Vapor Pressure Gradient Research Articles

High-aspect-ratio single-crystalline AlN nanowires: Free-catalytic PVT growth and field-emission studies

Nanowires (NWs) with high aspect ratios (HARs) have great advantages for the fabrication of nanodevices. Herein, a high-efficiency and simple physical vapor transport (PVT) method is utilized to synthesize the uniform HAR aluminum nitride (AlN) NWs on a tungsten substrate without any catalysts. Synergistic effect of high surface energy of (0001), low saturated vapor pressure and large axial temperature gradient leads to the growth of HAR AlN NWs, which provides new insight for the growth of low-dimensional AlN nanostructures. The as-obtained AlN NWs with super HAR have hexagonal wurtzite structure, the diameters are about 100 nm and the lengths are over 200 μm. The AlN NWs have an intensive deep ultraviolet (DUV) absorption peak at 5.94 eV and exhibit a relatively high electrical conductivity (1.29 × 10−3 Ω−1 cm−1), low turn-on field (6.2 V μm−1) and threshold field (8.5 V μm−1). These results indicate that PVT method is efficient to fabricate HAR AlN NWs and the AlN NWs not only play an important role in DUV photoelectric devices but also have tremendous potential as a candidate for field-emission nanodevices.

An optimization of water transport through polyurethane silica-nanocomposite membrane

Analysis of water transport through polyurethane silica nano-composite membranes were performed by numerical simulation and experiment. Simulation work was performed using a finite element analysis (FEA) software, COMSOL Multiphysics. Experimental work was performed to validate the simulation model. Through simulation we suggest the minimum membrane-to-membrane distance for stacked membranes to be 2-mm to avoid the membrane concentration polarization effect. The method used to create a concentration gradient between the membrane feed side (humid air) and the membrane permeate side (connected to vacuum) involved with creating a pressure gradient. Water vapor pressure gradient creates a concentration gradient which drives water vapor molecules to diffuse from the membrane feed side to the membrane permeate side. Different membrane configurations including rectangular parallel membranes, perforated cylinder with circular membranes, perforated group of cylinders with circular membranes, and perforated cylinder with curved rectangular membranes were compared in terms of water vapor removal from humid air while connecting all membrane configurations to 47-mm vacuum port. The parallel 22 membranes exhibited maximum water removal. Based on the total membrane area, water vapor removal was maximized at the maximum membrane effective area in contact with humid area. The maximum total water removal was obtained for the 11 cassettes with 22 rectangular membranes with a value of 142 g/(m2·s) while the relative humidity dropped from 60% at the duct inlet to 25.6% at the duct outlet.

Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

AbstractThe stable isotopes of hydrogen and oxygen (δ2H and δ18O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run‐off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation.

Картографирование современных изменений климата на территории Урала

The author describes the methods and results of current climate change mapping in the Ural region. The maps of temperature and water vapor pressure dynamics (for 1951–2010) and also precipitation amount changes (for 1966–2010) are presented. The maps of temperature and water vapor pressure are created basing upon spline interpolation of weather stations data, taking into account altitude-dependent regression. The altitude gradient of temperature and water vapor pressure was estimated using reanalysis data. The maps of precipitation amount were created basing upon spline interpolation combined with multiple linear regression model. Altitude and large-scale slope and aspect are used as independent variables. Accuracy assessment is carried out by cross-validation method. Basing upon the created maps, it is shown that the average annual temperature increased by 0,7–1,2° between 1951–1980 and 1981–2010 throughout the Ural region. In the North-Western Ural (i. e. in Komi Republic) the rise of average annual temperature is less significant, than in other parts of the mentioned area. The increase of water vapor pressure is also observed across the Urals. The changes of precipitation amount between 1966– 1995 and 1981–2010 are multidirectional. In general, the precipitation amount increased in the western part of Ural. At the same time, the precipitation amount decreased in some regions of Eastern Urals.

Stomatal Conductance Measurement for Toxicity Assessment in Zero-Effluent Constructed Wetlands: Effects of Landfill Leachate on Hydrophytes.

In this research, we explore for the first time the use of leaf stomatal conductance (gs) for phytotoxicity assessment. Plants respond to stress by regulating transpiration. Transpiration can be correlated with stomatal conductance when the water vapor pressure gradient for transpiration is constant. Thus, our working hypothesis was that the gs measurement could be a useful indicator of the effect of toxic compounds on plants. This lab-scale study aimed to test the measurement of gs as a phytotoxicity indicator. Our model plants were two common hydrophytes used in zero-effluent constructed wetlands for treating landfill leachate. The toxic influence of two types of leachate from old landfills (L1, L2) on common reed (Phragmites australis (Cav.) Trin. ex Steud.) and sweet flag (Acorus calamus L.) was tested. The gs measurements correlated well with plant response to treatments with six solutions (0 to 100%) of landfill leachate. Sweet flag showed higher tolerance to leachate solutions compared to common reed. The estimated lowest effective concentration (LOEC) causing the toxic effect values for these leachates were 3.94% of L1 and 5.76% of L2 in the case of reed, and 8.51% of L1 and 10.44% of L2 in the case of sweet flag. Leachate L1 was more toxic than L2. The leaf stomatal conductance measurement can be conducted in vivo and in the field. The proposed approach provides a useful parameter for indicating plant responses to the presence of toxic factors in the environment.

Membrane stripping enables effective electrochemical ammonia recovery from urine while retaining microorganisms and micropollutants

Ammonia recovery from urine avoids the need for nitrogen removal through nitrification/denitrification and re-synthesis of ammonia (NH3) via the Haber-Bosch process. Previously, we coupled an alkalifying electrochemical cell to a stripping column, and achieved competitive nitrogen removal and energy efficiencies using only electricity as input, compared to other technologies such as conventional column stripping with air. Direct liquid-liquid extraction with a hydrophobic gas membrane could be an alternative to increase nitrogen recovery from urine into the absorbent while minimizing energy requirements, as well as ensuring microbial and micropollutant retention. Here we compared a column with a membrane stripping reactor, each coupled to an electrochemical cell, fed with source-separated urine and operated at 20 A m−2. Both systems achieved similar nitrogen removal rates, 0.34 ± 0.21 and 0.35 ± 0.08 mol N L−1 d−1, and removal efficiencies, 45.1 ± 18.4 and 49.0 ± 9.3%, for the column and membrane reactor, respectively. The membrane reactor improved nitrogen recovery to 0.27 ± 0.09 mol N L−1 d−1 (38.7 ± 13.5%) while lowering the operational (electrochemical and pumping) energy to 6.5 kWhe kg N−1 recovered, compared to the column reactor, which reached 0.15 ± 0.06 mol N L−1 d−1 (17.2 ± 8.1%) at 13.8 kWhe kg N−1.Increased cell concentrations of an autofluorescent E. coli MG1655 + prpsM spiked in the urine influent were observed in the absorbent of the column stripping reactor after 24 h, but not for the membrane stripping reactor. None of six selected micropollutants spiked in the urine were found in the absorbent of both technologies.Overall, the membrane stripping reactor is preferred as it improved nitrogen recovery with less energy input and generated an E. coli- and micropollutant-free product for potential safe reuse. Nitrogen removal rate and efficiency can be further optimized by increasing the NH3 vapor pressure gradient and/or membrane surface area.

Directional Water Collection in Nanopore Networks.

The development of artificial nanosystems that mimic directional water-collecting ability of evolved biological surfaces is eagerly awaited. Here we report a new type of addressable water collection that is induced by coupling both vapor gradients, like a road drawn, and the temperature-tuned condensation in nanopores as step signals. What distinguishes the motion described here from the motions reported earlier is the fact that neither bulk liquid infiltration nor displacement of droplet is required. Instead, the motion results from a scanned water capture because of the temperature-dependent condensation command acting on the vapor pressure gradient track originated by a droplet without a bulk fluidic connection with a mesoporous film. This novel working principle demands only a small-range surface temperature control, which was entirely generated by a thermoelectric cell integrated to the mesoporous substrates. The strategy opens the route to achieving precise control over wetting location (from a few to hundreds of micrometers) and hence over the direction of water collected by these widely employed nanomaterials. Furthermore, as water is collected from condensation into the pores, the system naturally involves purification and subsequent delivery of clean water, which provides an added value to the proposed strategy.

Modeling porosity migration in LWR and fast reactor MOX fuel using the finite element method

An engineering-scale finite element simulation of pore migration in oxide fuel is presented. The porosity field is governed by an advection-diffusion equation which is coupled to the fuel temperature and stress fields through the thermal conductivity and volumetric heat source term. The engineering-scale porosity equation models the microscopic process of vapor transport of fuel across pores, taking into account thermal and vapor pressure gradients within the fuel. In the simulations, the porosity is initialized to a constant value at every point in the domain, and as the temperature gradient is increased by application of a heat source, the pores move up the thermal gradient and accumulate at the center of the fuel in a time frame that is consistent with experimental observations. Results from representative simulations are provided to demonstrate the new capability, and we show that a sufficiently high power ramp rate limits restructuring and leads to a corresponding increase in fuel temperature. We also discuss the finite element mesh density required to compute pore migration and present multidimensional results.

Effect of Latent Heat Released by Freezing Droplets during Frost Wave Propagation.

Frost spreads on nonwetting surfaces during condensation frosting via an interdroplet frost wave. When a supercooled condensate water droplet freezes on a hydrophobic or superhydrophobic surface, neighboring droplets still in the liquid phase begin to evaporate. Two possible mechanisms govern the evaporation of neighboring water droplets: (1) The difference in saturation pressure of the water vapor surrounding the liquid and frozen droplets induces a vapor pressure gradient, and (2) the latent heat released by freezing droplets locally heats the substrate, leading to evaporation of nearby droplets. The relative significance of these two mechanisms is still not understood. Here, we study the significance of the latent heat released into the substrate by freezing droplets, and its effect on adjacent droplet evaporation, by studying the dynamics of individual water droplet freezing on aluminum-, copper-, and glass-based hydrophobic and superhydrophobic surfaces. The latent heat flux released into the substrate was calculated from the measured droplet sizes and the respective freezing times ( tf), defined as the time from initial ice nucleation within the droplet to complete droplet freezing. To probe the effect of latent heat release, we performed three-dimensional transient finite element simulations showing that the transfer of latent heat to neighboring droplets is insignificant and accounts for a negligible fraction of evaporation during microscale frost wave propagation. Furthermore, we studied the effect of substrate thermal conductivity on the transfer of latent heat transfer to neighboring droplets by investigating the velocity of ice bridge formation. The velocity of the ice bridge was independent of the substrate thermal conductivity, indicating that adjacent droplet evaporation during condensation frosting is governed solely by vapor pressure gradients. This study not only provides key insights into the individual droplet freezing process but also elucidates the negligible role of latent heat released into the substrate during frost wave propagation.

Evaporation from a temperate closed-basin lake and its impact on present, past, and future water level

Lakes provide enormous economic, recreational, and aesthetic benefits to citizens. These ecosystem services may be adversely impacted by climate change. In the Twin Cities Metropolitan Area of Minnesota, USA, many lakes have been at historic low levels and water augmentation strategies have been proposed to alleviate the problem. White Bear Lake (WBL) is a notable example. Its water level declined 1.5 m during 2003–2013 for reasons that are not fully understood. This study examined current, past, and future lake evaporation to better understand how climate will impact the water balance of lakes within this region. Evaporation from WBL was measured from July 2014 to February 2017 using two eddy covariance (EC) systems to provide better constraints on the water budget and to investigate the impact of evaporation on lake level. The estimated annual evaporation losses for years 2014 through 2016 were 559 ± 22 mm, 779 ± 81 mm, and 766 ± 11 mm, respectively. The higher evaporation in 2015 and 2016 was caused by the combined effects of larger average daily evaporation and a longer ice-free season. The EC measurements were used to tune the Community Land Model 4 – Lake, Ice, Snow and Sediment Simulator (CLM4-LISSS) to estimate lake evaporation over the period 1979–2016. Retrospective analyses indicate that WBL evaporation increased during this time by about 3.8 mm year−1, which was driven by increased wind speed and lake-surface vapor pressure gradient. Using a business-as-usual greenhouse gas emission scenario (RCP8.5), lake evaporation was modeled forward in time from 2017 to 2100. Annual evaporation is expected to increase by 1.4 mm year−1 over this century, largely driven by lengthening ice-free periods. These changes in ice phenology and evaporation will have important implications for the regional water balance, and water management and water augmentation strategies that are being proposed for these Metropolitan lakes.

Geothermal Membrane Distillation in Industrial Greenhouse Applications: Membrane Fabrication and Characterization

Abstract Membrane distillation is reported for the first time in the literature being used to desalinate geothermal brackish groundwater for industrial greenhouse applications. Asymmetric hollow fiber membranes were fabricated and characterized for their direct contact membrane distillation (DCMD) application to provide irrigation water at Masson Greenhouse, New Mexico. Membranes exhibited an asymmetric configuration with an external sponge layer and an internal macrovoid layer. The intention was to prevent pore wetting from the hot saline fluid with the sponge layer, while using the macrovoid layer to increase the water flux and thermal efficiency of the membrane. A water flux of 55 liters/m2-h (LMH) with a salt rejection greater than 99.99% was observed for the fabricated polyvinylidene fluoride (PVDF) hollow fiber membranes at a water vapor pressure gradient of 3.2 MPa/cm (shell-side inlet temperature of 80°C). Further investigation, however, revealed that pore wetting existed in the macrovoid layer at...

Advanced modelling of vapor resistance of moisture dependent vapor retarder

Moisture dependent vapor retarders (smart vapor retarders) are widely used in building construction especially roofs in order to prevent moisture accumulation or to support drying out processes. The current modelling approaches mainly use only a relative humidity dependent vapor resistance. Recent studies have shown more complex behavior like direction depending flow or vapor pressure gradient depending flow. This paper shows a new procedure for including all these effects into physical model. Furthermore description of necessary measurements with evaluation of results and selection of a proper model with parameter adjustment will be shown based on three different example membranes. Finally the implementation of this model into a hygrothermal simulation code is described

Hygrothermal Dynamic and Mould Growth Risk Predictions for Concrete Tiles by Using Least Squares Support Vector Machines

The hygrothermal analysis of roofs is relevant due to the large areas exposed to a wide range of weather conditions, these directly affecting the energy performance and thermal comfort of buildings. However, after a long life service, the solar absorptivity coatings of roofs can be altered by mould accumulation. Based on two well established mathematical models, one that adopts driving potentials to calculate temperature, moist air pressure and water vapor pressure gradients, and the other to estimate the mould growth risk on surfaces, this research introduces an approach to predict mould growth considering a reduced computational effort and simulation time. By adopting multiple MISO (Multiple-Input, Single-Output) Nonlinear AutoRegressive with eXogenous inputs (NARX) models, a machine learning technique known as Least Squares Support Vector Machines (LS-SVM), a maximum margin model based on structural risk minimization, was used to predict vapor flux, sensible heat flux, latent heat flux, and mould growth risk on roof surfaces. The proposed model was validated in terms of the Multiple Correlation Coefficient (R2R2R2), Mean Square Error (MSE) and Mean Absolute Error (MAE) performance indices considering as input the weather file from Curitiba city—Brazil, showing consistent precision when compared to the results of a validated numerical model.

Evaporation and the subcanopy energy environment in a flooded forest

AbstractThe combination of tree canopy cover and a free water surface makes the subcanopy environment of flooded forested wetlands unlike other aquatic or terrestrial systems. Subcanopy vapour fluxes and energy budgets represent key controls on water level and understorey climate but are not well understood. In a permanently flooded forest in south‐eastern Louisiana, USA, an energy balance approach was used to address (a) whether evaporation from floodwater under a forest canopy is solely energy limited and (b) how energy availability was modulated by radiation and changes in floodwater heat storage. A 5‐month continuous measurement period (June–November) was used to sample across seasonal changes in canopy activity and temperature regimes. Over this period, the subcanopy airspace was humid, maintaining saturation vapour pressure for 28% of the total record. High humidity coupled with the thermal inertia of surface water altered both seasonal and diel energy exchanges, including atypical phenomena such as frequent day‐time vapour pressure gradients towards the water surface. Throughout the study period, nearly all available energy was partitioned to evaporation, with minimal sensible heat exchange. Monthly mean evaporation ranged from 0.7 to 1.7 mm/day, peaking in fall when canopy senescence allowed greater radiation transmission; contemporaneous seasonal temperature shifts and a net release of stored heat from the surface water resulted in energy availability exceeding net radiation by 30% in October and November. Relatively stable energy partitioning matches Priestley–Taylor assumptions for a general model of evaporation in this ecosystem.

Sap flow, leaf-level gas exchange and spectral responses to drought in Pinus sylvestris, Pinus pinea and Pinus halepensis

Abstract: In a climate change scenario, Mediterranean forest species such as pines may be endangered by rising temperatures and reduced precipitation, thus calling for studies on the transpiration and water balance in pines. In this paper, the response of young plants of Pinus sylvestris L., Pinus pinea L. and Pinus halepensis Mill. to different irrigation treatments has been studied. Significant differences were found in water potential, sap flow, leaf-level gas exchange and spectral variables. P. sylvestris had higher pre-dawn and midday water potentials, sap flow rates and leaf-level gas exchange rates compared to the other two species in well-watered conditions. Vapor pressure gradient correlated with stomatal conductance, net assimilation and transpiration, but the association between stomatal conductance and sap flow was weak. The environmental variables more strongly associated with sap flow were solar radiation and reference evapo-transpiration, especially in the well-watered plants, but those associations were weaker in the stressed plants. All three pine species showed the isohydric, drought-avoiding strategy common in the genus Pinus, maintaining relatively high water potentials in dry conditions. Nevertheless, P. halepensis showed a water-saving strategy, with a stomatal closure behavior under drought. Stomatal regulation was less strict in P. sylvestris, closer to a water-spending pattern, while P. pinea showed an intermediate behavior. Significant differences were recorded among species in spectral reflectance in the visible and infra-red regions. Photochemical Reflectance Index, Normalized Difference Vegetation Index and combinations of other ratios permitted the discrimination among the three pine species. These spectral variables showed association with sap flow rate, water potential and leaf-level gas exchange variables. Both cluster analysis and k-means classification discriminated Scots pine and Aleppo pine in two different groups. On the other hand, Stone pine showed differences in spectral behavior depending on the hydric status of the plants. Well-watered Stone pine plants had the same spectral behavior as Scots pine, while the plants subjected to drought stress were closer to Aleppo pine plants in spectral response. These findings may help to quantify the impacts of early and mid-summer water deficit on Mediterranean pines in future climate regimes.

Modeling biosolids drying through a laminated hydrophobic membrane

The adaptation of the membrane distillation process as a low-cost and sustainable approach to biosolids drying and stabilization is investigated, which may have application in container-based sanitation systems proposed in low-income urban regions. Three-layer laminated, breathable, hydrophobic membranes enclose the biosolids, facilitating drying but preventing transport of contaminants. The membranes used in this process are non-wetting with pore spaces that only allow vapor transport. Water vapor can be expelled due to a moderate vapor pressure gradient. Other constituents, including both particulate and dissolved matter are retained. The permeate, therefore, is expected to be of high purity and pathogen-free. This study presents experimental results showing usable rates of moisture transfer through the laminated hydrophobic membranes with temperature gradients, ΔT = −2 °C, corresponding to the condition that biosolids do not receive external heating in which laminate-enclosed biosolids are 2 °C cooler than outside, as well as conditions that samples are 2 °C and 10 °C warmer than the ambient temperature (ΔT = 2 and 10 °C, respectively). The conditions result in reduction in the moisture content of the laminate-enclosed biosolids from about 97% to 12–30% and the permeate is observed to be free of fecal coliforms, indicator organisms for pathogens. The initial constant-rate drying period is described well with a stagnant film model that accounts for different temperature gradients, laminate surface area, and ambient relative humidity. The proposed model may be used to assess the feasibility of incorporating laminated hydrophobic membranes to enhance biosolids drying in container-based sanitation systems as well as other applications.

Effects of Atmospheric Surface Layer Stability on Turbulent Fluxes of Heat and Water Vapor across the Water–Atmosphere Interface

Abstract Widely used numerical models to estimate turbulent exchange of latent heat flux (LE) and sensible heat flux H across the water–atmosphere interface are based on the bulk transfer relations linked indirectly to atmospheric stability, even though the accurate prediction of the influence of stability on fluxes is uncertain. Here eddy covariance data collected over the water surface of Ross Barnett Reservoir, Mississippi, was analyzed to study how atmospheric stability and other variables (wind speed, vapor pressure gradient, and temperature gradient) in the atmospheric surface layer (ASL) modulated LE and H variations in different stability ranges. LE and H showed right-skewed, bell-shaped distributions as the ASL stability shifted from very unstable to near neutral and then stable conditions. The results demonstrate that the maximum (minimum) LE and H did not necessarily occur under the most unstable (stable) conditions, but rather in the intermediate stability ranges. No individual variables were able to explain the dependence of LE and H variations on stability. The coupling effects of stability, wind speed, and vapor pressure gradient (temperature gradient) on LE (H) primarily caused the observed variations in LE and H in different stability ranges. These results have important implications for improving parameterization schemes to estimate fluxes over water surfaces in numerical models.

A New Temperature-Vegetation Triangle Algorithm with Variable Edges (TAVE) for Satellite-Based Actual Evapotranspiration Estimation

The estimation of spatially-variable actual evapotranspiration (AET) is a critical challenge to regional water resources management. We propose a new remote sensing method, the Triangle Algorithm with Variable Edges (TAVE), to generate daily AET estimates based on satellite-derived land surface temperature and the vegetation index NDVI. The TAVE captures heterogeneity in AET across elevation zones and permits variability in determining local values of wet and dry end-member classes (known as edges). Compared to traditional triangle methods, TAVE introduces three unique features: (i) the discretization of the domain as overlapping elevation zones; (ii) a variable wet edge that is a function of elevation zone; and (iii) variable values of a combined-effect parameter (that accounts for aerodynamic and surface resistance, vapor pressure gradient, and soil moisture availability) along both wet and dry edges. With these features, TAVE effectively addresses the combined influence of terrain and water stress on semi-arid environment AET estimates. We demonstrate the effectiveness of this method in one of the driest countries in the world—Jordan, and compare it to a traditional triangle method (TA) and a global AET product (MOD16) over different land use types. In irrigated agricultural lands, TAVE matched the results of the single crop coefficient model (−3%), in contrast to substantial overestimation by TA (+234%) and underestimation by MOD16 (−50%). In forested (non-irrigated, water consuming) regions, TA and MOD16 produced AET average deviations 15.5 times and −3.5 times of those based on TAVE. As TAVE has a simple structure and low data requirements, it provides an efficient means to satisfy the increasing need for evapotranspiration estimation in data-scarce semi-arid regions. This study constitutes a much needed step towards the satellite-based quantification of agricultural water consumption in Jordan.

On Localized Vapor Pressure Gradients Governing Condensation and Frost Phenomena.

Interdroplet vapor pressure gradients are the driving mechanism for several phase-change phenomena such as condensation dry zones, interdroplet ice bridging, dry zones around ice, and frost halos. Despite the fundamental nature of the underlying pressure gradients, the majority of studies on these emerging phenomena have been primarily empirical. Using classical nucleation theory and Becker-Döring embryo formation kinetics, here we calculate the pressure field for all possible modes of condensation and desublimation in order to gain fundamental insight into how pressure gradients govern the behavior of dry zones, condensation frosting, and frost halos. Our findings reveal that in a variety of phase-change systems the thermodynamically favorable mode of nucleation can switch between condensation and desublimation depending upon the temperature and wettability of the surface. The calculated pressure field is used to model the length of a dry zone around liquid or ice droplets over a broad parameter space. The long-standing question of whether the vapor pressure at the interface of growing frost is saturated or supersaturated is resolved by considering the kinetics of interdroplet ice bridging. Finally, on the basis of theoretical calculations, we propose that there exists a new mode of frost halo that is yet to be experimentally observed; a bimodal phase map is developed, demonstrating its dependence on the temperature and wettability of the underlying substrate. We hope that the model and predictions contained herein will assist future efforts to exploit localized vapor pressure gradients for the design of spatially controlled or antifrosting phase-change systems.

Effect of DOC on evaporation from small Wisconsin lakes

Summary Evaporation (E) dominates the loss of water from many small lakes, and the balance between precipitation and evaporation (P–E) often governs water levels. In this study, evaporation rates were estimated for three small Wisconsin lakes over several years using 30-min data from floating evaporation pans (E-pans). Measured E was then compared to the output of mass transfer models driven by local conditions over daily time scales. The three lakes were chosen to span a range of dissolved organic carbon (DOC) concentrations (3–20 mg L −1 ), a solute that imparts a dark, tea-stain color which absorbs solar energy and limits light penetration. Since the lakes were otherwise similar, we hypothesized that a DOC-mediated increase in surface water temperature would translate directly to higher rates of evaporation thereby informing climate response models. Our results confirmed a DOC effect on surface water temperature, but that effect did not translate to enhanced evaporation. Instead the opposite was observed: evaporation rates decreased as DOC increased. Ancillary data and prior studies suggest two explanatory mechanisms: (1) disproportionately greater radiant energy outflux from high DOC lakes, and (2) the combined effect of wind speed ( W ) and the vapor pressure gradient ( e s − e z ), whose product [ W ( e s − e z )] was lowest on the high DOC lake, despite very low wind speeds ( −1 ) and steep forested uplands surrounding all three lakes. Agreement between measured (E-pan) and modeled evaporation rates was reasonably good, based on linear regression results ( r 2 : 0.6–0.7; slope: 0.5–0.7, for the best model). Rankings based on E were similar whether determined by measured or modeled criteria (high DOC −1 (C.V. 9%), but statistically significant differences between lakes resulted in substantial differences in cumulative E that were consistent from year to year. Daily water budgets for these lakes show that inputs were dominated by P and outputs by E; and our findings indicate that subtle changes in the variables that drive E can have measurable effects on water levels by shifting the balance between P and E.