Changes In Hydraulic Properties Research Articles

Postconstruction Changes in the Hydraulic Properties of Water Balance Cover Soils

Hydraulic properties of soils used for water balance covers measured at the time of construction and one to four years after construction are compared to assess how the hydraulic properties of cover soils change over time as a result of exposure to field conditions. Data are evaluated from ten field sites in the United States that represent a broad range of environmental conditions. The comparison shows that the saturated hydraulic conductivity Ks can increase by a factor of 10,000, saturated volumetric water content s by a factor of 2.0, van Genuchten's parameter by a factor of 100, and van Genuchten's n parameter can decrease by a factor of 1.4. Larger changes occur for denser or more plastic fine-textured soils that have lower as-built Ks, , and s and higher as-built n, resulting in a reduction in the variation in hydraulic properties that can be attributed to compaction. After two to four years, many water balance cover soils can be assumed to have Ks between 10 5 and 10 3 cm/s, s between 0.36 and 0.40, between 0.002 and 0.2 kPa 1 , and n between 1.2 and 1.5. The data may be used to estimate changes in hydraulic properties for applications such as waste containment, where long-term maintenance of hydraulic properties in shallow engineered soil layers is important.

Using a pore‐scale model to quantify the effect of particle re‐arrangement on pore structure and hydraulic properties

AbstractA pore‐scale model based on measured particle size distributions has been used to quantify the changes in pore space geometry of packed soil columns resulting from a dilution in electrolyte concentration from 500 to 1 mmol l−1 NaCl during leaching. This was applied to examine the effects of particle release and re‐deposition on pore structure and hydraulic properties. Two different soils, an agricultural soil and a mining residue, were investigated with respect to the change in hydraulic properties. The mining residue was much more affected by this process with the water saturated hydraulic conductivity decreasing to 0·4% of the initial value and the air‐entry value changing from 20 to 50 cm. For agricultural soil, there was little detectable shift in the water retention curve but the saturated hydraulic conductivity decreased to 8·5% of the initial value. This was attributed to localized pore clogging (similar to a surface seal) affecting hydraulic conductivity, but not the microscopically measured pore‐size distribution or water retention. We modelled the soil structure at the pore scale to explain the different responses of the two soils to the experimental conditions. The size of the pores was determined as a function of deposited clay particles. The modal pore size of the agricultural soil as indicated by the constant water retention curve was 45 µm and was not affected by the leaching process. In the case of the mining residue, the mode changed from 75 to 45 µm. This reduction of pore size corresponds to an increase of capillary forces that is related to the measured shift of the water retention curve. Copyright © 2007 John Wiley & Sons, Ltd.

Seasonal changes of hydraulic properties of a Chromic Luvisol under different soil management

In the present work the effect of five tillage methods on the hydraulic properties and water regime of a brown forest soil was studied. In each treatment, measurements of bulk density and soil water retention characteristics were carried out 3 times (March, June and August) within the vegetation period. Near-saturated hydraulic conductivity and soil water content measurements were performed five and eight times, respectively. Statistically valuable differences were obtained between the soil properties, measured in different tillage treatments. The effect of the tillage treatments on the water retention curves was significant in the low suction range (pF < 2.0) only. Differences between the soil water retention curves, measured at the end of the vegetation period reflected the indirect effect of different tillage systems on soil hydraulic properties. The seasonal variability of both the soil hydraulic functions was proofed. Saturated hydraulic conductivity values, evaluated in the ploughing treatment at the beginning and end of the vegetation period differed up to 4-times. The near-saturated hydraulic conductivity values measured in March were nearly two times higher in all the treatments, except no till, than those, measured in August. The applied tillage systems did not influence the potential amount of water available for the plant; still, valuable differences between the soil water contents were measured. According to the soil hydraulic properties and measured soil water regime, ploughing and deep loosening created the most favourable soil conditions for the plants. The biological activity, however, was the highest in the no till treatment. Further studies on the application of the soil conserving tillage systems under Hungarian conditions are recommended.

Changes of permeability characteristics dependent on damage process in granites

Fracture properties are closely related to the permeability characteristics of massive crystalline rocks. In order to understand the changes of rock permeability related to the damage process, this study performed characterization on changes of hydraulic properties with both in-situ tests and laboratory tests. The results of in-situ tests showed that permeability coefficients have differences on each section from the distance of tunnel wall. The differences are related to the conditions of fracture distribution due to rock damage in the process of tunneling. This study performed uniaxial compressive tests (UCT) and water injection tests in order to understand the relationship between the damage process and changes of permeability characteristics by the laboratory tests. Crack distribution on each damage grade was observed by the acetate peel. According to the results of the tests, the strongly damaged specimens with crack length density of more than 0.6 cm−1 showed higher values of permeability coefficients than the weakly damaged specimens. Moreover, the former specimens have persistent cracks and good connectivity throughout the rock specimens. It indicates that rock damage influences on the permeability characteristics in rocks.

Responses to water stress in two Eucalyptus globulus clones differing in drought tolerance.

We evaluated drought resistance mechanisms in a drought-tolerant clone (CN5) and a drought-sensitive clone (ST51) of Eucalyptus globulus Labill. based on the responses to drought of some physiological, biophysical and morphological characteristics of container-grown plants, with particular emphasis on root growth and hydraulic properties. Water loss in excess of that supplied to the containers led to a general decrease in growth and significant reductions in leaf area ratio, specific leaf area and leaf-to-root area ratio. Root hydraulic conductance and leaf-specific hydraulic conductance decreased as water stress became more severe. During the experiment, the drought-resistant CN5 clone maintained higher leaf water status (higher predawn and midday leaf water potentials), sustained a higher growth rate (new leaf area expansion and root growth) and displayed greater carbon allocation to the root system and lower leaf-to-root area ratio than the drought-sensitive ST51 clone. Clone CN5 possessed higher stomatal conductances at moderate stress as well as higher hydraulic conductances than Clone ST51. Differences in the response to drought in root biomass, coupled with changes in hydraulic properties, accounted for the clonal differences in drought tolerance, allowing Clone CN5 to balance transpiration and water absorption during drought treatment and thereby prolong the period of active carbon assimilation.

Stomatal control and hydraulic conductance, with special reference to tall trees.

A better understanding of the mechanistic basis of stomatal control is necessary to understand why modes of stomatal response differ among individual trees, and to improve the theoretical foundation for predictive models and manipulative experiments. Current understanding of the mechanistic basis of stomatal control is reviewed here and discussed in relation to the plant hydraulic system. Analysis focused on: (1) the relative role of hydraulic conductance in the vicinity of the stomatal apparatus versus whole-plant hydraulic conductance; (2) the influence of guard cell inflation characteristics and the mechanical interaction between guard cells and epidermal cells; and (3) the system requirements for moderate versus dramatic reductions in stomatal conductance with increasing evaporation potential. Special consideration was given to the potential effect of changes in hydraulic properties as trees grow taller. Stomatal control of leaf gas exchange is coupled to the entire plant hydraulic system and the basis of this coupling is the interdependence of guard cell water potential and transpiration rate. This hydraulic feedback loop is always present, but its dynamic properties may be altered by growth or cavitation-induced changes in hydraulic conductance, and may vary with genetically related differences in hydraulic conductances. Mechanistic models should include this feedback loop. Plants vary in their ability to control transpiration rate sufficiently to maintain constant leaf water potential. Limited control may be achieved through the hydraulic feedback loop alone, but for tighter control, an additional element linking transpiration rate to guard cell osmotic pressure may be needed.

Tillage Effects on Subsurface Drainage

Tillage near the soil surface may greatly influence drainage discharge and pressure head values in the subsurface zone. In this study, a controlled comparative experiment was conducted in the field under natural weather conditions, using a tilled and an untilled column, to evaluate the effects of tillage on subsurface drainage discharge and pressure head values. There were no significant differences between the two columns in subsurface drainage before tillage treatment before the study, as checked by a preliminary experiment. After tilling one column of the two columns, cumulative subsurface drainage discharge was larger and occurred earlier for the tilled column than for the untilled column. The measured drainage discharge and pressure head values were evaluated using the water movement model, HYDRUS version 6.0, which numerically solves the Richards equation. The HYDRUS model reproduced measured values well for subsurface drainage discharge and pressure head values after the tillage, as determined by root mean squared error (RMSE), mean bias error (MBE), and R2 Therefore, it is concluded that the model, which includes the effects of tillage on the hydraulic properties, can explain the reasons for differences in observed drainage in the two columns. Moreover, the effects of tillage depth on subsurface drainage discharge were simulated. The results implied that the effect of tillage on subsurface drainage were induced by the conditions of pressure head just before the first rainfall, and this effect was equal to, or greater than, the effects of changes in hydraulic properties due to tillage.

Field Evaluation of Soil Physical, Chemical and Hydraulic Property Changes Caused By Surface Water Application: A Case Study of Ruaha-Mbuyuni Irrigation Scheme, Iringa, Tanzania

Field Evaluation of Soil Physical, Chemical and Hydraulic Property Changes Caused By Surface Water Application: A Case Study of Ruaha-Mbuyuni Irrigation Scheme, Iringa, Tanzania

Modelling leachate quality and quantity in municipal solid waste landfills.

The operational phase of landfills may last for 20 years or more. Significant changes in leachate quality and generation rate may occur during this operational period. A mathematical model has been developed to simulate the landfill leachate behaviour and distributions of moisture and leachate constituents through the landfill, taking into consideration the effects of time-dependent landfill development on the hydraulic characteristics of waste and composition of leachate. The model incorporates governing equations that describe processes influencing the leachate production and biochemical processes taking place during the stabilization of wastes, including leachate flow, dissolution, acidogenesis and methanogenesis. To model the hydraulic property changes occurring during the development stage of the landfills, a conceptual modelling approach was proposed. This approach considers the landfill to consist of cells or columns of cells, which are constructed at different times, and considers each cell in the landfill to consist of several layers. Each layer is assumed to be a completely mixed reactor containing uniformly distributed solid waste, moisture, gases and micro-organisms. The use of the proposed conceptual model enables the incorporation of the spatial changes in hydraulic properties of the landfill into the model and also makes it possible to predict the spatial and temporal distributions of moisture and leachate constituents. The model was calibrated and partially verified using leachate data from Keele Valley Landfill in Ontario, Canada and data obtained from the literature. Ranges of values were proposed for model parameters applicable for real landfill conditions.

Fault compaction and overpressured faults: results from a 3-D model of a ductile fault zone

SUMMARY A model of a ductile fault zone is incorporated into a forward 3-D earthquake model to better constrain fault-zone hydraulics. The conceptual framework of the model fault zone was chosen such that two distinct parts are recognized. The fault core, characterized by a relatively low permeability, is composed of a coseismic fault surface embedded in a visco-elastic volume that can creep and compact. The fault core is surrounded by, and mostly sealed from, a high permeability damaged zone. The model fault properties correspond explicitly to those of the coseismic fault core. Porosity and pore pressure evolve to account for the viscous compaction of the fault core, while stresses evolve in response to the applied tectonic loading and to shear creep of the fault itself. A small diffusive leakage is allowed in and out of the fault zone. Coseismically, porosity is created to account for frictional dilatancy. We show in the case of a 3-D fault model with no in-plane flow and constant fluid compressibility, pore pressures do not drop to hydrostatic levels after a seismic rupture, leading to an overpressured weak fault. Since pore pressure plays a key role in the fault behaviour, we investigate coseismic hydraulic property changes. In the full 3-D model, pore pressures vary instantaneously by the poroelastic effect during the propagation of the rupture. Once the stress state stabilizes, pore pressures are incrementally redistributed in the failed patch. We show that the significant effect of pressure-dependent fluid compressibility in the no in-plane flow case becomes a secondary effect when the other spatial dimensions are considered because in-plane flow with a near-lithostatically pressured neighbourhood equilibrates at a pressure much higher than hydrostatic levels, forming persistent high-pressure fluid compartments. If the observed faults are not all overpressured and weak, other mechanisms, not included in this model, must be at work in nature, which need to be investigated. Significant leakage perpendicular to the fault strike (in the case of a young fault), or cracks hydraulically linking the fault core to the damaged zone (for a mature fault) are probable mechanisms for keeping the faults strong and might play a significant role in modulating fault pore pressures. Therefore, fault-normal hydraulic properties of fault zones should be a future focus of field and numerical experiments.

Permeability of Triaxially Compressed Sandstone: Influence of Deformation and Strain-rate on Permeability

— The influence of differential stress on the permeability of a Lower Permian sandstone was investigated. Rock cylinders of 50 mm in diameter and 100 mm length of a fine-grained (mean grain size 0.2 mm), low-porosity (6–9%) sandstone were used to study the relation between differential stress, rock deformation, rock failure and hydraulic properties, with a focus on the changes of hydraulic properties in the pre-failure and failure region of triaxial rock deformation. The experiments were conducted at confining pressures up to 20 MPa, and axial force was controlled by lateral strain with a rate ranging from 10−6 to 10−7 sec−1. While deforming the samples, permeability was determined by steady-state technique with a pressure gradient of 1 MPa over the specimen length and a fluid pressure level between 40 and 90% of the confining pressure. The results show that permeability of low-porosity sandstones under increasing triaxial stress firstly decreases due to compaction and starts to increase after the onset of dilatancy. This kind of permeability evolution is similar to that of crystalline rocks. A significant dependence of permeability evolution on strain rate was found. Comparison of permeability to volumetric strain demonstrates that the permeability increase after the onset of dilatancy is not sufficient to regain the initial permeability up to failure of the specimen. The initial permeability, which was determined in advance of the experiments, usually was regained in the post-failure region. After the onset of dilatancy, the permeability increase displays a linear dependence on volumetric strain.

Tillage and traffic effects on soil hydraulic properties and evaporation

Evaporation is a major component of the water loss from the soil whose structure is modified by traffic and tillage. This study was undertaken to analyse, in field conditions, the effect of tillage and traffic on soil structure and evaporation, and to determine the role of the change in hydraulic properties on soil drying using water transfer model. Three structures of the ploughed layer were formed in a loess soil (Luvisol Orthique) and a calcareous soil (Rendzina): a fragmentary structure created by deep soil tillage in autumn or in spring (rotary tiller at 30 cm depth), a compacted ploughed layer created by compaction under wet conditions. The bulk density varied from 1.16 to 1.63 Mg m −3 in the loess soil, from 1.00 to 1.45 Mg m −3 in the calcareous soil. Evaporation was calculated from the change in soil water content and matric water potential profiles measured during the spring season. Soil hydraulic properties were estimated using an inverse modelling method applied to field measurements of water content and water potential or the Wind method. Soil structure greatly affected the drying of the calcareous soil: the evaporation of the compacted plot was about two times that of the tilled plot. The compacted plot dried out homogeneously, with a soil surface which remained wet. Evaporation mainly concerned the first 15 cm of the ploughed layer created by autumn or spring tillage. This effect of soil structure on evaporation was not observed in the loess soil. The unsaturated hydraulic conductivity was higher in the compacted plot than in the tilled plots in the calcareous soil. It was similar in the three plots of the loess soil, because of the formation of relict structural pores by compaction. Experimental and numerical results showed that unsaturated hydraulic conductivity is of major concern in soil drying and that the albedo and surface roughness have minor effects if any. The possible relict structural pores have to be characterised in various soils, as a function of soil sensitivity to compaction, traffic and tillage conditions.

Effect of Application of Surfactants on Hydraulic Properties of Soils

This study explores the effect of surfactants, commonly found in detergents, on the hydraulic properties of soils. The soil properties examined included hydraulic conductivity, infiltration characteristics, and effective suction at the wetting front, capillary rise and soil penetrability. Two agricultural soils—a loam and a sandy loam, and three surfactants—one anionic surfactant (Sulphonic) and two non-ionic surfactants (Rexol and Rexonic), were used in the study. Changes in hydraulic properties with the application of surfactants were compared with properties obtained with deionised water (control). The results showed that Sulphonic, the anionic surfactant, had a significant effect on hydraulic properties of both soils. Applications of Sulphonic caused decreases in the capillary rise and penetrability, and an increase in the solid–liquid contact angle, shape factor and sorptivity. Except for a slight decrease in hydraulic conductivity resulting from the application of Rexol, the non-ionic surfactants did not reveal significant impact on the hydraulic characteristics of test soils.

Monitoring Disturbances in the Repository System during Construction Process: Experiences from Natural Decay Series Studies at the Pyhäsalmi Mine (Central Finland)

ABSTRACTPrior to the selection of the site for nuclear waste disposal, the performance of the repository is assessed based on general geological data of sites under investigation. Once the site has been selected, the results of the site investigations are used to establish the baseline condition to monitor and evaluate the induced processes during the construction of the repository. One of the processes considered is the change in hydraulic properties in the bedrock due to excavation of tunnels and other open spaces. Within this framework, the results of natural decay series studies in rock samples from the Pyhäsalmi mine collected in the nearby of a water-conducting fracture at the depths of 1275 and 1325 m may give insights on the influence of the excavation-induced changes, if any, in the groundwater flow. The spatial and temporal scale of these changes is also evaluated. The methodology used in this work could be applied to add confidence in the establishment of the baseline conditions to better assess the evolution of changes in groundwater flow patterns during and after construction of a repository system.

Relating Electrical Response and Petrophysical Properties of Sands Subjected to Stress Changes

The spectral electrical responses (SER) and hydraulic properties of clean sands saturated with deionized water and subjected to varying stress levels in a laboratory environment are investigated and analyzed. A variation of the multi-Cole-Cole dispersion model that predicts the behavior of the SER of sands is suggested for the analysis. This dispersion model is described by circuit elements [Formula: see text], the bulk resistivity, chargeability [Formula: see text], time constant [Formula: see text], and exponent [Formula: see text], representing various conduction mechanisms. The values of [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text], which describe the SER are directly affected by changes in the applied stress and the attendant changes in hydraulic properties: porosity and hydraulic conductivity. It is observed that changes in the hydraulic properties of the sands due to changes in the applied effective stress are related to the circuit parameters characterizing this model. In general, a decrease in porosity results in an increase in the relaxation time constant [Formula: see text]. The rate of increase in [Formula: see text] is higher for variable size sands than for monosize sands. The time constant is generally greater for larger monosize sands than for smaller monosize sands, and thus [Formula: see text] may be related directly to the mean grain size. Changes in the applied effective stress are shown to result in changes in the chargeability of the soils. The chargeability [Formula: see text] of the soil decreases with increases in hydraulic conductivity [Formula: see text] and porosity [Formula: see text]. Increases in [Formula: see text] and [Formula: see text] tend to increase the frequency dependence exponent [Formula: see text] of the soil.

Seasonal changes in tissue elasticity and water transport efficiency in three co-occurring Mediterranean shrubs under natural long-term CO2 enrichment.

Seasonal changes in hydraulic properties and tissue elasticity were evaluated in Erica arborea L., Myrtuscommunis L. and Juniperus communis L., three Mediterranean shrubs that differ in adaptations to drought. These parameters were analysed over 12 months under field conditions, by comparing plants grown in the proximity of a natural CO2 spring (about 700 μmol mol-1 atmospheric CO2 concentration, [CO2]) with plants in ambient conditions. Plants at the CO2-spring site have been growing for generations at elevated [CO2]. At both sites, stem hydraulic and structural properties followed the prevailing climatic constraints. However, these shrub species co-occurring in the same environment differed in their capacity to tolerate water deficits, in xylem efficiency, and in strategies for regulating water movement between plant compartments. Either an increase or a decrease in tissue elasticity was effective in promoting resistance to drought stress, depending on the species. Long-term elevated [CO2] influenced all the studied parameters. Species-dependent differences existed in hydraulic architecture between the CO2-spring plants and control plants of E. arborea and M. communis, while J. communis plants rarely showed differences between sites. Less distinct differences between sites were observed for wood structure. The three species showed somewhat lower tissue elasticity under elevated [CO2], in particular during stress periods. The effects of elevated [CO2] on stem hydraulic pathway and structure and shoot elastic properties persist in the long term, but differ in absolute values and sign among the studied species and with the seasonal course, and thus might alter competitive relations between these shrubs.

A forward model for earthquake generation on interacting faults including tectonics, fluids, and stress transfer

We present a forward model of interacting faults for systems of any geometry. The model generalizes that of Miller et al. [1996, 1999] to a fully three‐dimensional model where faults of any strike and geometry interact through an elastic matrix using the general solutions of Okada [1992]. The model includes large‐scale plate motion loading and increasing pore pressures from a source term, undrained poroelastic effects, large coseismic hydraulic property changes, and porosity creation through dilatant slip. To illustrate the basic behavior and utility of the model, results are presented of the long‐term evolution (≈9300 years) for a generic case of a blind, dipping fault and a subvertical strike‐slip fault in a transpressional environment. We show the stress state evolution along both faults, seismicity time lines, quasi‐static rupture propagation including rake angle changes, local and regional stress buildup and rotations, static and dynamic fault interactions, and ΔCFS (changes in Coulomb Failure Stress) within the fault system. Large compartments of varying overpressure result on both faults from coseismic pore pressure changes and contribute to the complexity of the stress state. For the considered case, we find that the poroelastic effects on the receiver fault are about twice the change in the shear stress, providing a significant contribution to the ΔCFS. Regional stress rotations in response to the model seismicity indicate that further model developments must include dynamic generation of new faults in response to the evolving tectonic regime.

Effect of compaction on the porosity of a silty soil: influence on unsaturated hydraulic properties

SummaryTillage and traffic modify soil porosity and pore size distribution, leading to changes in the unsaturated hydraulic properties of the tilled layer. These changes are still difficult to characterize. We have investigated the effect of compaction on the change in the soil porosity and its consequences for water retention and hydraulic conductivity. A freshly tilled layer and a soil layer compacted by wheel tracks were created in a silty soil to obtain contrasting bulk densities (1.17 and 1.63 g cm−3, respectively). Soil porosity was analysed by mercury porosimetry, and scanning electron microscopy was used to distinguish between the textural pore space and the structural pore space. The laboratory method of Wind (direct evaporation) was used to measure the hydraulic properties in the tensiometric range. For water potentials &lt; −20 kPa, the compacted layer retained more water than did the uncompacted layer, but the relation between the hydraulic conductivity and the water ratio (the volume of water per unit volume of solid phase) was not affected by the change in bulk density. Compaction did not affect the textural porosity (i.e. matrix porosity), but it created relict structural pores accessible only through the micropores of the matrix. These relict structural pores could be the reason for the change in the hydraulic properties due to compaction. They can be used as an indicator of the consequences of compaction on unsaturated hydraulic properties. The modification of the pore geometry during compaction results not only from a decrease in the volume of structural pores but also from a change in the relation between the textural pores and the remaining structural pores.

Numerical modeling of coupled variably saturated fluid flow and reactive transport with fast and slow chemical reactions

The couplings among chemical reaction rates, advective and diffusive transport in fractured media or soils, and changes in hydraulic properties due to precipitation and dissolution within fractures and in rock matrix are important for both nuclear waste disposal and remediation of contaminated sites. This paper describes the development and application of LEHGC2.0, a mechanistically based numerical model for simulation of coupled fluid flow and reactive chemical transport, including both fast and slow reactions in variably saturated media. Theoretical bases and numerical implementations are summarized, and two example problems are demonstrated. The first example deals with the effect of precipitation/dissolution on fluid flow and matrix diffusion in a two-dimensional fractured media. Because of the precipitation and decreased diffusion of solute from the fracture into the matrix, retardation in the fractured medium is not as large as the case wherein interactions between chemical reactions and transport are not considered. The second example focuses on a complicated but realistic advective–dispersive–reactive transport problem. This example exemplifies the need for innovative numerical algorithms to solve problems involving stiff geochemical reactions.

Stem hydraulic properties and xylem vulnerability to embolism in three co-occurring Mediterranean shrubs at a natural CO2 spring

Seasonal changes in hydraulic properties and vulnerability to xylem embolism of Erica arborea L., Myrtus communis L. and Juniperus communis L. were analysed by comparing plants at two locations in a Mediterranean environment. A distinct atmospheric CO2 concentration ([CO2]) gradient exists between the two sites with higher [CO2] in the proximity of a natural CO2 spring (700 mol mol–1). Changes in native embolism in E. arborea and M. communis indicated rather clear seasonal segregation by species and by the growth [CO2]. J. communis had constantly lower percentage embolism than the other two species (the effect of site being not consistent). Differences in summer embolism among species and between sites were in accordance with vulnerability curves. Volumetric fractions also showed seasonal and site-dependent changes. Mean specific hydraulic conductivity was strongly affected by site in E. arborea and M. communis (in opposite directions). Hydraulic properties varied as a function of shoot biomass and leaf area, and the latter increased with increasing sapwood area; differences between sites were somewhat significant in M. communis. Foliage biomass increased with stem biomass; E. arborea had higher values of foliage biomass at similar values of stem biomass at the control site. Altering branch biomass allocation may influence or not (depending on the species) hydraulic adjustment. Plant responses to resource imbalances caused by increasing [CO2] tend to compensate for the imbalance by changes in hydraulic properties and biomass allocation. However, the plasticity or compensation ability of any particular species appears limited, and effective compensation for large changes in resource balance caused by environmental forcing factors may require changes in species composition.