids generated during higher flow rates. Quartz, kaolinite, and hydroxy-interlayered vermiculite were enriched by at least 32% in colloids generated during faster flow rates. Mobile colloid surface charge was greatly enhanced by organic carbon (OC) coatings. Concentrations of OC associated with mobile colloids were higher than or equal to the OC concentrations existing in the bulk soils from which the mobile colloids were derived. The profound effects of pore water flow rate and OC on mobile colloid generation introduces complexity to this potentially critical, yet poorly understood, component of subsurface contaminant transport. 41 refs., 8 figs., 1 tab.

Thigmomorphogenesis: the effect of mechanical perturbation on plants.

Abstract: Thigmomorphogenetic responses occur in many environmental settings. The most pronounced effects are found under conditions of extremely high rates of turbulent wind or water flow. However, it is an ubiquitous phenomenon, since mechanical perturbations are to be encountered under all but the most stringent laboratory conditions. Our present understanding of these phenomena is the result of studies at the ecological, anatomical, physiological, biochemical, biophysical and molecular biological levels.

Unstable Wetting Fronts in Water-Repellent Field Soils

Abstract: Study objectives are to (i) investigate the effects of a water-repellent top layer on water flow and solute transport and (ii) examine whether unstable wetting front theories can be used to predict the occurrence of preferential flow in water-repellent field soils. The study took place on two adjacent plots with grasscover near Ouddorp (The Netherlands)-one plot with a water-repellent top layer, the other plot with a wettable top layer. The soil at the experimental site is a sand soil of marine orgin-mesic Typic Psammaquent []

A fully coupled model for water flow and airflow in deformable porous media

Abstract: A fully coupled model is developed to simulate the slow transient phenomena (consolidation) involving flow of water and air in deforming porous media. The model is of the Biot type and incorporates the capillary pressure relationship. The finite element method is used for the discrete approximation of the partial differential equations governing the problem. The temporal discretization error, iteration error and stability error are evaluated. The model is validated with respect to a documented experiment on semisaturated soil behavior. Other examples involving an air storage problem in an aquifer and a flexible footing resting on a semisaturated soil are also presented.

Continuous Measurements of Ground‐Water Seepage Using an Automatic Seepage Meter

Abstract: An automatic seepage meter using a heat pulse method was developed to obtain a continuous measurement of ground-water seepage rates. According to calibrations of the automatic seepage meter fitted with a 50 cm diameter collection funnel, seepage rates from 2 X 10-5 to 5 X 10-4 cm/sec can be obtained by measuring the time when the temperature as measured by a thermistor peaks after applying a heat pulse. The automatic seepage meter was used to measure continuous seepage rates into Lake Biwa, Japan. The ground-water seepage rate measured by the automatic seepage meter in Lake Biwa changed by six times within 12 hours. The automatic seepage meter is useful for surface-/ground-water studies, because a continuous seepage rate can be obtained without errors caused by the resistance of a collection bag to water flow.

Mediterranean Outflow Mixing and Dynamics

Abstract: The Mediterranean Sea produces a salty, dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern Gulf of Cadiz. The current accelerates to 1.3 meters per second, which raises the internal Froude number above 1, and is intensely turbulent through its full thickness. The outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline North Atlantic Central water. Within 100 kilometers downstream, the current is turned by the Coriolis force until it flows nearly parallel to topography in a damped geostrophic balance. The mixed Mediterranean outflow continues westward, slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass.

Transport of Water and Solutes across Maize Roots Modified by Puncturing the Endodermis (Further Evidence for the Composite Transport Model of the Root).

Abstract: The effects of puncturing the endodermis of young maize roots (Zea mays L.) on their transport properties were measured using the root pressure probe. Small holes with a diameter of 18 to 60 [mu]m were created 70 to 90 mm from the tips of the roots by pushing fine glass tubes radially into them. Such wounds injured about 10-2 to 10-3% of the total surface area of the endodermis, which, in these hydroponically grown roots, had developed a Casparian band but no suberin lamellae. The small injury to the endodermis caused the original root pressure, which varied from 0.08 to 0.19 MPa, to decrease rapidly (half-time = 10-100 s) and substantially to a new steady-state value between 0.02 and 0.07 MPa. The radial hydraulic conductivity (Lpr) of control (uninjured) roots determined using hydrostatic pressure gradients as driving forces was larger by a factor of 10 than that determined using osmotic gradients (averages: Lpr [hydrostatic] = 2.7 x 10-7 m s-1 MPa-1; Lpr [osmotic] = 2.2 x 10-8 m s-1 MPa-1; osmotic solute: NaCl). Puncturing the endodermis did not result in measurable increases in hydraulic conductivities measured by either method. Thus, the endodermis was not rate-limiting root Lpr: apparently the hydraulic resistance of roots was more evenly distributed over the entire root tissue. However, puncturing the endodermis did substantially change the reflection ([sigma]sr) and permeability (Psr) coefficients of roots for NaCl, indicating that the endodermis represented a considerable barrier to the flow of nutrient ions. Values of [sigma]sr decreased from 0.64 to 0.41 (average) and Psr increased by a factor of 2.6, i.e. from 3.8 x 10-9 to 10.1 x 10.-9 m s-1(average). The roots recovered from puncturing after a time and regained root pressure. Measurable increases in root pressure became apparent as soon as 0.5 to 1 h after puncturing, and original or higher root pressures were attained 1.5 to 20 h after injury. However, after recovery roots often did not maintain a stable root pressure, and no further osmotic experiments could be performed with them. The Casparian band of the endodermis is discontinuous at the root tip, where the endodermis has not yet matured, and at sites of developing lateral roots. Measurements of the cross-sectional area of the apoplasmic bypass at the root tip yielded an area of 0.031% of the total surface area of the endodermis. An additional 0.049% was associated with lateral root primordia. These areas are larger than the artificial bypasses created by wounding in this study and may provide pathways for a "natural bypass flow" of water and solutes across the intact root. If there were such a pathway, either in these areas or across the Casparian band itself, roots would have to be treated as a system composed of two parallel pathways (a cell-to-cell and an apoplasmic path). It is demonstrated that this "composite transport model of the root" allows integration of several transport properties of roots that are otherwise difficult to understand, namely (a) the differences between osmotic and hydrostatic water flow, (b) the dependence of root hydraulic resistance on the driving force or water flow across the root, and (c) low reflection coefficients of roots.

Water-facilitated transport of bacteria in unsaturated soil columns: Influence of cell surface hydrophobicity and soil properties

Abstract: The numbers of bacteria, surface applied to soil columns which were subsequently irrigated, decreased exponentially with depth. The numbers of surface-inoculated bacteria decreased by a factor of 10 every 1.1–6.0 cm of soil depth after irrigating the columns with 5cm of water at a rate of 2.5cm h−1. Major differences were observed between the examined test strains. The migration of hydrophobic strains was 2–3 times slower compared to that of hydrophilic strains. This was correlated with an increased adhesion of the hydrophobic strains to soil particles. Increasing the bulk density of the soil from 1.27 to 1.37 g cm−3 reduced the volume of the pores with a diameter of 16–30 μm by ca 80%. This corresponded with a decrease in the migration of bacteria by 30–60%. Migration was reduced by ca 40–80% in a sandy soil mixed with 20% clay soil. The reduction was more pronounced for hydrophilic bacteria.

Fuzzy rule-based models for infiltration

Abstract: Modeling water movement in the unsaturated zone requires a great number of parameters, such as the initial water content, the saturated moisture content, and the saturated hydraulic conductivity, which can relatively easily be assessed. Other functions such as the suction head and the relative hydraulic conductivity, which are highly nonlinear functions of the actual water content are more difficult to find. Furthermore, infiltration experiments carried out at a few locations can only define these parameters at selected locations, and a reasonable interpolation of these parameters is extremely difficult. In this paper, two fuzzy logic-based models are presented for the infiltration process. Basic definitions of fuzzy sets are given, and basic operations are explained. Principles of the rule-based infiltration models are presented. The rules for the first model are based on the moisture content of the adjacent layers and the depth of the wetting front. Using these rules, the flows can be calculated as exact quantities. The rules are derived from a training set which was obtained from several test runs of a Green and Ampt (GA) infiltration model. Separate rules are derived for the flow dynamics in the upper layer (runoff and infiltration), and for water flow between the layers. Examples of the application of the model are given. The model performs with similar accuracy as the GA model, using only 40% of its parameters and running 2 orders of magnitude faster. A training set obtained by numerical solution of the Richards equation was used to create a second fuzzy rule-based model. Here the rules are based on the moisture content of the adjacent layers. Results obtained by the use of the models are compared with measured values, showing a very accurate reproduction of the values.

Geometry, bed topography and drainage system structure of the Haut Glacier d'Arolla, Switzerland

Abstract: As part of an integrated study of the hydrology, meltwater quality and dynamics of the Haut Glacier d'Arolla, Switzerland, the glacier's drainage network structure was determined from patterns of dye recovery in 342 injection experiments conducted from 47 moulins distributed widely across the glacier. This structure was compared with theoretical predictions based upon reconstructed patterns of water flow governed by (a) the subglacial hydraulic potential surface, and (b) the subglacial bedrock surface. These reconstructions were based on measurements of ice surface and bedrock topography obtained by a combination of ground survey and radio-echo sounding techniques. The two reconstructions simulate the drainage system structures expected for (a) closed channels, in which water is pressurized by the overlying ice, and (b) gravity-driven, open-channel flow. The closed-channel model provides the best fit to the observed structure, even though theoretical calculations suggest that, under summer discharge conditions, open-channel flow may be widespread beneath the glacier. Possible reasons for this apparent discrepancy are discussed.

A Numerical Model for Water Flow and Chemical Transport in Variably Saturated Porous Media

Abstract: A two-dimensional numerical model is developed for the simulation of water flow and chemical transport through variably saturated porous media. The nonlinear flow equation is solved using the Calerkin finite-element technique with either the Picard or the Newton iteration scheme. A continuous velocity field is obtained by separate application of the Galerkin technique to the Darcy's equation. A two-site adsorption-desorption model with a first-order loss term is used to describe the chemical behavior of the reactive solute. The advective part of the transport equation is solved with one-step backward particle tracking while the dispersive part is solved by the regular Galerkin finite-element technique. A precondi tioned conjugate gradient-like method is used for the iterative solution of the systems of linear simultaneous equations to save on computer memory and execution time. The model is applied to a few flow and transport problems, and the numerical results are compared with observed and analytic values. The model is found to duplicate the analytic and observed values quite well, even near very sharp fronts.

Hydraulic resistance in Acer saccharum shoots and its influence on leaf water potential and transpiration.

Abstract: A new method is presented for measuring whole-shoot hydraulic conductance, K(T) (kg s(-1) MPa(-1)). The method was also used to determine other conductance values in maple (Acer saccharum Marsh.) stem segments of differing diameter including: K(h) (absolute conductance or conductance per unit pressure gradient, kg s(-1) m MPa(-1)), K(s) (specific conductance or K(h) per unit wood area, kg s(-1) m(-1) MPa(-1)), and LSC (leaf specific conductance or K(h) per unit leaf area, kg s(-1) m(-1) MPa(-1)). A regression of K(T) versus stem basal diameter, D (m), gave K(T) = 5.998 x 10(-2) D(1.402) (R(2) = 0.986 for D from 0.001 to 0.1 m) and a regression for leaf area, A(L) (m(2)), gave A(L) = 4.667 x 10(3) D(2.007) (R(2) = 0.981 for D from 0.001 to 0.3 m). More than 50% of the resistance to water flow in large shoots (0.1 m in diameter and 8 to 10 m long) was contained in branches less than 0.012 m in diameter, i.e., in the distal 1.5 m of branches. We used the regressions to predict the steady state difference in pressure potential, P, between the base of a shoot of diameter D and the average pressure potential at the apices of the shoot; the relation is given by P = 7.781 x 10(4) E D(0.605), where E is the average evaporative flux density (kg s(-1) m(-2)) in the leaves attached to the shoot. After comparing the predictions of this equation to field observations of E and leaf water potential and stomatal conductance, we concluded that the hydraulic conductance of large maple shoots is sufficiently low to prevent maximum stomatal conductance in maple leaves.

Development and evaluation of test methods for benthic invertebrates and sediments: Effects of flow rate and feeding on water quality and exposure conditions

Abstract: In order to ensure among-laboratory comparability in the results of sediment toxicity tests, it is necessary to characterize the influence of variations in test regimes on organism responses and exposure conditions. The objective of these studies was to develop and document an optimized combination of overlying water renewal (flow) and feeding rates for sediment tests with three commonly used benthic species (midges, Chironomus tentans; amphipods, Hyalella azteca; oligochaetes, Lumbriculus variegatus). Optimal conditions were defined by a number of chemical and biological considerations including: (1) flow rate through the system, (2) amount of food added, (3) acceptable responses (survival, growth, reproduction) of the organisms over the course of a 10-day test, and (4) maintenance of an adequate concentration of dissolved oxygen in overlying water. The goal was to minimize factors (1) and (2), while maximizing criteria (3) and (4). The major reason for minimizing (1) and (2) was the concern that excessive water flow or addition of food could reduce exposure of the test organisms to sediment-associated contaminants. To evaluate this, interstitial (pore) water concentrations of contaminants (ammonia, zinc, copper, dieldrin) were measured over the course of 10 day tests conducted with a number of different sediments under various flow and feeding regimes. The different combinations of flow/feeding had variable effects upon pore water concentrations of contaminants; for example under our optimized regime, in some instances slight decreases in interstitial water contaminant concentrations were observed, whereas in other cases contaminant concentrations remained constant or even increased. Overall, the use of minimal water renewal and feeding rates should result only in small changes in exposure of benthic organisms to contaminants in pore water over the course of 10-day tests.

Xylem development and hydraulic conductance in sun and shade shoots of grapevine (Vitis vinifera L.) : evidence that low light uncouples water transport capacity from leaf area

Abstract: Morphology, water relations, and xylem anatomy of high-light (sun)- and low-light (shade)-grown Vitis vinifera L. shoots were studied to determine the effects of shading on the hydraulic conductance of the pathway for water flow from the roots to the leaves. Shade shoots developed leaf area ratios (leaf area: plant dry weight) that were nearly threefold greater than sun shoots. Water-potential gradients (ΔΨ·m−1) in the shoot xylem accounted for most of the ΔΨ·m−1 between soil and shoot apex at low and high transpiration rates in both sun and shade shoots, but the gradients were two- to fourfold greater in shade-grown plants. Low light reduced xylem conduit number in petioles, but had an additional slight effect on conduit diameter in internodes. The hydraulic conductance per unit length (Kh) and the specific hydraulic conductivity (ks, i.e. Kh per xylem cross-sectional area) of internodes, leaf petioles, and leaf laminae at different developmental stages leaf plastochron index was calculated from measurements of water potential and water flow in intact plants, from flow through excised organs, and from vessel and tracheid lumen diameters according to Hagen-Poiseuille's equation. For all methods and conductance parameters, the propensity to transport water to sink leaves was severalfold greater in internodes than in petioles. The Kh and ks increased logarithmically until growth ceased, independent of treatment and measurement method, and increased further in pressurized-flow experiments and Hagen-Poiseuille predictions. However, the increase was less in shade internodes than in sun internodes. Mature internodes of shade-grown plants had a two- to fourfold reduced Kh and significantly lower ks than sun internodes. Except very early in development, leaf lamina conductance and ks from shade-grown plants was also reduced. The strong reduction in Kh with only a slight reduction in leaf area (17% of sun shoots) in the shade shoots indicated a decoupling of water-transport capacity from the transpirational surface supplied by that capacity. This decoupling resulted in strongly reduced leaf specific conductivities and Huber values for both internodes and petioles, which may increase the likelihood of cavitation under conditions of high evaporative demand or soil drought.

Water flow in soils

Abstract: The new edition of a bestseller, Water Flow in Soils bridges the fields of soil physics-where descriptions of water flow tend to be microscopic- and hydrology - where they tend to be macroscopic. Unlike other physics laden texts, this work conveys the fundamental concepts of water flow in soils with clear and essentially nonmathematical explanation

Kinetic parameters of nitrate uptake by different catch crop species: effects of low temperatures or previous nitrate starvation

Abstract: The pollution of aquifers by NO−3 in temperate environments is aggravated by farming practices that leave the ground bare during winter. The use of catch crops during this time may decrease nitrate loss from the soil. Nitrate uptake by several catch crop species (Brassica napus L., Sinapis alba L., Brassica rapa L., Raphanus sativus L., Trifolium alexandrinum L., Trifolium incarnatum L., Phacelia tanacetifolia Benth., Lolium perenne L., Lolium multiflorum Lam. and Secale cereale L.) was here studied in relation to transpiration rate and low temperatures applied to the whole plant or to roots only. The Michaelis constant (Km), maximum uptake rate (Vmax), time of induction and contributions of inducible and constitutive mechanisms were estimated from measurements of NO−3 depletion in the uptake medium. There were large differences between species, with Km (μM) values ranging between 5.12 ± 0.64 (Trifolium incarnatum) and 36.4 ± 1.97 (Lolium perenne). Maximum NO−3 uptake rates expressed per unit root weight were influenced by ageing, temperature and previous NO−3 nutrition. They were also closely correlated with water flow through the roots and with shoot/root ratio of these species. The combined results from all species and treatments showed that Vmax increased with shoot/root ratio, suggesting a regulatory role for the shoots in NO−3 uptake. Overall, the results showed a great diversity in NO−3 uptake characteristics between species in terms of kinetic parameters, contribution of the constitutive system (100% of total uptake in ryegrass, nil in Fabaceae) and time of induction.

The importance of fluvial hydraulics to fish-habitat restoration in low-gradient alluvial streams

Abstract: SUMMARY 1 A major cause of degradation and loss of stream fish is alteration of physical habitat within and adjacent to the channel. We describe a potentially efficient approach to fish restoration based upon the relationship between fluvial hydraulics, geomorphology, and those habitats important to fish. 2 The aquatic habitat in a low-gradient, alluvial stream in the Ozark Plateaus physiographical province was classified according to location in the channel, patterns of water flow, and structures that control flow. The resulting habitat types were ranked in terms of their temporal stability and ability to be manipulated. 3 Delineation and quantification of discrete physical spaces in a stream, termed hydraulic habitat units, are shown to be useful in stream restoration programmes if the ecological importance of each habitat unit is known, and if habitats are defined by fluvial dynamics so that restoration is aided by natural forces. 4 Examples, using different taxa, are given to illustrate management options.