Water Vapor Pressure Difference Research Articles

Analysis of drought response thresholds and drought-causing factors of central Asian vegetation

Understanding the threshold responses of vegetation to drought is essential for predicting and mitigating drought risk. Across Central Asia, extreme weather is expected to increase by the end of the 21st century. However, the threshold responses of different Central Asian vegetation types are not well understood. Here, we calculated drought indices for Central Asia based on meteorological data and remotely sensed vegetation indices from 2001 to 2020, used trend analysis to assess changes in drought and vegetation in Central Asia over the past 20 years, evaluated the threshold responses of five vegetation types to drought, and explored the drought causative factors of different vegetation types using coexistence analysis. All five Central Asian vegetation types responded to increasing drought in a nonlinear manner, and thus exhibited drought-responsive thresholds. Forests were the first responders to increasing drought, and wetlands were the least responsive overall. Grasslands, cultivated lands, shrublands, and forests crossed the drought threshold in different years between 2001 and 2020, with shrublands exhibiting the largest percentage of area crossing the threshold. Finally, the drought causative factor for grasslands, shrublands, and wetlands was soil moisture; for cultivated lands was precipitation; and for forests was saturated water vapor pressure difference. This study uncovers the characteristic responses of different Central Asian vegetation types to drought, and lays a theoretical foundation for drought assessment and ecological restoration in Central Asia.

Measurement of microclimates in a warming world: problems and solutions.

As the world warms, it will be tempting to relate the biological responses of terrestrial animals to air temperature. But air temperature typically plays a lesser role in the heat exchange of those animals than does radiant heat. Under radiant load, animals can gain heat even when body surface temperature exceeds air temperature. However, animals can buffer the impacts of radiant heat exposure: burrows and other refuges may block solar radiant heat fully, but trees and agricultural shelters provide only partial relief. For animals that can do so effectively, evaporative cooling will be used to dissipate body heat. Evaporative cooling is dependent directly on the water vapour pressure difference between the body surface and immediate surroundings, but only indirectly on relative humidity. High relative humidity at high air temperature implies a high water vapour pressure, but evaporation into air with 100% relative humidity is not impossible. Evaporation is enhanced by wind, but the wind speed reported by meteorological services is not that experienced by animals; instead, the wind, air temperature, humidity and radiation experienced is that of the animal's microclimate. In this Commentary, we discuss how microclimate should be quantified to ensure accurate assessment of an animal's thermal environment. We propose that the microclimate metric of dry heat load to which the biological responses of animals should be related is black-globe temperature measured on or near the animal, and not air temperature. Finally, when analysing those responses, the metric of humidity should be water vapour pressure, not relative humidity.

Appropriate Water and Nitrogen Regulation Improves the Production of Wolfberry (Lycium barbarum L.)

Wolfberry (Lycium barbarum L.) production in arid and semi-arid areas is drastically affected by the low utilization rate of soil and water resources and the irrational application of water and nitrogen fertilizers. Thus, this study explored a high-yielding, high-quality, and efficient irrigation and nitrogen regulation model to promote the production efficiency of wolfberry and rational utilization of water and land resources in arid and semi-arid areas. We compared and analyzed the effects of different soil water treatments (the upper and lower limits of soil water were estimated as the percentage of soil water content to field water capacity (θf), with the following irrigation regimen: adequate irrigation (W0, 75–85% θf), mild water deficit (W1, 65–75% θf), moderate water deficit (W2, 55–65% θf), and severe water deficit (W3, 45–55% θf)) and nitrogen levels (no nitrogen (N0, 0 kg·ha−1), low nitrogen (N1, 150 kg·ha−1), moderate nitrogen (N2, 300 kg·ha−1), and high nitrogen (N3, 450 kg·ha−1)) on the growth, physiology, and production of wolfberry. The results showed that water regulation, nitrogen application level, and their interaction significantly affected plant height and stem diameter growth amount (p < 0.05). Additionally, the relative chlorophyll content of wolfberry leaves first increased and then decreased with increasing nitrogen levels and water deficit. The average net photosynthetic rate (Pn), stomatal conductance (gs), intercellular carbon dioxide concentration, and transpiration rate (Tr) reached the highest values in plants exposed to W0N2 (19.86 μmmol·m−2·s−1), W1N1 (182.65 mmol·m−2·s−1), W2N2 (218.86 μmol·mol−1), and W0N2 (6.44 mmol·m−2·s−1) treatments, respectively. Pn, gs, and Tr were highly correlated with photosynthetically active radiation and water vapor pressure difference (goodness-of-fit: 0.366–0.828). Furthermore, water regulation and nitrogen levels exhibited significant effects on the yield and water- (WUE), and nitrogen-use efficiency (NUE) (p < 0.01), and their interactions exhibited significant effects on the yield, WUE, and nitrogen partial productivity of wolfberry plants (p < 0.05). Moreover, the contents of total sugar, polysaccharides, fats, amino acids, and proteins were the highest in W1N2, W1N2, W1N2, W2N3, and W0N2 treatments, respectively, which were increased by 3.32–16.93%, 7.49–54.72%, 6.5–45.89%, 11.12–86.16%, and 7.15–71.67%, respectively. Under different water regulations (except for the W3 condition) and nitrogen level treatments, the net income and input–output ratio of wolfberry were in the order W1 > W0 > W2 > W3 and N2 > N3 > N1 > N0. The TOPSIS method also revealed that the yield, quality, WUE, NUE, and economic benefits of wolfberry improved under the W1N2 treatment, suggesting that WIN2 might be the most suitable irrigation and nitrogen regulation model for wolfberry production in regions with scarce land and water resources such as the Gansu Province and areas with similar climate.

Variation in canopy conductance of Cunninghamia lanceolata forest and its response to environmental factors after an extreme rainfall event

AbstractCanopy conductance (gc) is a crucial parameter in simulating evapotranspiration and modulating water exchange, but its variation mechanism has regional uncertainties and complex environmental co‐controls. In addition, the effect of extreme rainfall on gc cannot be ignored under the changing climate. Here, we investigated the variation and environmental controls on gc and the effect of extreme rainfall events in a Cunninghamia lanceolata forest across the subtropical area of Southern China. In July 2020, an extreme rainstorm hit the source area of the Xin'an River, with the cumulative rainfall on July 7 and 8 reaching 216.6 mm. The thermal diffusion probe method was used to measure the density of sap flow, and the environmental factors such as air temperature (Ta), net solar radiation (Rn) and soil water content (SWC) were monitored during the growing seasons of 2020 and 2021. Ultimately, gc obtained by the Penman‐Monteith equation was adopted since the result from the Köstner equation was overestimated. gc showed a unimodal curve on the diurnal scale, and this characteristic was more obvious after the extreme rainfall. Daily gc appeared a fluctuating pattern with a maximum in summer. gc was simultaneously affected by Ta, Rn, water vapour pressure difference (VPD), SWC, among which Ta was the most significant driving factor at both the diurnal and daily timescales. The regulation of Ta, VPD and SWC on gc had obvious thresholds, and the most definite response mode was VPD (2020: 1.25 kPa; 2021: 0.95 kPa). SWC and Ta were the dominant factors after the rainfall period, and the promotion effect of VPD on post‐rainy days turned to inhibition effect on typical sunny days. These findings will further reveal the water exchange mechanism between atmosphere and vegetation and impacts of environmental factors in subtropical coniferous forests, especially after the extreme rainfall events.

Assessment of satellite images terrestrial surface temperature and WVP using GNSS radio occultation data

Abstract Land Surface Temperature (LST) and Water Vapor Pressure (WVP) contour maps can be produced using cameras aboard satellites, for instance, under the name “Remote Sensing (RS)”. Satellite image observations should be verified before using based on a reliable data. Global Navigation Satellite System Radio Occultation (GNSS-RO) method is observing accurate Earth atmosphere parameters continuously. In the present research, LST and WVP differences between Landsat 8 (LC08), Sentinel-3 (S3), and MODIS (Terra and Aqua) images and GNSS-RO are assessed in Egypt depending on the satellites operating periods and data availability during the years from 2015 to 2020. Statistically, S3 and Terra have insignificant differences with RO temperature with an average bias of 3.48 °C and 1.47 °C, respectively, but LC08 and Aqua have significant differences with it. For WVP, Aqua and LC08 have insignificant differences with an average bias of 0.02 kg/m2 and 2.31 kg/m2, respectively, but S3 and Terra have significant differences with RO observations. When comparing LC08 LST data to other satellites, it was found that there were insignificant differences between LC08 and S3 as well as Terra. However, significant differences were observed when comparing LC08 LST data to Aqua. Additionally, significant differences were noted when comparing LC08 WVP data to other satellites. In response to these differences, improvement models have been developed to enhance the estimation of terrestrial data through remote sensing, particularly for satellites that exhibited significant disparities when compared to reference observations (RO).

Spatial and Temporal Variations of Carbon Dioxide Fluxes in Urban Ecosystems of Changsha, China

Understanding the spatial and temporal variations of urban carbon dioxide fluxes (FCO2) and their influencing factors is crucial for solving urban climate problems and promoting the development of low-carbon cities. In this study, the carbon dioxide flux (FCO2) in Changsha City, China, was analyzed using the eddy covariance technique and flux footprint model. The results showed that the extent of the flux footprint within the observation site was mostly limited to 500 m. Diurnal variation of FCO2 showed a regular pattern influenced by plant photosynthesis and traffic flow. Meanwhile, photosynthesis was directly regulated by photosynthetically active radiation and indirectly regulated by air temperature and water vapor pressure differences. The average value of FCO2 was lower during the daytime than at night, indicating the high vegetation cover (43.5%) in the study area. In addition, there were spatial characteristics of FCO2 in each wind direction due to different surface land use in the study area. Notably, a decreasing trend in carbon dioxide content was observed after the area covered by vegetation was 1.8 times the area of buildings and major roads combined. These findings guide climate management, urban planning, and sustainable development toward a low-carbon society.

Arid AREAS Water-Piled Photovoltaic Prevents Evaporation Effects Research

(1) Background: In arid and semi-arid reservoirs, water surface evaporation is the main method of water dissipation in order to inhibit the evaporation of water and enhance economic efficiency. The evaporation inhibition rate of water-piled PV at different times of the year is derived from the anti-evaporation test of water-piled PV, and a new idea is proposed for water conservation in plains reservoirs in arid areas. (2) Methods: The test was conducted by dividing the area into groups A and B, with and without PV panel shading. In situ observation and numerical calculation were used to measure the atmosphere’s temperatures, test group, and PV module. The saturated water vapor pressure difference was then calculated according to Dalton’s principle to analyze the economic benefits of water saving. (3) Results: Based on the test results, it was found that the shading of PV panels had a cooling effect on the water body, the PV module, and the atmosphere. Group A showed a 44.2% decrease in the saturation water vapor pressure difference compared to Group B. The maximum evaporation suppression rate of 40.2% was observed in July, while the minimum rate of 12.2% was observed in January. The average evaporation suppression rate for the entire year was 29.2%. By utilizing the annual water savings for agricultural irrigation, it is possible to cover 38 hm2 of land and generate a revenue of 39,000 CNY. (4) Conclusions: The photovoltaic water cover can effectively reduce water evaporation and generate economic benefits.

Sap flow of two typical woody halophyte species responding to the meteorological and irrigation water conditions in Taklimakan Desert

Understanding plant water consumption is crucial for artificial afforestation under drought environments and water stress in desert regions. However, the water consumption characteristics of desert species responding to the irrigation regimes are often neglected. By conducting a field test in the Taklimakan Desert Highway shelterbelt, this study examines the sap flow traits of two typical woody halophyte species (Calligonum mongolicum and Haloxylon ammodendron) and how they react to weather conditions and watering practices. Under the same irrigation treatment, the stem flux of C. mongolicum on sunny days was 1.5–5.3 times that on dusty days, while the stem flux of H. ammodendron on sunny days was 3.5–5.5 times that on dusty days. Both species demonstrated some sap flow during the night, representing 14.3%–24.9% and 7.3%–10.4% of the total sap flow for C. mongolicum and H. ammodendron, respectively. H. ammodendron maintained a higher stem flow during daytime and was more drought resistant than C. mongolicum. The daily sap flow patterns of these two species varied, showing both ‘single’ and ‘double peak’ curves depending on the watering conditions. A delay was also observed between the sap flow of these two species and the environmental factors. The factors influencing plant sap flow were found to be in the order of solar radiation, temperature, relative humidity, and saturated water vapor pressure difference. A BP-neural network proved highly effective for accurately simulating the sap flow of these two species. This research provides insights into how two common desert tree species adapt their water use in response to drought conditions, which is vital for artificial forest creation in desert areas.

Spatial variations and mechanisms for the stability of water use efficiency in China.

A clearer understanding of the stability of water use efficiency (WUE) and its driving factors contributes to improving water use efficiency and strengthening water resource management. However, the stability of WUE is unclear. Based on the EEMD method, this study analyses the spatial variations and mechanisms for the stability of WUE in China, especially in the National Forest Protection Project (NFPP) areas. It is found that the stable WUE was dominated by non-significant trends and increasing trends in China, accounting for 33.59% and 34.19%, respectively. The non-significant trend of stable WUE was mainly located in the Three-North shelterbelt program area, and the increasing trend of stable WUE was in Huaihe and Taihu, Taihang Mountains, and Pearl River shelterbelt program areas. Precipitation and soil moisture promoted the stable WUE in these project areas. The unstable WUE was dominated by positive reversals or negative reversals of WUE trends. The positive reversals of unstable WUE were mainly located in the Yellow River shelterbelt program areas, which was promoted by temperature and radiation, while the negative reversals of unstable WUE were mainly distributed in the Yangtze River and Liaohe shelterbelt program areas, which were mainly induced by saturation water vapor pressure difference (VPD). Our results highlight that some ecological restoration programs need to be improved to cope with the negative climate impact on the stability of WUE.

The effect of time scales on the distribution of evapotranspiration and driving factors in desert grasslands

Promoting the reasonable allocation of evaporation (E) and transpiration (T) at a certain time scale is important for controlling the grassland desertification. Nevertheless, evapotranspiration (ET) and its components on a single time scale often cannot guarantee the dynamic and reasonable distribution of E and T. This study quantified ET and its component distribution in different time scales of the plant-growing season in desert grassland ecosystems to address this contradiction. Further the main driving factors of ET and its components at different time scales were clarified under multi-factor coupling. Results indicated that E was the leading component of ET in the ecosystem, with an average T/ET of 39.64%. The dominant factors of T/ET on different time scales were air temperature (Ta) and surface temperature (Ts), which showed significant negative correlations (p < 0.05). ET and its components showed obvious unimodal changes at the hourly scale. The peaks occurred at 12:00–14:00, and they all increased with increasing radiation (Ra), Ts, Ta, and saturated water vapor pressure difference (VPD). Ta was the dominant factor with a correlation coefficient of 0.76, followed by VPD> Ra>Ts. At the daily scale, ET and its components showed clear seasonal changes, ET was 1.94 mm d-1and T was 0.77 mm d-1. The dominant factors were soil moisture content (SWC) of the 0–10 cm soil layer and Ts. The dominant factors affecting ET and its components in the early, middle, and late stages of growth were relative humidity (RH), SWC, and Ts. At the monthly scale, the cumulative values of ET and its components were the largest in August and the lowest in October, mainly regulated by precipitation (P), whereas, T/ET had no significant correlation with P. The results from this study can provide a scientific basis for vegetation restoration and environmental protection in desert grassland ecological function areas.

Research on mass transfer characteristics of the flue gas dehydration using ceramic membrane transport condensers

• Building an experimental platform for flue gas dehydration with the ceramic membrane tube. • Circulating water and purge gas are used as the cooling medium. • The operating parameters are divided into flue gas and cooling medium parameters. • Correlation analysis of permeate flux and intrinsic factors influenced by operating parameters. • Excessive transmembrane pressure difference can damage the selective permeability of ceramic membranes. The direct discharge of flue gas from the coal fired boiler can result in significant water loss. Membrane separation technology is an effective means to solve the flue gas dehydration problem. Based on the permeation characteristics of porous membrane materials, ceramic membrane method flue gas dehydration is a complex mass transfer process. In this paper, the ceramic membrane transport condensers were built with circulating water, positive pressure purge gas and negative pressure purge gas as the cooling medium, respectively. Changing flue gas parameters and cooling medium parameters, the correlation coefficients of water vapor pressure difference, transmembrane pressure difference and Reynolds number with permeate flux were investigated. The results showed that the key factors that determine the permeate flux were slightly different with the different experimental conditions. Changing the flue gas temperature or the cooling medium temperature has an obvious effect on the water vapor pressure difference. There is a significant correlation between the water vapor pressure difference and the permeate flux. Changing the flue gas flow or the cooling medium flow, the water vapor pressure difference is less variable, and the transmembrane pressure difference is not the key factor affecting the permeate flux, and the Reynolds number is the key factor affecting the permeate flux. Meanwhile, the experimental process revealed that an excessive transmembrane pressure difference can cause non-condensable gases in the flue gas to enter the cooling water tank.

Analysis and Experimental Study on Water Vapor Partial Pressure in the Membrane Distillation Process.

In membrane distillation, the vapor pressure difference is the driving force of mass transfer. The vapor pressure is generally assumed by the saturation pressure and calculated by the Antoine equation. However, in the actual operation process, the feed solutions usually flow in a non-equilibrium state, which does not meet the theoretical and measurement conditions of the vapor-liquid equilibrium (VLE) state. This study tested the actual water vapor pressure of the pure water, lithium bromide (LiBr) solution, lithium chloride (LiCl) solution, and calcium chloride (CaCl2) solution under different flow conditions. The results showed that the actual water vapor pressure was lower than the saturation pressure overall, and the difference increased with temperature but decreased with the mass concentration. Therefore, in vacuum membrane distillation (VMD), air gap membrane distillation (AGMD), and sweeping gas membrane distillation (SGMD), the membrane flux calculated by water vapor saturation pressure was higher than the actual membrane flux, and the relative difference decreased and was less than 10% after 60 °C. In direct contact membrane distillation (DCMD), the water vapor pressure difference on both membrane sides was almost the same by using the saturation vapor pressure or the tested data since the pressure errors were partially offset in parallel flow or counter-flow modes.

A Modified Evaporation Model Indicates That the Effects of Air Warming on Global Drying Trends Have Been Overestimated

AbstractPotential evapotranspiration (PET) is a key input to hydrologic models simulating the atmospheric water demand. Thus, uncertainties in PET would result in considerable uncertainty in model outputs. Among numerous PET algorithms, the Penman‐Monteith equation (PM equation) is most widely used because of its clear physical basis and acceptable estimation. However, PM equation has a physical limitation resulting from the linear approximation of the saturation water vapor pressure differences between the evaporation surface and the surrounding air, which can affect markedly the responses of PET to air warming. By considering a modified PM‐type evaporation model, we revisited the global drying trends based on the widely used Palmer Drought Severity Index (PDSI). We demonstrate that the studies, which based on the classic PM equation, often overestimate the effects of air warming on global drying trends. Particularly, the PET_PM (PET calculated using the classic PM equation) driven PDSI, overestimates global mean drought‐affected areas by 60%, drought intensity by 65%, and drought duration by 35% on average over the globe, and tends to overestimate drying trends in areas that are drying and underestimate wetting trends in areas that are becoming wetter.

Energy flux observation in a shrub ecosystem of a gully region of the Chinese Loess Plateau

Flux observations are important for understanding the basic processes of carbon and water cycling in an ecosystem. In 2020 (1 January 2020 to 31 December 2020), we continuously monitored the surface energy fluxes of a shrub ecosystem using an eddy covariance system. Additionally, meteorological variables, such as temperature and rainfall, were measured. The study site was located in Chunhua of China, which is a gully region of the Chinese Loess Plateau. The diurnal and seasonal dynamic changes in the energy balance of this region were analysed, Bowen and energy balance ratios were calculated, and environmental factors affecting the variations in latent (LE) and sensible heat fluxes (H) were explored. The results showed that the annual net radiation (Rn), LE, H, and soil heat flux (G) were 2385.6, 1059.1, 787.6, and - 48.3 MJ/m2, respectively, with 490.58 MJ/m2 energy remaining undetected. The main part of the annual energy of the shrub ecosystem was latent heat, accounting for 44.4% of Rn. A seasonal variation in each energy component was evident, and the diurnal variation showed a single peak curve. LE in the growing season was dominant, and the proportion of LE and H in the net radiation in the non-growing season showed little difference. The Bowen ratio changed monthly and significantly fluctuated from 0.2 in July to 4.0 in December. It was high in winter and low in summer, with little fluctuation. The annual energy balance ratio was 0.8. The energy balance closure in the non-growing season was greater than that in the growing season. Both H and LE were significantly correlated with net radiation, and H was negatively correlated with rainfall, soil moisture, and soil temperature. Unlike H, LE was significantly positively correlated with soil temperature. In addition, LE was positively correlated with air humidity gradient and water vapour pressure difference.

Variation Characteristics and Influencing Factors of Soil Moisture Content in the Lime Concretion Black Soil Region in Northern Anhui

Soil moisture content (SMC) is an important factor affecting crop growth. Based on the field SMC data measured at the Wudaogou experimental station from 1989 to 2019, two typical crop types, wheat and maize, were selected. By combining the groundwater depth, crop growth period, and meteorological factors in the same period, and using classical statistics and redundant analysis (RDA) methods, the characteristics and influencing factors of SMC changes in vertical profiles of different crops were compared and analyzed. The results showed that the SMC and average daily water storage of wheat were greater than that of maize. The crop growth mainly consumed 0–60 cm SMC. The SMC in this area was moderately variable; the SMC of 0–30 cm belongs to the active layer, and the SMC of 30–100 cm belongs to the sub-active layer. The RDA method identified ground temperature, groundwater depth, relative humidity, and the wheat growing period as the main factors affecting soil moisture variation in wheat fields; groundwater depth, relative humidity, and water vapor pressure differences were the main factors affecting soil moisture variation in maize fields. The results can provide a basis for accurate prediction of soil water dynamics and thus provide a reference for irrigation decision-makers.

Heat and moisture transfer of building envelopes under dynamic and steady-state operation mode of indoor air conditioning

Realizing dynamic thermal comfort can help improve building performance. The heat and moisture transfer of the envelope is an important part of building performance. Therefore, this paper takes a typical room in Shanghai as an example and uses simulation software WUFI-PLUS and DELPHIN to study the influence of dynamic/steady-state operation mode of indoor air conditioning on the heat and moisture transfer of the envelope structure. The results show that based on the same thermal comfort, the energy consumption of the full heat load in dynamic operation mode is 73.69% lower than that in steady-state operation mode at 0ACH night ventilation, and it is 65.10% lower at 5ACH night ventilation. And at the same night ventilation condition, the moisture flow through the wall in the dynamic operating mode is smaller than that in the steady-state operating mode. In the case of 0ACH, when the partial pressure difference of water vapor is positive, the moisture transfer capacity in the dynamic operation mode of the indoor thermal environment is 11.7% lower than that in the steady-state operation mode. Therefore, when the partial pressure difference of water vapor is positive, the dynamic change of the indoor thermal environment can reduce the latent heat load of the envelope under the same water vapor pressure difference.

Performance evaluation of water vapor permeation through perfluorosulfonic acid capillary membranes

Perfluorosulfonic acid membranes perform water vapor permeation at a high level. Few studies, however, have described the water vapor permeance of dehumidification membranes. In this study, therefore, perfluorosulfonic acid capillary membranes used in commercial membrane dehumidifiers were examined to clarify the measurement conditions that are used to evaluate water vapor permeation performance. The measurement conditions include an operating temperature of 10–40 °C, a feed gas relative humidity (RH) of 0–90%, and a total feed side pressure of 0.2 MPaG. Water vapor permeation was evaluated by measuring slight changes in the RH of sweep gas on the permeate sides using countercurrent modules with an effective membrane length of 80 mm. Regarding the RH of the feed gas, the difference between feed-in and sweep-in RH was set to 20% or less to minimize the water vapor pressure difference across the membranes. The water vapor flux was higher under high operating temperatures and high levels of feed gas RH. On the other hand, the water vapor permeance increased with increasing feed gas RH but decreased with increasing operating temperature. Under conditions of 10 °C and feed-in RH of 90%, a high water vapor permeance of 1.6 × 10−5 mol/(m2 s Pa) was obtained. To verify the dependence of water vapor permeation performance on the feed gas species used, measurements were carried out using air, N2, He, and CO2 as the feed gases. The water vapor permeation performance showed no dependence on feed gases of air, N2, or He, but permeation was decreased when CO2 gas was used.

Experimentation of a Membrane-based Concentration Gradient Energy Storage of Liquid Desiccant Solutions Driven by Solar Energy

Solar energy storage is an indispensable and sustainable utilization mode of renewable energy; environment friendly, large-capacity, low heat loss, and long-term storage are critical to improving the integration of solar energy supply. Traditional thermal energy storage mode cannot achieve long-term storage due to the heat loss even under the excellent thermal insulation measures. In this work, a solar-powered membrane-based concentration gradient energy storage of liquid desiccant solutions is presented. In the membrane distillation process driven by solar energy under the right solar radiation conditions, the liquid desiccant solution is concentrated gradually and long-term stored as the concentration gradient energy. To this end, the measured temperature of solar hot water is in the range of 40°C to 90°C from May to September, 2018, in Xi’an, China. And then, the LiBr solution (50 wt%), the LiCl solution (35 wt%), and the CaCl2 solution (40 wt%) were membrane-based concentrated in the temperature range of 42°C to 63°C, separately. The results showed that the water vapor pressure difference decides the water vapor transferred across the membrane pores from the liquid desiccant side to the air side. The energy storage density of liquid desiccant solutions increases along with the increases in temperature and the membrane area. Consequently, when the LiBr, LiCl, and CaCl2 solutions are concentrated from 50% to 55%, from 35% to 40%, and from 40% to 45%, separately, the concentration energy storage density is 245 kJ/kg, 350 kJ/kg, and 306 kJ/kg, which is equivalent to or even higher than ice storage capacity. Due to the two independent closed cycle of the liquid desiccant solution and air, the liquid desiccant solution’s concentration gradient energy storage can be long-term stored environment-friendly without any insulation measures.

基于Penman-Monteith方程模拟青海云杉生长季日蒸腾过程

青海云杉（Picea crassifolia）作为我国黄土高原与青藏高原地区的主要造林树种，对其林分蒸腾耗水特征的研究，能够更合理的指导该地区植被重建与林分调控，以加强林分的稳定性，提高水分利用效率。为了揭示青海云杉在生长季内的冠层蒸腾规律以及冠层整体气孔阻力与环境因子的响应，评价Penman-Monteith方程在青海云杉冠层尺度上的适用性，采用探针式热扩散茎流计（TDP）进行测定，同步长期监测了环境数据，利用反推法建立冠层整体气孔阻力（r_sT）与环境因子之间的回归模型，结合Penman-Monteith方程模拟出青海云杉生长季的日蒸腾量，采用均方根误差、平均绝对误差和平均相对误差对蒸腾量的实测值与模拟值进行误差分析，以验证模型的准确性。得出的主要结论有：（1）生长季内青海云杉日蒸腾量随月份变化呈先增高后降低的趋势；各月蒸腾量占潜在蒸散量的比例为7月（79.68%） > 8月（72.71%） > 6月（72.53%） > 5月（67.08%） > 9月（66.48%） > 10月（64.29%）；（2）树干液流对气象因子的滞后时间为0.5 h；（3）不同月份云杉冠层整体气孔阻力（r_sT）与空气相对湿度（RH）呈正相关关系，与大气温度（T）、饱和水汽压差（VPD）呈负相关关系；（4）应用所建立的多因素回归模型结合Penman-Monteith方程对青海云杉蒸腾量进行模拟验证，累计平均相对误差为14.381%，平均绝对误差为0.160 mm，均方根误差为0.2。综上所述，Penman-Monteith方程在林分冠层尺度上有较好适用性根据所建立的多因素回归模型并结合Penman-Monteith方程，可以利用饱和水汽压差、温度和空气相对湿度三个气象因子较好地模拟日蒸腾过程。;Picea crassifolia is the main afforestation tree species in the Loess Plateau in China. Vegetation reconstruction and forest stand regulation can be guided reasonably by studying water consumption characteristics in this area. It is important for forest stand to strengthen stability and to improve water use efficiency (WUE). In order to reveal the canopy transpiration characteristics of Picea crassifolia during the growing season and the response of the overall canopy stomatal resistance to environmental factors, and to understand the equation of Penman-Monteith whether or not suitable for scale of canopy of Picea crassifolia, we used thermal dissipation probe (TDP) to measure sap flow of Picea crassifolia and monitored the factor of environment by weather station (DAVIS). Then we framed the regression model between Canopy overall stomatal resistance (r_sT) and the environmental factors by inverse method. Lastly, we used equation of Penman-Monteith to predict the transpiration of Picea crassifolia during the growing season. The Root Mean Squared Error, Mean Absolute Deviation and Mean Relative Error would be the indexes to test and verify the accuracy of model we framed when we compared the predicted value with the measured value. The results show that (1) the daily evapotranspiration of Picea crassifolia increases firstly and then decreases with the monthly changes during the growth season. The ratio of monthly evapotranspiration to potential evapotranspiration is Jul (79.68%) > Aug (72.71%)> Jun (72.53%) > May (67.08%) > Sep (66.48%) > Oct (64.29%). (2) There is an obvious Lag-time phenomenon between sap flow and meteorological factors about 0.5 h. (3) The r_sT is positively correlated with relative humidity (RH), and negatively correlated with atmospheric temperature (T) and saturated water vapor pressure difference (VPD). (4) We applied the established multi-factor regression model and Penman-Monteith equation to simulate and verify transpiration of Picea crassifolia. The results indicated that the cumulatively average relative error is 14.381%, the average absolute error is 0.160 mm, and the root mean square error is 0.2. The simulation effect is basically feasible. In summary, the equation of Penman-Monteith has good applicability on the stand canopy scale. We can accord to the established multi-factor regression model combined with the equation of Penman-Monteith, three meteorological factors of saturated water VPD, T and RH can be used to simulate the evapotranspiration on any day, avoiding the inconvenience of long-term observation.

北京山区侧柏林冠层-大气蒸腾导度模拟及环境因子响应

蒸腾导度模型是衡量冠层-大气界面水汽输出的重要阻力模型，研究其特征及对环境因子的响应，为揭示森林冠层-大气界面水汽输出阻力机制提供理论依据。以首都圈森林生态系统定位观测研究站侧柏林为研究对象，采用TDP热探针法测定侧柏林树干液流密度，同步监测光合有效辐射、饱和水汽压差、气温、风速等主要环境因子，分析冠层导度和空气动力学导度的动态变化，构建冠层-大气蒸腾导度模型并模拟，明确冠层-大气蒸腾导度对各环境因子的响应关系。结果表明：蒸腾导度季节变化表现为非生长季与冠层导度趋势一致，生长季与空气动力学导度趋势一致，全年均为单峰趋势。冬季蒸腾导度与冠层导度保持较稳定差值（45 mol m^-2 s^-1左右），其他季节蒸腾导度与冠层导度、空气动力学导度的最大差值，均在各季节冠层导度、空气动力学导度的峰值水平。全年日均蒸腾导度冬季最大（86.92 mol m^-2 s^-1），其他季节较小且稳定（40-50 mol m^-2 s^-1之间）。在非生长季各环境因子对蒸腾导度的影响与对冠层导度的影响基本一致，温度为主要影响因子（r=-0.198），其他环境因子影响较小（r<0.1）；在生长季中风速为主要影响因子（r=0.488），光合有效辐射（r=0.228）和饱和水汽压差（r=-0.299）的影响明显升高，温度的影响降低（r=0.114）。蒸腾导度模型较好的模拟了冠层-大气界面侧柏蒸腾不同季节的变化规律，阐明了各环境因子和冠层导度、空气动力学导度对蒸腾导度的影响机制，证实在生长季应重视空气动力学导度对蒸腾的影响。;Transpiration conductance model is an important resistance model to measure water vapor output at the canopy atmosphere interface. Our study foucused on its characteristics and response to environmental factors, which provided a theoretical basis for clarifying the water vapor output mechanism at the canopy atmosphere interface. In this paper, Platycladus orientalis forest observation station of forest ecosystem in the capital circle was taken as the research object. Through analyzing the dynamic changes of canopy conductance and aerodynamics conductance by TDP thermoprobe, we measured the density of sap flow in the trunk of Platycladus orientalis forest and monitoring the main environmental factors such as photosynthetic effective radiation, saturated water vapor pressure difference, air temperature, and wind speed. And the model of canopy-atmospheric transpiration conductance was derived and calculated so as to build a simulation, which clarified the response of canopy atmospheric transpiration conductance to various environmental factors. The results showed that the trend of transpiration conductance in non-growing season was consistent with that in canopy; the growing season was consistent with the trend of aerodynamic conductance, and the trend was single peak throughout the year. The difference between transpiration conductance and canopy conductance in winter remained stable (around 45 mol m^-2 s^-1);the maximum difference value among transpiration conductance, canopy conductance and aerodynamic conductance in other seasons was at the peak level of canopy conductance and aerodynamic conductance; the annual average transpiration conductance was the largest in winter (86.92 mol m^-2 s^-1) and it was smaller and stable in other seasons (40-50 mol m^-2 s^-1); in the non-growing season, the effect of environmental factors on transpiration conductance was basically the same as that on canopy conductance, and temperature was the most important factor (r=-0.198) while other environmental factors had little impact (r<0.1); in the growing season, wind speed was the most important factor (r=0.488); the influence of photosynthetic effective radiation (r=0.228) and vapor pressure deficit (r=-0.299) was significantly increased, and the influence of temperature was reduced (r=0.114). The model of transpiration conductance simulated the change rule of Platycladus orientalis transpiration in different seasons, clarified the influence mechanism of environmental factors, canopy conductance and aerodynamic conductance on transpiration conductance, and confirmed that we should pay attention to the influence of aerodynamic conductance on transpiration in growing season.