Net Water Flux Research Articles

Interface nanoparticle control of a nanometer water pump.

Nanoparticles are highly versatile and exhibit broad applications in tuning material properties. Herein, we show through molecular dynamics simulations the possibility of a nanometer water pump, driven by the motion of nanoparticles (NPs) on a membrane surface. Surprisingly, considerable net water flux can be induced through a carbon nanotube (CNT) that is perpendicular to the NP motion. The water transport can occur in a highly controllable fashion, not only by using a single NP with different forces, but also by varying the CNT length or the NP number. Specifically, for a single NP, the water flow and flux are found to increase linearly with an increase in force, following the same behavior of NP velocity. Inversely, the water translocation time exhibits a linear decrease. We further revealed the unique relation between the water flow and occupancy divided by the translocation time. The CNT length can significantly screen the thermal fluctuation of an outside water reservoir, leading to an increase in the water flux and subsequent unidirectional transport. More interestingly, under moderate force, the water flow and flux demonstrate maximum behaviors with an increase in NP number, co-determined by the NP velocity and water occupancy. The maximum location shifts to the lower NP number region for a larger force. We also identify two CNT states that correspond to low water flow. Our results provide a significant new method to pump water molecules through a CNT channel, which is helpful for the design of controllable nanofluidic devices.

Regional water budgets and hydroclimatic trend variations in Xinjiang from 1951 to 2000

Xinjiang is located in arid northwestern China where water cycle has accelerated due to increased precipitation and temperature. However, the regional water budget characteristics vary due to the complex topography and spatial heterogeneities of hydroclimatology. This study uses atmospheric forcing constrained by observation from 90 meteorological stations in Xinjiang as input for the optimized Community Land Model version 3.5 (CLM 3.5) to investigate Xinjiang’s regional water budgets from 1951 to 2000 between the northern and southern part divided by the Tianshan Mountain. Results show that precipitation, evapotranspiration and runoff increased in Xinjiang from 1951 to 2000, particularly after the climate shift around 1987, and the net water flux (P-E) gap between North and South Xinjiang was widened. Rapid, intense wetting occurred in North Xinjiang in response to regional climate change, whereas South Xinjiang experienced relatively small changes. North and South Xinjiang exhibited opposite trends in water table depth (WTD), which became shallower in North Xinjiang, particularly after 1987. The WTD in South Xinjiang gradually became deeper. These results suggest that water resources in North Xinjiang are more sensitive to the warmer and wetter climate than South Xinjiang, and the serious water shortage in South Xinjiang did not improve during the second half of the twentieth century.

Dimensionless Model Analysis of PEFC Cathode

INTRODUCTION Although extensive studies have been conducted on PEFC behavior and a lot of knowledge has been published, understanding the PEFC is still not easy since the system is considerably complicated. By investigating a dimensionless form of equations describing the cathode catalyst layer (CL), the cathode model was simplified and a few intrinsic moduli which control the dimensionless rate profile were proposed (1). The impact of convection on the oxygen reduction reaction (ORR) rate and some case studies are demonstrated. DIMENSIONLESS MODEL The ORR rate is proportional to the oxygen partial pressure at fixed cathode emf. The emf dependency follows a Tafel equation. Oxygen balance in cathode catalyst layer is expressed as follows: d(N g y O – C g D eO dy O/dz) / dz = –r vc[1] where N g is the total gas flux, y O is the oxygen mole fraction, and D eOis the effective oxygen diffusivity (2). Proton flux can be calculated by the reaction stoichiometry. Proton potential follows the Ohm’s law. With the 1D model of cathode CL, 12 variables including boundary conditions have to be specified so as to determine the profiles of partial pressure, emf, reaction rate, etc. Dimensionless forms of model equations were derived, which include the following dimensionless moduli we propose: M O (C) m = δ (C) (k vc m RT / D eO)1/2[2] M p (C) m = δ (C) {4F k vc m p Oc / (σ ep b c)}1/2 [3] where σ ep is the effective proton conductivity, b c is the Tafel slope, δ (C) is the CL thickness, k vcm is the ORR rate constant per unit volume of cathode CL at the PEM–CL boundary and p Oc is oxygen partial pressure at the CL–GDL boundary. k vcm is the greatest rate constant and p Oc is the highest partial pressure in the CL. Therefore, k vcm p Oc represents the intrinsic ORR rate without transport resistance. M O (C) m and M p (C) m respectively represent the ratio of reaction performance to oxygen transport performance and the ratio of reaction to proton transport. A Peclet number which represents the ratio of convection to diffusion of oxygen is also required for formulation: P O (C) m = δ (C) N A(M) /(C g D eO) [4] where N A (M)is the net water flux through the PEM, which equals the total gas flux at the PEM–CL boundary. The dimensionless ORR rate profile is determined by only 4 dimensionless parameters, M O (C) m , M p (C) m , P O (C) m , and y Oc . The dimensionless system greatly reduces the complication of system. RESULTS AND DISCUSSION Define the cathode effectiveness factor F ec as a ratio of the ORR rate to the intrinsic ORR rate without transport resistance. Figure 1 shows that increase in transport resistance reduces the effectiveness factor. Typical values of M O (C) m and M p (C) m in usual cells are around 1. When M p (C) m is high, proton transport is slow, which increases the cathode emf and reduces the ORR rate. If M O (C) m >> M p (C) m , oxygen transport is suppressed and ORR takes place near the oxygen inlet, the GDL side. As the convective flow from PEM to GDL increases, oxygen distribution is shifted towards the GDL, reducing the effectiveness factor as shown in Fig. 2. Even at quite low M O (C) m and M p (C) m of 0.01, the convection can reduce the effectiveness factor. When M O (C) m and M p (C) m are high, the impact of convection is not remarkable since the reaction takes place only near the boundaries. When back diffusion of water in the PEM is dominant, P O (C) m can be negative. F ec reaches 1 under reaction control. Figure 3 shows a case study in which the CL thickness δ (C) is varied with a fixed Pt loading per volume. As δ (C) increases, the Pt amount increases proportionally but the transport resistance increases M O (C) m and M p (C) m and the effectiveness factor decreases as shown in Fig. 1. As a result, the increase in the current density is limited. Since the increase in δ (C)raises both moduli, it overtakes the increase of Pt; a maximum current appears in Fig. 3. Employing the proposed dimensionless moduli, the effects of dimensions and operating conditions can be estimated quantitatively without expensive simulation. Acknowledgement This work is a part of the project by the New Energy and Industrial Technology Development Organization (NEDO), Japan. REFERENCES 1. M. Kawase et al., 24th Int. Symp. Chem. React Eng. (Minneapolis, June 2016). 2. M. Kawase et al., ECS Trans. 16(2), 563–573 (2008). Figure 1

Analytical calculation and evaluation of water transport through a proton exchange membrane fuel cell based on a one-dimensional model

A one-dimensional model accounting for chief water transport phenomena in proton exchange membrane (PEM) fuel cell is developed. The water transfer process in catalyst layers (membrane water absorption/desorption) is explicitly taken into account and the mathematical descriptions of the Schroeder's paradox are presented in this model. In addition, to solve two-phase problems in gas diffusion layers (GDLs) without using complex numerical approach, the assumption of an infinite phase change rate is applied and the analytical solutions to the two-phase model equations are derived. Based on the model and the analytical solutions, an identification procedure using the iterative approach is proposed to (1) determine the net water flux through the membrane, (2) obtain exact water profiles in all components of PEM fuel cell and (3) predict the dehydration and flooding. The AC impedance technique is used to analyze the cell performance and the predicting capability of the model-based approach is validated. The measurements and predictions both reveal the effect of electro-osmotic drag on drying the anode at high current and the trade-off between membrane dehydration and water flooding in the cell.

Fully coupled atmospheric‐hydrological modeling at regional and long‐term scales: Development, application, and analysis of WRF‐HMS

AbstractA closed description of the regional water balance requires hydro‐meteorological modeling systems which represent the atmosphere, land surface, and subsurface. We developed such a mesoscale modeling system, extending the atmospheric model WRF with the distributed hydrological model HMS in a fully coupled way. It includes explicit lateral groundwater and land surface flow parameterization schemes and two‐way groundwater‐unsaturated zone interaction by replacing the free drainage bottom boundary of WRF's Noah‐LSM with a Fixed‐head or Darcy‐flux boundary condition. The system is exemplarily applied for the Poyang Lake basin (160,000 km2) and the period 1979–1986 using a two‐nest approach covering East Asia (30 km) and the Poyang Lake basin (10 km) driven by ERA Interim. Stand‐alone WRF effectively simulates temperature (bias 0.5°C) and precipitation (bias 21–26%). Stand‐alone HMS simulations provide reasonable streamflow estimates. A significant impact on the regional water balance was found if groundwater‐unsaturated zone interaction is considered. But the differences between the two groundwater coupling approaches are minor. For the fully coupled model system, streamflow results strongly depend on the simulation quality for precipitation. Two‐way interaction results in net upward water fluxes in up to 25% of the basin area after the rainy season. In total, two‐way interaction increases basin averaged recharge amounts. The evaluation with CPC and GLEAM indicates a better performance of the fully coupled simulation. The impact of groundwater coupling on LSM and atmospheric variables differs. Largest differences occur for the variable recharge (26%), whereas for atmospheric variables, the basin‐averaged impact is minor (<1%). But locally, a spatial redistribution up to ±5% occurs for precipitation.

Hydraulic and Biochemical Gradients Limit Wetland Mercury Supply to an Adirondack Stream

Net fluxes (change between upstream and downstream margins) for water, methyl mercury (MeHg), total mercury (THg), dissolved organic carbon (DOC), and chloride (Cl) were assessed twice in an Adirondack stream reach (Sixmile Brook, USA), to test the hypothesized importance of wetland-stream hydraulic and chemical gradients as fundamental controls on fluvial mercury (Hg) supply. The 500 m study reach represented less than 4%of total upstream basin area. During a snowmelt high-flow event in May 2009surface water, DOC, and chloride fluxes increased by 7.1±1.3%, 8.0±1.3%, and 9.0±1.3%, respectively, within the reach, demonstrating that the adjacent wetlands are important sources of water and solutes to the stream. However, shallow groundwater Hg concentrations lower than in the surface water limited groundwater-surface water Hg exchange and no significant changes in Hg (filtered MeHg and THg) fluxes were observed within the reach despite the favorable hydraulic gradient. In August 2009, the lack of significant wetland-stream hydraulic gradient resulted in no net flux of water or solutes (MeHg, THg, DOC, or Cl) within the reach. The results are consistent with the wetland- Hg-source hypothesis and indicate that hydraulic and chemical gradient (direction and magnitude) interactions are fundamental controls on the supply of wetland Hg to the stream.

Electro-Osmotic Driven Kinetics of Cyclodextrin through the CymA Channel

Trans membrane voltage is applied to understand the permeation of uncharged molecules into and across nano pores in Electrophysiology. The permeating molecule blocks the ion flow causing ion current fluctuations. However the fluctuation density is dependent on the magnitude and direction of the applied voltage. This dependency may be assumed due to electro-osmotic (EOF) flow, electrically driven ion – associated water flow. Here we investigate the contribution of the electro-osmotic flow (EOF) as a potential cause for an external voltage driven substrate permeation. As an example, we quantified the permeation of α-cyclodextrin through the cyclodextrin-specific channel-forming porin CymA from the Gram-negative bacterium Klebsiella oxytoca. To further elucidate these effects, substrate interaction studies were performed at various external voltages in presence of three different electrolyte solutions: KCl, NaCl and MgCl2. To demonstrate the significance of the EOF at an atomistic level, the net water flux was calculated and its effect on the binding affinity of substrates was studied by employing extensive molecular dynamics simulations.[1] B. van den Berg et al., Proceedings of the National Academy of Sciences, 112, E2991-E2999 (2015).[2] F. Orlik et al., Biophysical journal, 85, 876–885 (2003).[3] L.-Q. Gu et al., Proceedings of the National Academy of Sciences, 100, 15498–15503 (2003).

Measuring intestinal fluid transport in vitro: Gravimetric method versus non-absorbable marker

The gut sac is a long-standing, widely used in vitro preparation for studying solute and water transport, and calculation of these fluxes requires an accurate assessment of volume. This is commonly determined gravimetrically by measuring the change in mass over time. While convenient this likely under-estimates actual net water flux (Jv) due to tissue edema. We evaluated whether the popular in vivo volume marker [14C]-PEG 4000, offers a more representative measure of Jvin vitro. We directly compared these two methods in five teleost species (toadfish, flounder, rainbow trout, killifish and tilapia). Net fluid absorption by the toadfish intestine based on PEG was significantly higher, by almost 4-fold, compared to gravimetric measurements, compatible with the latter under-estimating Jv. Despite this, PEG proved inconsistent for all of the other species frequently resulting in calculation of net secretion, in contrast to absorption seen gravimetrically. Such poor parallelism could not be explained by the absorption of [14C]-PEG (typically <1%). We identified a number of factors impacting the effectiveness of PEG. One was adsorption to the surface of sample tubes. While it was possible to circumvent this using unlabelled PEG 4000, this had a deleterious effect on PEG-based Jv. We also found sequestration of PEG within the intestinal mucus. In conclusion, the short-comings associated with the accurate representation of Jv by gut sac preparations are not overcome by [14C]-PEG. The gravimetric method therefore remains the most reliable measure of Jv and we urge caution in the use of PEG as a volume marker.

Water Management in Polymer Electrolyte Fuel Cells through Asymmetric Thermal and Mass Transport Engineering of the Micro-Porous Layers

For polymer electrolyte fuel cells (PEFCs) operating at very high current, prevention of anode dry-out through enhanced back flux of water and restriction of evaporation is required. In this work, back flux of water to the anode is engineered using an asymmetric anode and cathode micro-porous layer (MPL) configuration. Extensive experimental tests have been conducted to study the impact of thermal and mass transport resistances on the net water flux coefficient for extremes of wet and dry operating conditions. The net water drag coefficient was measured in the range of −0.17 to +0.18 depending on the operating conditions and material configurations. A simplified model has also been developed to investigate the effect of temperature gradient on the net water drag coefficient. It is shown that with an asymmetric configuration, the net flux of water can be reversed under certain conditions, greatly enhancing high current density performance. For wet operating conditions, the cell configuration with asymmetric mass transport resistance can be utilized to tailor the back flux of water. For dry operating conditions, the thermal resistance is the key controlling parameter to affect the net water drag.

Tuning water transport through nanochannels by changing the direction of an external electric field.

The transport properties of water through a nanochannel influenced by the direction of an external electric field has been investigated by using molecular dynamics simulations. Water molecules flow unidirectionally across the nanochannel under a uniform external electric field without an osmotic pressure. It is found that the direction of the external field plays an important role in the interactions and dipole orientations of water molecules in the nanochannel, accordingly changing the net water flux dramatically. Most importantly, a critical angle (θC) between the external field and the nanochannel axis is found. The average net water flux increases as θ increases for θ≤θC but decreases sharply to a near-zero value for a further increase of θ. The maximum value of the average net water flux is 7.33 times as high as the value when the electric field is along the nanochannel axis. Our findings are of great practical importance for nanomolecular engineering, which provide a possible strategy for designing novel controllable water nanopumps.

Variation of oxygen isotopic ratio during wine dealcoholization by membrane contactors: Experiments and modelling

Due to the increasing trend of ethanol level in wines produced in different regions, partial dealcoholization (up to 2% v/v) has become a common practice in wine industry. Membrane contactors employing water as stripping agent represent a promising technology for implementing dealcoholization at the industrial level. This work investigates the effect of the treatment on the oxygen isotopic ratio (18O/16O) of the wine water. Notably, European Union prescribes the use of stable isotopic ratio analyses as official methods to detect the illegal additions of compounds in wines and authentication of geographic origin, and year of vintage. Experiments were performed with wines and water with different 18O/16O ratio and in the presence of non-volatile solute (glycerol) to alter the net water flux through the membrane. Experimental results clearly showed that the water flux through the membrane does not contribute significantly to the isotopic ratio change, which can be adequately explained by the diffusion of the isotope, according to the models commonly used to describe the exchange of ethanol or other components. A model has been developed herein, which accounts for the diffusion of ethanol, water and isotopic. The role of concentration polarization as well as the transport mechanisms are also discussed.

RETRACTED: Limitations of osmotic gradient resource and hydraulic pressure on the efficiency of dual stage PRO process

Dual stage PRO process has been proposed for power generation from a salinity gradient across a semi-permeable membrane. Closed-loop and open-loop dual stage PRO system were evaluated using 2 M NaCl and Dead Sea as draw solutions whereas the feed solution was either fresh water or seawater. The impact of feed salinity gradient resource and feed pressure on the net power generation and water flux was evaluated. The results showed that power density in stage one reached a maximum amount atΔP = π/2 but the maximum net power generation occurred atΔP < π/2. This was mainly attributed to the variation of net driving pressure in stage one and two of the PRO process. The dual stage PRO process was found to perform better at high osmotic pressure gradient across the PRO membrane, for example when Dead Sea brine or highly concentrated NaCl is the draw solution. Total power generation in the dual stage PRO process is up to 40% higher than that in the conventional PRO process. This was achieved through harvesting the rest of energy remaining in the diluted draw solution. Therefore, dual stage PRO process has the potential of maximizing power generation from a salinity gradient resource.

Impact of Sea-Level Rise on Saltwater Intrusion in the Pearl River Estuary

Yuan, R.; Zhu, J., and Wang, B., 2015. Impact of sea-level rise on the saltwater intrusion in the Pearl River Estuary.A three-dimensional numerical model was established based on the Finite Volume Coastal Ocean Model to study the impact of sea-level rise (SLR) on saltwater intrusion in the Pearl River Estuary. The model domain covered the entire river network, estuary, and offshore sea, and the model grid fit well with the coastlines, with a high resolution of ∼100 m in the river mouths. The model validation process showed that the model results fit the observed data fairly well. Results showed that saltwater intrusion in the river mouths was highly sensitive to SLR. Higher SLR enhanced saltwater intrusion, especially in the Modaomen waterway during neap tide. SLR enhanced the landward residual current in the Hongwan waterway and the landward bottom residual current in the Modaomen waterway, both of which carried high salinity water landward and thus made salinity higher. Isohaline of 0.45 (which is the salinity standard for drinking water) moved seaward in the Jiaomen and Hongqili outlets, whereas it moved landward in the Modaomen and Hutiaomen outlets in response to SLR. This pattern was well explained by the net water-flux changes in this branching estuary.

Electromanipulating Water Flow in Nanochannels

AbstractIn sharp contrast to the prevailing view that a stationary charge outside a nanochannel impedes water permeation across the nanochannel, molecular dynamics simulations show that a vibrational charge outside the nanochannel can promote water flux. In the vibrational charge system, a decrease in the distance between the charge and the nanochannel leads to an increase in the water net flux, which is contrary to that of the fixed‐charge system. The increase in net water flux is the result of the vibrational charge‐induced disruption of hydrogen bonds when the net water flux is strongly affected by the vibrational frequency of the charge. In particular, the net flux is reaches a maximum when the vibrational frequency matches the inherent frequency of hydrogen bond inside the nanochannel. This electromanipulating transport phenomenon provides an important new mechanism of water transport confined in nanochannels.

Electromanipulating water flow in nanochannels.

In sharp contrast to the prevailing view that a stationary charge outside a nanochannel impedes water permeation across the nanochannel, molecular dynamics simulations show that a vibrational charge outside the nanochannel can promote water flux. In the vibrational charge system, a decrease in the distance between the charge and the nanochannel leads to an increase in the water net flux, which is contrary to that of the fixed-charge system. The increase in net water flux is the result of the vibrational charge-induced disruption of hydrogen bonds when the net water flux is strongly affected by the vibrational frequency of the charge. In particular, the net flux is reaches a maximum when the vibrational frequency matches the inherent frequency of hydrogen bond inside the nanochannel. This electromanipulating transport phenomenon provides an important new mechanism of water transport confined in nanochannels.

New Estimates of Variations in Water Flux and Storage over Europe Based on Regional (Re)Analyses and Multisensor Observations

Abstract Precipitation minus evapotranspiration, the net flux of water between the atmosphere and Earth’s surface, links atmospheric and terrestrial water budgets and thus represents an important boundary condition for both climate modeling and hydrological studies. However, the atmospheric–terrestrial flux is poorly constrained by direct observations because of a lack of unbiased measurements. Thus, it is usually reconstructed from atmospheric reanalyses. Via the terrestrial water budget equation, water storage estimates from the Gravity Recovery and Climate Experiment (GRACE) combined with measured river discharge can be used to assess the realism of the atmospheric–terrestrial flux in models. In this contribution, the closure of the terrestrial water budget is assessed over a number of European river basins using the recently reprocessed GRACE release 05 data, together with precipitation and evapotranspiration from the operational analyses of high-resolution, limited-area NWP models [Consortium for Small-Scale Modelling, German version (COSMO-DE) and European version (COSMO-EU)] and the new COSMO 6-km reanalysis (COSMO-REA6) for the European Coordinated Regional Climate Downscaling Experiment (CORDEX) domain. These closures are compared to those obtained with global reanalyses, land surface models, and observation-based datasets. The spatial resolution achieved with the recent GRACE data allows for better evaluation of the water budget in smaller river basins than before and for the identification of biases up to 25 mm month−1 in the different products. Variations of deseasoned and detrended atmospheric–terrestrial flux are found to agree notably well with flux derived from GRACE and discharge data with correlations up to 0.75. Finally, bias-corrected fluxes are derived from various data combinations, and from this, a 20-yr time series of catchment-integrated water storage variations is reconstructed.

Vadose-zone moisture dynamics under radiation boundary conditions during a drying process

In order to better understand the soil moisture dynamics during a drying process, a soil column experiment is conducted in the laboratory, followed by the numerical modeling with consideration of the coupled liquid water, water vapor and heat transport in the vadose zone. Results show that there are three distinct subzones above the water table according to the temporally dynamic variation of the water content profiles. Zone 1 sees a decrease in the water contents in the upper profiles (0 m-0.05 m) due to a negative net water flux in this zone where the upward isothermal water vapor flux becomes the main flow mechanism in the soils. In contrast, the water content within Zone 2 in the depth ranging from 0.05 m to 0.37 m sees an apparent increase over time, resulting from the positive net thermal water-vapor and isothermal liquid-water fluxes into this layer. Zone 3 (0.37 m-0.65 m) also sees an apparent decrease in the water content since the isothermal liquid water flux carries the liquid water either upward out of this region for vaporization or downward to the water table as a recharge to the groundwater.

Myosin 5b loss of function leads to defects in polarized signaling: implication for microvillus inclusion disease pathogenesis and treatment.

Microvillus inclusion disease (MVID) is an autosomal recessive condition resulting in intractable secretory diarrhea in newborns due to loss-of-function mutations in myosin Vb (Myo5b). Previous work suggested that the apical recycling endosomal (ARE) compartment is the primary location for phosphoinositide-dependent protein kinase 1 (PDK1) signaling. Because the ARE is disrupted in MVID, we tested the hypothesis that polarized signaling is affected by Myo5b dysfunction. Subcellular distribution of PDK1 was analyzed in human enterocytes from MVID/control patients by immunocytochemistry. Using Myo5b knockdown (kd) in Caco-2BBe cells, we studied phosphorylated kinases downstream of PDK1, electrophysiological parameters, and net water flux. PDK1 was aberrantly localized in human MVID enterocytes and Myo5b-deficient Caco-2BBe cells. Two PDK1 target kinases were differentially affected: phosphorylated atypical protein kinase C (aPKC) increased fivefold and phosohoprotein kinase B slightly decreased compared with control. PDK1 redistributed to a soluble (cytosolic) fraction and copurified with basolateral endosomes in Myo5b kd. Myo5b kd cells showed a decrease in net water absorption that could be reverted with PDK1 inhibitors. We conclude that, in addition to altered apical expression of ion transporters, depolarization of PDK1 in MVID enterocytes may lead to aberrant activation of downstream kinases such as aPKC. The findings in this work suggest that PDK1-dependent signaling may provide a therapeutic target for treating MVID.

Electricity resonance-induced fast transport of water through nanochannels.

We performed molecular dynamics simulations to study water permeation through a single-walled carbon nanotube with electrical interference. It was found that the water net flux across the nanochannel is greatly affected by the external electrical interference, with the maximal net flux occurred at an electrical interference frequency of 16670 GHz being about nine times as high as the net flux at the low or high frequency range of (<1000 GHz or >80,000 GHz). The above phenomena can be attributed to the breakage of hydrogen bonds as the electrical interference frequency approaches to the inherent resonant frequency of hydrogen bonds. The new mechanism of regulating water flux across nanochannels revealed in this study provides an insight into the water transportation through biological water channels and has tremendous potential in the design of high-flux nanofluidic systems.

Influence of tree cover on herbaceous layer development and carbon and water fluxes in a Portuguese cork-oak woodland

Facilitation and competition between different vegetation layers may have a large impact on small-scale vegetation development. We propose that this should not only influence overall herbaceous layer yield but also species distribution and understory longevity, and hence the ecosystems carbon uptake capacity especially during spring. We analyzed the effects of trees on microclimate and soil properties (water and nitrate content) as well as the development of an herbaceous community layer regarding species composition, aboveground biomass and net water and carbon fluxes in a cork-oak woodland in Portugal, between April and November 2011.The presence of trees caused a significant reduction in photosynthetic active radiation of 35molm−2d−1 and in soil temperature of 5°C from April to October. At the same time differences in species composition between experimental plots located in open areas and directly below trees could be observed: species composition and abundance of functional groups became increasingly different between locations from mid April onwards. During late spring drought adapted native forbs had significantly higher cover and biomass in the open area while cover and biomass of grasses and nitrogen fixing forbs was highest under the trees. Further, evapotranspiration and net carbon exchange decreased significantly stronger under the tree crowns compared to the open during late spring and the die back of herbaceous plants occurred earlier and faster under trees. This was most likely caused by interspecific competition for water between trees and herbaceous plants, despite the more favorable microclimate conditions under the trees during the onset of summer drought.