Water Vapor Exchange Research Articles

Carbon and water fluxes and their coupling in an alpine meadow ecosystem on the northeastern Tibetan Plateau

Alpine meadow is one of the most widely distributed vegetation types on the Tibetan Plateau—the Earth’s Third Pole. Characterizing the carbon and water vapor fluxes in alpine meadow ecosystems is of particular importance for elucidating the mechanisms underlying the carbon budget and water cycle in high-altitude areas, especially under changing climatic conditions. Thus, the quantitative relationships between carbon and water fluxes and environmental drivers were examined based on a continuous eddy covariance (EC) dataset from 2013 to 2015 over the alpine Kobresia meadow on the northeastern Tibetan Plateau. The results show that (1) the net ecosystem CO2 exchange (NEE) was − 152.89 g C m−2 year−1, − 197.69 g C m−2 year−1, and − 160.09 g C m−2 year−1 from 2013 to 2015, respectively, suggesting that this alpine meadow ecosystem is a strong and consistent carbon sink. (2) Both the multiple stepwise regression analysis (MSRA) and the structural equation model (SEM) analysis confirmed the dominant role of Ts in controlling the carbon flux and that of Rn in controlling the water vapor flux. (3) The inherent water use efficiency (IWUE = GPP×VPD/ET) and underlying water use efficiency (uWUE = GPP×VPD0.5/ET), which incorporates the vapor pressure deficit (VPD) effect, better described the carbon-water coupling characteristics at daily and hourly scales than did the traditional water use efficiency (WUE = GPP/ET). These findings highlight the dominant climatic factors controlling CO2 and water vapor exchanges and contribute to our knowledge of the land surface-atmosphere exchange in alpine meadows on the Tibetan Plateau.

Changes in Soil Cohesion Due to Water Vapor Exchange: A Proposed Dry‐Flow Trigger Mechanism for Recurring Slope Lineae on Mars

AbstractRecurring slope lineae (RSL) are seasonal flows on steep slopes on Mars. Their formation mechanism is unknown, but dry granular flows are a likely explanation. Any proposed trigger for these flows must be consistent with the observed temperature dependence of RSL: more active in warmer months or when sun‐facing. Here, we use atmospheric modeling and laboratory experiments to explore a potential mechanism that involves both wet and dry processes at Hale Crater, a known RSL location. We propose that dry flows are triggered by changes in soil cohesion due to the loss of water. When surface temperature and humidity were experimentally simulated, salts likely found in the soil only completely dehydrated during the active season for RSL. We propose that the loss of water from soil in warmer months (or when illuminated) lowers soil cohesion and maximum stability angle. Slope failure may occur, exposing darker underlying material and creating RSL.

Changes in leaf stomatal traits of different aged temperate forest stands

Stomata control carbon and water vapor exchange between the leaves and the atmosphere, thus influencing photosynthesis and transpiration. Combinations of forest patches with different stand ages are common in nature, however, information of which stomatal traits vary among these stands and how, remains limited. Here, seven different aged forest stands (6, 14, 25, 36, 45, 55, and 100 years) were selected in typical temperate, mixed broadleaf-conifer forests of northeast China. Stomatal density, size and relative area of 624 species, including the same species in stands of different ages were selected. Stomatal density, size and relative area were distributed log-normally, differing across all species and plant functional groups. Stomatal density ranged from 4.2 to 1276.7 stomata mm–2, stomatal size ranged from 66.6 to 8315.7 μm2, and stomatal relative area 0.1–93.3%. There was a significant negative relationship between density and size at the species and functional group levels, while the relative stomatal area was positively correlated with density and size. Stomatal traits of dominant species were relatively stable across different stand ages but were significantly different for herbs. The results suggest that stomatal traits remain relatively stable for dominant species in natural forests and therefore, spatial variation in stomatal traits across forest patches does not need to be incorporated in future ecological models.

Factors controlling the latent and sensible heat fluxes over Erhai Lake under different atmospheric surface layer stability conditions

The stratification of the atmospheric surface layer (ASL) plays an important role in regulating the water vapor and heat exchange across the lake–air interface. Based on one year of data measured b...

Small Circular DNA Molecules as Triangular Scaffolds for the Growth of 3D Single Crystals

DNA is a very useful molecule for the programmed self-assembly of 3D (three dimension) nanoscale structures. The organised 3D DNA assemblies and crystals enable scientists to conduct studies for many applications such as enzymatic catalysis, biological immune analysis and photoactivity. The first self-assembled 3D DNA single crystal was reported by Seeman and his colleagues, based on a rigid triangle tile with the tile side length of two turns. Till today, successful designs of 3D single crystals by means of programmed self-assembly are countable, and still remain as the most challenging task in DNA nanotechnology, due to the highly constrained conditions for rigid tiles and precise packing. We reported here the use of small circular DNA molecules instead of linear ones as the core triangle scaffold to grow 3D single crystals. Several crystallisation parameters were screened, DNA concentration, incubation time, water-vapour exchange speed, and pH of the sampling buffer. Several kinds of DNA single crystals with different morphologies were achieved in macroscale. The crystals can provide internal porosities for hosting guest molecules of Cy3 and Cy5 labelled triplex-forming oligonucleotides (TFOs). Success of small circular DNA molecules in self-assembling 3D single crystals encourages their use in DNA nanotechnology regarding the advantage of rigidity, stability, and flexibility of circular tiles.

An Impact Crater Origin for the InSight Landing Site at Homestead Hollow, Mars: Implications for Near Surface Stratigraphy, Surface Processes, and Erosion Rates

AbstractThe InSight mission to Mars landed within Homestead hollow on an Early Amazonian lava plain. The hollow is a 27‐m‐diameter, 0.3‐m‐deep quasi‐circular depression that shares morphologic and sedimentologic characteristics to degraded impact craters. Unlike the intercrater plains outside of the hollow, the interior lacks cobbles and is dominated by loose sand, granules, and pebbles. Fresher craters near the landing site exhibit meter‐scale bedforms in their ejecta and on their floors due to sediment trapping. The sedimentology of the interior fill of Homestead hollow suggests similar trapping. The hollow falls along a morphologic continuum that requires low rates of rim degradation and fill. Crater degradation rates (rim erosion plus filling) in the landing site decline nonlinearly through time from 10−2 to 10−4 m/Myr as craters evolve to a hollow‐like form. Rim erosion rates are lower initially, at 10−3 m/Myr, but converge with degradation rates to 10−4 m/Myr. This implies that while filling plays an important role soon after crater formation, it is limited in later stages. Crater statistics indicate that the bulk of the fill occurred in the first ~50 Myr for Homestead hollow. The estimated maximum age of the hollow is ~400 to 700 Myr. This requires near‐zero fill aggradation and long‐term soil stability for the bulk of the crater's history. Fill stability manifests in Homestead hollow as a ~5‐ to 10‐cm‐thick duricrust, formed by exchanges of atmospheric water vapor with soil. The estimated degradation in the hollow requires ~2 to 3 m of sedimentary fill beneath the lander.

Edible coating based on achira starch containing garlic/oregano oils to extend the shelf life of double cream cheese

Edible coatings provide food products with a barrier to gases and water vapor exchange; additionally, when complemented with antimicrobial agents, they can be suitable to extend food shelf life. This study aimed to evaluate the effect of using edible coatings based on achira starch (Canna indica L.), microcrystalline cellulose, and natural antimicrobial compounds (garlic and oregano oils) on the quality of double cream cheese during storage at 5 °C for 42 days. The physicochemical characteristics of the cheeses, such as weight loss, hardness, water activity, and color, were evaluated on days 1, 8, 21, and 42. The microbiological analyses were carried out on days 1, 21, and 42, and the sensorial analysis on days 1 and 42. The coated cheese samples maintained the pH value of fresh products during storage, whereas the pH of the uncoated samples progressively decreased. No effect (P≥0.05) was observed at the different storage times on the weight and color of the coated samples, as compared to the control. The hardness of the coated samples was lower (50% for oregano oil treatment and 18% for garlic oil treatment) at the end of the storage, with a significant difference (P≤0.05) from the control. Additionally, the use of coatings containing garlic or oregano oil prevented the growth of pathogenic or contaminating microorganisms on the product during 42-day storage. The results indicated that the use of edible coatings incorporating garlic or oregano oil as antimicrobial compounds are an alternative to extend the shelf life of double cream cheese.

Dispersed Association of Single-Component Short-Alkyl Chains toward Thermally Programmable and Malleable Multiple-Shape Hydrogel.

In pursuit of intelligent hydrogel-based devices, it is imperative to concurrently enhance the shape programmability and customization of hydrogels in order to realize sophisticated actuation and implantation. Until now, multiple temporary shapeshifting of hydrogels has required either multiple external stimuli or distinct thermal-transition phases. In addition, reprocessing the permanent shape of hydrogel mostly relies on the change in their components. These complex prerequisites present challenges to programmability and customization of application-related shapes for hydrogels. This paper reports a type of thermally programmable and malleable multiple-shape hydrogel. The network of this hydrogel is solely composed of a type of polyvinyl alcohol derivative, which is synthesized by substituting hydroxyl groups of polyvinyl alcohol with single-component octyl chains. Through water-vapor exchange and heating in water, these single-component octyl side chains form dispersed clusters with a large strength gradient. Such broadly dispersed clusters serve as switchable segments and dynamic net-points to orthotopically offer multiple (e.g., quintuple) temporary shapes and editable permanent shapes, respectively, thereby realizing sophisticated and designed shape changes. This hydrogel system can act as a smart device for on-demand bidirectional twining around a 1D substrate. Such a capability potentially enables the self-mounting and self-detaching behavior of soft devices on tissues with minimal invasion.

Partitioning evapotranspiration with concurrent eddy covariance measurements in a mixed forest

Plants have an important effect on our climate: as they assimilate atmospheric CO2 through the process of photosynthesis, they also transpire water to the atmosphere and thereby influence surface temperatures. It is, however, difficult to quantify transpiration from ecosystems due to measurement limitations. Direct eddy covariance (EC) measurements are currently the best available approach to observe interactions linked to biosphere–atmosphere CO2 and water vapor exchange. While there are well-established methods to partition CO2 fluxes into the component fluxes of photosynthesis and respiration, there is still no standardized method to partition water vapor fluxes (evapotranspiration, ET) into the component fluxes of evaporation and transpiration.In this study, we used two years of concurrent below and above canopy EC measurements in a mixed deciduous forest in Switzerland to partition water vapor fluxes into the components of transpiration (biological) and evaporation (physical). We compare our results with transpiration from the ecosystem demographic (ED2) model as well as derived from plot-level sap flow measurements. EC-derived transpiration accounted on average for 74% of ET, emphasizing a considerably lower contribution from evaporation. EC and sap flow measurements showed mid-afternoon reductions in transpiration during periods of high vapor pressure deficit in summer. Reductions in ET and transpiration were found under limiting soil moisture conditions, while the ratio of transpiration to ET remained constant over the years due to the low and rather constant evaporation in this closed canopy forest. Stomatal regulation in response to enhanced atmospheric evaporative demand was also found under water-stressed conditions in the afternoon in summer. When comparing our EC-derived evaporation with the ED2 model, we found large discrepancies linked to the challenge of modeling evaporation in a light limited, yet variable environment below the canopy. A strong correlation was found for transpiration from ED2 with the EC-based estimates. Our results show the potential of concurrent below and above canopy EC measurements to partition ecosystem ET in forests.

Biophysical controls of evapotranspiration in cowpea cultivation under different water regimes

ABSTRACT The objective of this study was to analyze the variations of the decoupling factor and the aerodynamic and stomatal conductances for cowpea subjected to different levels of water availability during the reproductive phenological stages of the crop. The study was conducted in the municipality of Castanhal located in the northeast of the state of Pará, Brazil, in an area planted with cowpea, in 2013, 2014 and 2015 with four irrigation treatments: T100 - replacement of 100% of the crop evapotranspiration (ETc); T50 - replacement of 50% ETc; T25 - replacement of 25% ETc; and T0 - without any replacement (without irrigation). The interaction between the crop and the atmosphere in water vapor exchange was estimated through the decoupling factor (Ω), which was between 0.5 and 1, except for the treatment without irrigation, indicating a moderate to high decoupling between the crop and the atmosphere, so that the water vapor exchange process was controlled by the energy supply, instead of stomatal control.

Characteristics of CO2, water vapor, and energy exchanges at a headwater wetland ecosystem of the Qinghai Lake

Ecosystem carbon dioxide (CO2), water vapor, and heat exchanges in alpine wetlands on the Qinghai–Tibetan Plateau are not comprehensively understood. Thus, we studied variability of net ecosystem CO2 exchange (NEE), ecosystem respiration (Re), gross primary production (GPP), evapotranspiration (ET), and heat fluxes over a headwater wetland ecosystem in the Qinghai Lake region. Results showed that the headwater wetland ecosystem was net CO2 absorption on the annual scale, in which monthly NEE, GPP, and Re in two consecutive years varied from −165.16 to 93 g CO2 m−2 mo−1, 6.66 to 384.45 g CO2 m−2 mo−1, and 6.9 to 232.02 g CO2 m−2 mo−1, respectively. The monthly ET from June to September was smaller than precipitation; these results reversed in the remaining months. Annual ET was 362.1 mm in 2015 and 324.96 mm in 2016. The net radiation (Rn), sensible heat (H), latent heat (LE), and ground heat (G) fluxes showed similar monthly patterns. Values of monthly average half-hour Rn, H, LE, and G at the daytime showed Rn > LE > H > G, and the time of the monthly half-hour G peak obviously lagged the Rn, H, and LE. Monthly average Bowen ratios were <1 from May to October, but it reversed in the rest of the months.

Correlation-based flux partitioning of water vapor and carbon dioxide fluxes: Method simplification and estimation of canopy water use efficiency

The partitioning of water vapor and carbon dioxide (CO2) exchange between vegetation and the atmosphere remains a current research priority. A technique that has been proposed to simultaneously partition these fluxes, based on the correlation between their high-frequency concentration time series, has been the subject of recent empirical evaluations and theoretical advances. The method assumes that flux-variance similarity can be applied separately to stomatal exchange (transpiration for water vapor and net photosynthesis for CO2) and non-stomatal exchange (direct evaporation for water vapor and soil and stem respiration for CO2). Here, we present a mathematical simplification of this approach, from which the partitioned fluxes can be derived from routine eddy covariance measurements. The simplification arises from the fact that the transpiration and net photosynthesis fluxes are linearly related in solution space with respect to variable canopy water use efficiency, W. Conditions that are amenable to successful partitioning can now be determined a priori for a given averaging period. The simplified framework also has the benefit of providing a means for estimating W based on optimization theory. This allow for the estimation of W without any preconceptions of how the intercellular CO2 concentration, ci, varies as a function of ambient conditions. The simplified partitioning framework is applied to eddy covariance measurements collected over a mixed deciduous forests for three growing season. Aside from being more computationally efficient, the partitioned results exhibit less scatter compared with prior implementations.

Regional climate effects of plastic film mulch over the cropland of arid and semi-arid regions in Northwest China using a regional climate model

Plastic film mulch, which effectively maintains soil moisture by suppressing soil evaporation, has been widely used in cropland in Northwest China, where water scarcity severely restricts agricultural development. Plastic mulch affects surface albedo and blocks water vapor exchange between soil and atmosphere. Therefore, the large area of cropland covered by plastic film will have serious impacts on regional climate by modifying the surface-atmosphere partitioning of vapor and energy. In this study, a dynamic irrigation scheme was incorporated into the Weather Research and Forecasting model with Noah land surface scheme; two experiments (with and without plastic mulch) were designed to simulate the impacts of plastic mulch over cropland in Northwest China on regional climate. In irrigated cropland, the irrigation amount decreased substantially during the early stage of crop growth (ESCG; from 22 April to 1 July) with plastic mulch, especially in May, indicating that plastic mulch conserved agricultural water. In nonirrigated cropland, soil moisture in cropland with plastic mulch was clearly greater than that without plastic mulch. The influences of plastic mulch on land surface processes were mainly reflected in the ESCG, such that upward shortwave radiation increased and latent heat flux decreased. In cropland during the ESCG, after plastic mulch, the latent heat flux decreased significantly by 12–30 W m−2, the sensible heat flux increased significantly by 8–16 W m−2, and the surface air temperature increased significantly by 0.2–0.8 °C. The plastic mulch reduced water vapor in the lower atmosphere, which might lead to decrease precipitation in Northwest China.

An Advanced Multiple‐Layer Canopy Model in the WRF Model With Large‐Eddy Simulations to Simulate Canopy Flows and Scalar Transport Under Different Stability Conditions

AbstractForest canopies play a critical role in affecting momentum and scalar transfer. Although there have been recent advances in numerical simulations of turbulent flows and scalar transfer across plant canopies and the atmosphere interface, few models have incorporated all important physical and physiological processes in subcanopy layers. Here we describe and evaluate an advanced multiple‐layer canopy module (MCANOPY), which is developed based largely on the Community Land Model version 4.5 and then coupled with the Weather Research and Forecasting model with large‐eddy simulations (WRF‐LES). The MCANOPY includes a suite of subcanopy processes, including radiation transfer, photosynthesis, canopy layer energy balance, momentum drag, and heat, water vapor, and CO2 exchange between canopy layers and the canopy atmosphere. Numerical schemes for heat and water transport in soil, ground surface energy balance, and soil respiration are also included. Both the stand‐alone MCANOPY and the coupled system (the WRF‐LES‐MCANOPY) are evaluated against data measured in the Canopy Horizontal Array Turbulence Study field experiment. The MCANOPY performs reasonably well in reproducing vertical profiles of mean and turbulent flows as well as second‐order statistical quantities including heat and scalar fluxes within the canopy under unstable stability conditions. The coupled WRF‐LES‐MCANOPY captures major features of canopy edge flows under both neutral and unstable conditions. Limitations of the MCANOPY are discussed for our further work. Our results suggest that our model can be a promising modeling system for a variety of applications to study canopy flows and scalar transport (e.g., CO2).

Integration of eddy covariance and process-based model for the intra-annual variability of carbon fluxes in an Indian tropical forest

Forests play a crucial role by regulating the global and local weather through the exchange of atmospheric gases and water vapor. The present study aims to study the intra-annual variability of net ecosystem exchange (NEE) of carbon dioxide in a sal (Shorea robusta) dominated moist deciduous forest in India by integrating eddy covariance (EC) data and Biome-Biogeochemical Cycle (Biome-BGC) model. The study also attempts to address the spatial variability of NEE with respect to phenology. Monthly average NEE were estimated using a calibrated Biome-BGC model and the spatial NEE was mapped using random forest (RF) regression algorithm. Phenology metrics were generated using the moderate resolution imaging spectrometer (MODIS) enhanced vegetation index product (MOD13A2) and its relationship with the estimated NEE was studied. RF regression model for monthly average spatial NEE estimation was built with an R2 of 0.84 and % RMSE of 3.68%. The study revealed that the NEE at the regional scale can be estimated using the basic meteorological variables like mean temperature, vapor pressure deficit, minimum temperature and total precipitation. Biome-BGC model output showed that the sal forest of the study area acted as a net sink of carbon in almost all months of 2015, except April to June. Peak NEE value (− 2.80 to − 2.96 g C m−2 day−1) was observed during October month. Annual NEE of sal forest in 2015 was found to be − 526.87 g C m−2 year−1. With the start of season (end of June), sal forest showed an increasing trend in NEE while decreasing trend was observed at the end of season (end of October). The study showed the applicability of Biome-BGC model in Indian forest when integrated with EC data. The study also highlighted the utility of RF in capturing the spatial variability of NEE over large area.

Modeling experiments on seasonal lake ice mass and energy balance in the Qinghai–Tibet Plateau: a case study

Abstract. The lake-rich Qinghai–Tibet Plateau (QTP) has significant impacts on regional and global water cycles and monsoon systems through heat and water vapor exchange. The lake–atmosphere interactions have been quantified over open-water periods, yet little is known about the lake ice thermodynamics and heat and mass balance during the ice-covered season due to a lack of field data. In this study, a high-resolution thermodynamic ice model was applied in experiments of lake ice evolution and energy balance of a shallow lake in the QTP. Basal growth and melt dominated the seasonal evolution of lake ice, but surface sublimation was also crucial for ice loss, accounting for up to 40 % of the maximum ice thickness. Sublimation was also responsible for 41 % of the lake water loss during the ice-covered period. Simulation results matched the observations well with respect to ice mass balance components, ice thickness, and ice temperature. Strong solar radiation, negative air temperature, low air moisture, and prevailing strong winds were the major driving forces controlling the seasonal ice mass balance. The energy balance was estimated at the ice surface and bottom, and within the ice interior and under-ice water. Particularly, almost all heat fluxes showed significant diurnal variations including incoming, absorbed, and penetrated solar radiation, long-wave radiation, turbulent air–ice heat fluxes, and basal ice–water heat fluxes. The calculated ice surface temperature indicated that the atmospheric boundary layer stratification was consistently stable or neutral throughout the ice-covered period. The turbulent air–ice heat fluxes and the net heat gain by the lake were much lower than those during the open-water period.

Responses of Vegetation Productivity to Temperature Trends Over Continental United States From MODIS Imagery

Vegetation growth and productivity may record signals of global climate change since vegetation is sensitive to modifications in climatic variables, such as temperature and precipitation. Significant changes in vegetation growth and productivity can affect the exchange of energy, water vapor, and momentum between the land surface and atmosphere. Despite consensus on the vegetation growth enhancement by the warming of temperature, little attention has been given to the understanding of the impacts of cooling temperature trends on vegetation growth and productivity. Based on satellite-derived land surface temperatures and vegetation indices, this study shows that alternating temperature trends are more reasonable than a monotonic temperature trend for the study period from 2003 to 2016. More importantly, results reveal that both warming and cooling temperature trends can have a positive impact on vegetation growth over the continental United States. These findings are of importance to understand global carbon cycle sinks and sources accurately in different regions.

A Simplified Model of Water Vapor Exchange in the Amazon

Evapotranspiration is the exchange of water vapor between land and the atmosphere, and it is hard to measure and model. A new study shows promise for its estimation over large, vegetated landscapes.

Predicting light-induced stomatal movements based on the redox state of plastoquinone: theory and validation

Prediction of stomatal conductance is a key element to relate and scale up leaf-level gas exchange processes to canopy, ecosystem and land surface models. The empirical models that are typically employed for this purpose are simple and elegant formulations which relate stomatal conductance on a leaf area basis to the net rate of CO2 assimilation, humidity and CO2 concentration. Although light intensity is not directly modelled as a stomatal opening cue, it is well-known that stomata respond strongly to light. One response mode depends specifically on the blue-light part of the light spectrum, whereas the quantitative or ‘red’ light response is less spectrally defined and relies more on the quantity of incident light. Here, we present a modification of an empirical stomatal conductance model which explicitly accounts for the stomatal red-light response, based on a mesophyll-derived signal putatively initiated by the chloroplastic plastoquinone redox state. The modified model showed similar prediction accuracy compared to models using a relationship between stomatal conductance and net assimilation rate. However, fitted parameter values with the modified model varied much less across different measurement conditions, lessening the need for frequent re-parameterization to different conditions required of the current model. We also present a simple and easy to parameterize extension to the widely used Farquhar–Von Caemmerer–Berry photosynthesis model to facilitate coupling with the modified stomatal conductance model, which should enable use of the new stomatal conductance model to simulate ecosystem water vapour exchange in terrestrial biosphere models.

Interaction of urban heat islands and heat waves under current and future climate conditions and their mitigation using green and cool roofs in New York City and Phoenix, Arizona

Urban environments and heat waves interact synergistically and aggravate the thermal environment through the urban heat island (UHI) effect. Of concern is the potential for a projected warmer future climate to further compound heat waves in urban environments. The present study investigates the interaction of a 2006 heat wave in North America with two urban environments (Phoenix and New York City (NYC)) in current climate and future climate simulations. The future climate conditions were generated using the pseudo-global warming methodology. Multiple high-resolution (3 km) simulations were performed using the weather research and forecasting (WRF) model coupled with the single-layer urban canopy model to improve representation of urban processes and we explore how irrigated green roofs and cool roofs can mitigate heat wave amplification by UHIs. To quantify heat wave intensity, an analytical model is applied to the WRF model output that considers the urban surface heat and water vapor exchanges with the atmosphere. A future, warmer climate is found to amplify the UHI intensity during heat waves in both Phoenix (21%) and NYC (48%), but the amplification is of great uncertainty as its magnitude is comparable to the temporal variability of temperatures. The increase in urban heat index can be almost completely offset by adopting irrigated green roofs in urban areas, and partially offset by adopting cool roofs.