Average Annual Flux Research Articles

Yearly analysis of peak temperature, thermal amplitude, time lag and decrement factor of a building envelope in tropical climate

Two similar building structures, one is with macrocapsules of PCM called the experimental building structure (EBS), while the other is without macrocapsules of PCM called the reference building structure (RBS) were developed using conventional construction materials. Both the structures were then monitored for one year in the outdoor tropical type of environmental conditions to evaluate the indoor thermal behavior based on indoor peak temperature, indoor thermal amplitude, time lag, and decrement factor. Additionally, the annual average peak heat flux and corresponding cost-savings in peak cooling load were also analyzed. The results depict that EBS shows a reduction in the indoor peak temperature round the year, ranging from 0.2 °C to 4.3 °C and a percentage reduction in indoor thermal amplitude ranging from -2.43% to 51.3%. The annual average time delay of 97.5 min and the annual average percentage reduction of 24.69% in the decrement factor was obtained by the EBS. The annual average peak heat flux reduction of 17.37% and corresponding yearly average cost-saving in peak cooling load of 1.47 rupees/kWh/m2/day was obtained in the EBS.

Surface Energy Flux Changes and Budget in a Typical Coastal Reed Wetland (Liaohe Delta, China)

Jia, Q.; Zhou, L.; Yu, W.; Wang, X.; Wen, R., and Xie, Y., 2020. Surface energy flux changes and budget in a typical coastal reed wetland (Liaohe Delta, China). Journal of Coastal Research, 36(5), 1005–1012. Coconut Creek (Florida), ISSN 0749-0208.Coastal wetlands are located between the continents and oceans, and their surface energy flux changes and budget play an important role in regulating regional climate. Liaohe Delta is a typical coastal reed wetland in China. The latent heat, sensible heat, and net radiation flux of reed wetlands in Liaohe Delta were studied for four consecutive years (2012–2015) on the basis of the eddy correlation system. The average annual accumulation of latent heat flux was 1525.39 MJ m–2, with a large seasonal variation. The latent heat flux was positive, and the peak value did not exceed 320 W m–2 in the daytime. The latent heat flux increased with the increase of temperature each day. The annual average sensible heat flux was 550.96 MJ m–2, with the minimum sensible heat in June and July, and the daily average change peak did not exceed 160 W m–2. The annual net radiation accumulation was 2868.32 MJ m–2, and the net radiation in winter decreased. The maximum value of net radiation in summer reached 622 W m–2. Latent heat flux accounted for 41%–63% of solar radiation, followed by storage flux, and sensible heat flux had the lowest value. This typical reed wetland has a strong water vapor regulation ability. These results contribute to research on land surface processes in wetlands.

Aqueous fluids are effective oxidizing agents of the mantle in subduction zones

Aqueous fluids produced by dehydration of the downgoing slab facilitate chemical exchange in subduction zones, but the efficiency of fluid-mediated redox transfer as a mechanism to deliver oxidized material from the slab to the sub-arc mantle remains hotly debated. Here we report the first direct measurements of the oxidation state of experimentally produced slab fluids using in situ redox sensors. Our experiments show that the dehydration of natural antigorite serpentinite at shallow subduction zone conditions (1 GPa, 800 °C) produces moderately oxidizing fluids (QFM + 2) with elevated concentrations of Na, K, Ca, and Mg. The composition and redox of the experimental fluids are then used to parameterize a thermodynamic reactive transport model to investigate the interaction of slab fluid with the sub-arc mantle from 1–4 GPa and 700–900 °C. Recently determined equation of state parameters for aqueous fluids at high pressures now enables thermodynamic modeling of aqueous fluid–rock interactions at conditions relevant to deep subduction zones for the first time. Our thermodynamic modeling demonstrates that aqueous fluid can efficiently oxidize Fe in mantle minerals via the reduction of H+ to H2 in the fluid. We estimate that < 1–3 kg of serpentinite-derived fluid at 850–900 °C is required to increase the Fe3+/ΣFe in 1 kg of sub-arc mantle from MORB-like values (0.15) to those of primitive arc basalts (0.2–0.3). We calculate that a slab fluid flux of 1.4 × 109–1.4 × 1014 kg year−1 is required to oxidize sufficient sub-arc mantle to produce the average annual flux of magmas at arcs, which overlaps with the estimated range of H2O flux in subduction zones.

Daylighting performance simulation and analysis of translucent concrete building envelopes

A novel optical fiber (OF) embedded building envelope, namely translucent concrete (TC), was proposed and popularized for its light transmission property and unique artistic features, but there is still a lack of in-depth research on its daylighting performance analysis. To reveal the impacts of multi-factors on TC daylighting performance, an optical model of TC was established based on a ray-tracing method. In the model validation, both the simulated transmittance of OFs and illuminance of translucent concrete were in a good agreement with the experimental results. The simulation results showed when the numerical aperture (NA) of the OFs increased from 0.51 to 0.70, the annual average luminous flux of TC under seven cities conditions could be enhanced by up to 40.62%. In terms of effective illuminance, the optimum fiber volume ratio of the seven cities was determined, respectively. Besides, it was found that the core-cladding interface losses could lead to a relative deviation over 5% when the length of 1-mm OFs with 0.51 NA exceeded 259 mm, and the deviation could be larger with the decrease in diameter of OFs and the increase both in length and NA of OFs.

Magnitude and Edaphic Controls of Nitrous Oxide Fluxes in Natural Forests at Different Scales

Forest nitrous oxide (N2O) emission plays an important role in the greenhouse gas budget of forest ecosystems. However, spatial variability in N2O fluxes complicates the determination of key factors of N2O fluxes at different scales. Based on an updated database of N2O fluxes and the main edaphic factors of global forests, the magnitude of N2O fluxes from forests and the relationships between edaphic factors and N2O fluxes at different scales were analyzed. According to the results, the average annual N2O flux of the global forest was 142.91 ± 14.1 mg N m−2 year−1. The range of total forest estimated N2O emission was 4.45–4.69 Tg N in 2000. N2O fluxes from forests with different leaf traits (broadleaved and coniferous) have significant differences in magnitude, whereas the leaf habit (evergreen and deciduous) was an important characteristic reflecting different patterns of N2O seasonal variations. The main factors affecting N2O fluxes on the global scale were ammonium (NH4+) and nitrate (NO3−) concentrations. With an increasing scale (from the site scale to the regional scale to the global scale), the explanatory power of the five edaphic factors to N2O flux decreased gradually. In addition, the response curves of N2O flux to edaphic factors were diversified among different scales. At both the global and regional scales, soil hydrothermal condition (water filled pore space (WFPS) and soil temperature) might not be the main spatial regulation for N2O fluxes, whereas soil nutrient factors (particularly NO3− concentration) could contribute more on N2O flux spatial variations. The results of site-control analysis demonstrated that there were high spatial heterogeneity of the main N2O controls, showing N2O fluxes from low latitude forests being more likely associated with soil WFPS and temperature. Thus, our findings provide valuable insights into the regulatory edaphic factors underlying the variability in N2O emissions, when modeling at different scales.

A study on the water vapor transport trend and water vapor source of the Tibetan Plateau

The climatological characteristics of water vapor transport over the Tibetan Plateau (TP) were investigated in this study by using the ERA-interim and JRA55 monthly reanalysis dataset. The trends of water vapor budget and water vapor sources during the past 40 years were also revealed. The analyses show that the TP is a water vapor convergence area, where the convergence was enhanced from 1979 to 2018. In addition, the convergence is much stronger in JJA, with a linear trend that is twice the annual average trend. The climatological water vapor sources over the TP were identified mainly at the southern and western boundaries, with the vapor sources at the southern boundaries originating from the Arabian Sea and Bay of Bengal and the vapor sources at the western boundary being transported by mid-latitude westerlies. The TP is a moisture sink at a climatological mean, with an annual average net water vapor flux of 11.86 × 106kg ∙ s−1. Water vapor transport is much stronger in JJA than in other times of the year, and the net water vapor flux is 29.60 × 106kg ∙ s−1. The net water vapor flux in the TP increased with a linear trend of 0.12×106kg ∙ s−1 ∙ year−1 (α = 0.01), while the increase in the flux was more significant in JJA than in other times of the year with a linear trend of 0.30 ×106kg ∙ s−1 ∙ year−1 (α = 0.01). Detailed features in the water vapor flux and transport changes across the TP’s four boundaries were explored by simulating backward trajectories with a Lagrangian trajectory model (hybrid single-particle Lagrangian integrated trajectory model, HYSPLIT). In the study period, the water vapor contribution rate of western channel is increased. However, the Southern channel’s water vapor contribution decreased.

Partial pressure of CO2 and air-sea CO2 fluxes in the South China Sea: Synthesis of an 18-year dataset

This study synthesizes spatial and temporal variations in surface seawater pCO2 (partial pressure of CO2) and associated air-sea CO2 fluxes in the largest marginal sea of the North Pacific, the South China Sea (SCS), based on a large dataset collected from 47 surveys during 2000–2018. We categorized the SCS into five domains featuring different physical and biogeochemical characteristics to better understand the seasonality of SCS pCO2 dynamics and constrain the CO2 fluxes. The five domains are (A) the northern SCS shelf, (B) the northern SCS slope, (C) the SCS basin, (D) West of the Luzon Strait, and (E) the western SCS. We found a large spatial variability in sea surface pCO2 in the SCS, except during winter when values remained in a narrow range of 300 to 360 μatm. In general, seasonal variability was evident in surface water pCO2 values from the northern SCS (Domains A, B and D), with lower values during the cold seasons and higher values during the warm seasons, except in the Pearl River plume (150–650 μatm) and the area off northwest Luzon where winter upwelling occurred (370–470 μatm). In the SCS basin and the western SCS (Domains C and E), pCO2 in surface waters was generally higher than in the atmosphere (380–420 μatm). We also revealed large intra-seasonal variations in the northern SCS during monsoonal transitions in both spring and fall. In spring, pCO2 increased with temperature in the northern SCS, which was a CO2 sink in March but became a CO2 source in May with April as a transitional month. Fall is also a transitional season for the northern SCS, where it changes from a CO2 source back to a CO2 sink. The area-weighted CO2 fluxes across the entire SCS were −1.1 ± 2.2 mmol m−2 d−1 in winter, 0.9 ± 0.9 mmol m−2 d−1 in spring, 2.5 ± 1.4 mmol m−2 d−1 in summer and 1.9 ± 1.1 mmol m−2 d−1 in fall. Nevertheless, on an annual basis, the average CO2 flux from the SCS was 1.2 ± 1.7 mmol m−2 d−1. Enhanced carbon sink on the northern SCS shelf was observed in winter. The annual average CO2 flux was significantly lower than the previous estimate, which can largely be attributed to the addition of new datasets in the previously under-sampled seasons and regions.

Effect of water diversion from the Yangtze River to Lake Taihu on total phosphorus rebound after 2016

针对“引江济太”工程将总磷浓度偏高的长江水引入太湖后对2016年后太湖总磷反弹的影响，本文实测并收集整理了2016年前后“引江济太”调水入湖水量、磷通量及全太湖入湖水量、磷通量与太湖磷存量等数据，对2016年前后“引江济太”调水入湖水量、磷通量、磷形态与其他入湖河道水量、磷通量、磷形态以及全太湖的水质、受水区贡湖的水质进行了分析.结果表明：2016年前后，“引江济太”年均入湖磷通量为97.56 t，年均入湖水量为8.16亿m³，从调水量、入湖磷通量、调水后短期磷响应及各湖区磷增量来看，“引江济太”与2016年后太湖总磷反弹相关性不强.“引江济太”调水累计入湖磷通量为877.97 t，占太湖总入湖磷通量的4.58%，累计入湖水量占太湖累计入湖水量的7.36%，单位水量携带的磷通量仅为其他来水的一半左右，占比相对有限.与太湖主要入湖河流相比，“引江济太”调水属于优质来水，湖泊的入湖河流总磷浓度一般都高于湖泊本身的总磷浓度，“引江济太”调水总磷浓度偏高属于正常范围.目前“引江济太”工程在保证供水安全、缓解水华危机的同时对处于严重富营养化状态的太湖具有一定的改善效果，但未来引水量增加的情况下，必须继续关注引水带来的磷通量与太湖磷循环系统的关系，确保“引江济太”对太湖继续产生良性的影响.;In order to figure out the influence of the water diversion from the Yangtze River Project (WDP) on the rebound of total phosphorus (TP) concentration in Lake Taihu after 2016,this study analyzed the water quantity, phosphorus flux and phosphorus form of Yangtze River and other inflowing rivers including Lake Taihu and Gonghu Bay around 2016 based on the data of water quantity, phosphorus flux transfer from Yangtze River and the phosphorus stock in the lake. The result shows that the average annual phosphorus flux into the lake was 97.56 t and the average annual water inflow volume was 816 million m³ around 2016. From the perspective of water volume, phosphorus flux into the lake, short-term phosphorus response after water diversion and phosphorus increment in each lake area, the correlation between the WDP and the TP rebound in the lake after 2016 is not strong. The cumulative phosphorus flux from the WDP was 877.97 t, accounting for 4.58% of the total TP flux into the lake, and the accumulative amount of lake water accounted for 7.36% of the accumulative amount of lake water in Lake Taihu, the phosphorus flux per unit of water is only about half of that of other water, which accounts for a relatively limited proportion. Compared with the main inflow rivers of Lake Taihu, the water quality from Yangtze River is good. The TP concentration in the inflow rivers is generally higher than the TP concentration of the lake itself. At present, in addition to ensuring the safety of water supply and alleviating the water bloom crisis, the incoming water from the WDP has certain improvement effect when the lake is serious eutrophic. In the next step, we must pay attention to the relationship between the phosphorus flux brought by water diversion and the phosphorus circulation system in Lake Taihu, so as to ensure the ongoing WDP continue to have a positive impact.

Inverse Climate Modelling Study of the Planet Venus

The terrestrial planet Venus is classified by astronomers as an inferior planet because it is located closer to the Sun than the Earth. Venus orbits the Sun at a mean distance of 108.21 Million Km and receives an average annual solar irradiance of 2601.3 W/m², which is 1.911 times that of the Earth. A set of linked forward and inverse climate modelling studies were undertaken to determine whether a process of atmospheric energy retention and recycling could be established by a mechanism of energy partition between the solid illuminated surface and an overlying fully transparent, non-greenhouse gas atmosphere. Further, that this atmospheric process could then be used to account for the observed discrepancy between the average annual solar insolation flux and the surface tropospheric average annual temperature for Venus. Using a geometric climate model with a globular shape that preserves the key fundamental property of an illuminated globe, namely the presence on its surface of the dual environments of both a lit and an unlit hemisphere; we established that the internal energy flux within our climate model is constrained by a process of energy partition at the surface interface between the illuminated ground and the overlying air. The dual environment model we have designed permits the exploration and verification of the fundamental role that the atmospheric processes of thermal conduction and convection have in establishing and maintaining surface thermal enhancement within the troposphere of this terrestrial planet. We believe that the duality of energy partition ratio between the lit and unlit hemispheres applied to the model, fully accounts for the extreme atmospheric “greenhouse effect” of the planet Venus. We show that it is the meteorological process of air mass movement and energy recycling through the mechanism of convection and atmospheric advection, associated with the latitudinal hemisphere encompassing Hadley Cell that accounts for the planet’s observed enhanced atmospheric surface warming. Using our model, we explore the form, nature and geological timing of the climatic transition that turned Venus from a paleo water world into a high-temperature, high-pressure carbon dioxide world.

Biomass Burning Unlikely to Account for Missing Source of Carbonyl Sulfide

AbstractCarbonyl sulfide (OCS) provides a proxy for measuring photosynthesis and is the primary background source of stratospheric aerosols. OCS emissions due to biomass burning are a variable and substantial (over 10%) part of the current OCS budget. OCS emission ratios from open burning fires, coupled with 1997–2016 data from the Global Fire Emissions Database (GFED4), yield OCS biomass burning emissions with a global average annual flux of 60 ± 37 Gg(S) year−1. A global box model suggests these emissions are more consistent with observations from global atmospheric composition monitoring networks than fluxes derived from previous synthesis papers. Even after considering the uncertainty in emission factor observations for each category of emissions and the interannual variation in total burned dry matter, the total OCS emissions from open burning are insufficient to account for the large imbalance between current estimates of global OCS sources and sinks.

Spatial and temporal variations of the greenhouse gas emissions in coastal saline wetlands in southeastern China.

Coastal wetlands are crucial to global climate change due to their roles in modulating atmospheric concentrations of greenhouse gases (GHGs) (CO2, CH4, N2O). Under a warming climate, we investigated spatial and temporal variations of GHGs emissions over the coastal wetlands in southeastern China during 2012-2014. Five dominant land cover types in coastal wetlands have been considered, including the bare mud flat (BF), the Spartina alterniflora flats (SAF), the Suaeda glauca flats (SGF), the Phragmites australis flat (PAF), and the Scripus triqueter flat (STF). The results showed that the annual average CO2 fluxes were 305.8, 588.8, 370.2, and 136.5mgm-2h-1 from spring to winter. CH4 fluxes presented to be a sink in spring (- 0.02mgm-2h-1), and functioned as a source in the following seasons. Correlation analysis indicated that the surface air temperature and the cumulative precipitation could be two main factors that influenced the seasonal and inter-annual variations of GHGs emissions. In addition, we provided a regional budget of GHGs emissions that suggested the variations of GHGs emissions under a warming climate.

Understanding the effects of pasture type and stocking rate on the hydrology of the Southern Great Plains

Grassland is one of the major biomes in the United States (US) and the world. In the US, the majority of grasslands are concentrated in the Great Plains and has undergone through significant interventions or management changes over the last few decades. A key economy-driven intervention in the Southern Great Plains (SGP) include the introduction of new forage species and conversion of native grassland to introduced pasture to increase productivity and its nutritive value for improved cattle production. Since water is one of the fundamental resources needed to sustain grassland productivity, it is important to understand how such pasture conversion and prevailing cattle grazing practices affect water balance and biomass production in a given pasture system. In this study, the Nutrient Tracking Tool (NTT) with its core APEX (Agricultural Policy Environmental eXtender) model was used to assess the hydrological impacts of the pasture introduction, i.e., native pasture (little bluestem, Schizachyrium halapense) vs. introduced pasture (old world bluestem, Bothriochloa caucasica), and the stocking rate in the SGP. Monthly evapotranspiration (ET) and biomass estimates from NTT compared well with observed data at two USDA-ARS experimental pastures (native and introduced) near El Reno, Oklahoma, for the years 2015 and 2016. Simulated long-term average annual hydrologic fluxes (i.e., ET, runoff, and groundwater recharge) from the introduced pasture were slightly lower than the observed data but not significantly different than those from the native pasture under the current management conditions. NTT predicted higher water yield (runoff and recharge) and significantly lower ET for the introduced pasture than the native pasture. Results suggest that grazing has the potential to alter the hydrological balance in the SGP. For example, the increase in stocking rate within the carrying capacity of the farm decreases ET and increases runoff and groundwater recharge for both pastures. Comparison of estimated biomass production between native and introduced pastures indicated that introduced pastures are more efficient in using the available water and thus produce a higher forage biomass per unit of water in the SGP. This study highlighted the potential significance of considering hydrological and other biophysical impacts of new forage introduction and stocking rate changes for the sustainable management of grazing and pasture systems in the SGP.

Dynamic Changes in Hydrochemical Characteristics and Influencing Factors in the Karst Watershed Flood Process

The hydrochemistry of river water in a karst basin has a rapid response to the rainstorm/flood process, which is an important process of the karst carbon cycle and should not be ignored. Based on the dynamic monitoring of the hydrochemical characteristics of the flood process in the Yangshuo section on November 8-12, 2015, the dynamic change in the main ions and the influencing factors were analyzed, and the concentration and flux of inorganic carbon from different sources were calculated. The results showed that the hydrochemistry types in different stages of the flood area belonged to the Ca-HCO3 type. The ions were mainly sourced from carbonate weathering, and affected by silicate weathering, rainfall, and human activities. Because of the hydrological process, the weathering strength of carbonate rocks sharply weakened at the beginning of the flood, and then gradually increased. The concentrations of HCO3-, Ca2+, and Mg2+ sharply decreased at the beginning of the flood, then gradually increased, and continued to increase in the second flood process because of the waterlogging in the karst system. Because of the waterlogging, the reaction time between water and rock become longer; thus, the concentrations are higher. The dynamic changes in SO42-, Cl-, Na+, and K+ were mainly affected by precipitation and human activities. At the beginning of the flood, the concentrations of SO42-, Cl-, Na+, and K+ increased because the runoff takes more ions sourced from activities. The concentrations of SO42-, Cl-, Na+, and K+ decreased with the decrease of easily transported substances. At the lowest point of concentration, SO42- and Cl- were mainly sourced from precipitation, and Na+ and K+ were mainly sourced from precipitation and silicate weathering. The weathering of carbonates by carbonic acid was the main source of inorganic carbon, accounting for 74.3% of total inorganic carbon on average. Because of the input of sulfuric/nitric acid, the contribution of the weathering of carbonates by sulfuric/nitric acid to the inorganic carbon cannot be ignored, and the contribution increased significantly in the flood, up to 31.7%. The geological carbon sinks before the flood, and during the first and second flood processes in the Yangshuo section were 1.28×108, 5.28×108, and 11.52×108 g·d-1, respectively. The geological carbon sink before the flood was equal to the annual average flux, whereas the geological carbon sink in the flood process was several times that of the annual average flux. Moreover, because of the significant difference in the weathering strength of carbonate rocks during the two floods, there was also a significant difference in the amount of geological carbon sink under the same discharge.

Modeling sediment oxygen demand in a highly productive lake under various trophic scenarios.

Hypolimnetic oxygen depletion in lakes is a widespread problem and is mainly controlled by the sediment oxygen uptake (SOU) and flux of reduced substances out of the sediments (Fred). Especially in eutrophic lakes, Fred may constitute a major fraction of the areal hypolimnetic mineralization rate, but its size and source is often poorly understood. Using a diagenetic reaction-transport model supported by a large data set of sediment porewater concentrations, bulk sediment core data and lake monitoring data, the behavior of Fred was simulated in eutrophic Lake Baldegg. Transient boundary conditions for the gross sedimentation of total organic carbon and for hypolimnetic O2 concentrations were applied to simulate the eutrophication and re-oligotrophication history of the lake. According to the model, Fred is dominated by methanogenesis, where up to70% to the total CH4 is produced from sediments older than 20 years deposited during the time of permanent anoxia between 1890 and 1982. An implementation of simplified seasonal variations of the upper boundary conditions showed that their consideration is not necessary for the assessment of annual average fluxes in long-term simulations. Four lake management scenarios were then implemented to investigate the future development of Fred and SOU until 2050 under different boundary conditions. A comparison of three trophic scenarios showed that further reduction of the lake productivity to at least a mesotrophic state is required to significantly decrease Fred and SOU from the present state. Conversely, a termination of artificial aeration at the present trophic state would result in high rates of organic matter deposition and a long-term increase of Fred from the sediments of Lake Baldegg.

Atmospheric dry and bulk nitrogen deposition to forest environment in the North China Plain

Atmospheric nitrogen (N) deposition fluxes has dramatically increased in past few decades and has aroused great concerns of its impacts on forest ecosystems. However, complete quantitative information on dry and wet deposition of various reactive nitrogen (Nr) species is very limited for forest environments. Therefore, two-year (2014–2015) measurements of gaseous NH3, NO2, and HNO3 and particulate NH4+ and NO3− (pNH4+ and pNO3-) in air and/or precipitation were conducted in Jigong Mountain National Nature Reserve in the North China Plain (NCP). Our results showed that ambient concentrations of all Nr species were highest in winter and lowest in summer except for NH3; conversely, bulk deposition fluxes of each Nr species generally peaked in summer and were lowest in winter. Annual mean bulk Nr deposition fluxes was 15.5 kg N ha−1 yr−1 (7.9 kg N ha−1 yr−1 for NH4+ and 7.6 kg N ha−1 yr−1 for NO3−), with higher monthly fluxes occurring in spring and summer. Average annual dry Nr deposition fluxes was 44.3 kg N ha−1 yr−1 (30.1 kg N ha−1 yr−1 for reduced N (NH3 plus pNH4+) and 14.2 kg N ha−1 yr−1 for oxidized N (the sum of HNO3, NO2 and pNO3-). Total Nr deposition fluxes (dry plus bulk) reached 59.8 kg N ha−1 yr−1 with more contributions from reduced N than from oxidized N (64% vs 36%). When consider the N critical load (10–20 kg N ha−1 yr−1) in forest ecosystem, potential negative effects may cause by N deposition in the NCP which should be paid more attention.

Global satellite-driven estimates of heterotrophic respiration

Abstract. While heterotrophic respiration (Rh) makes up about a quarter of gross global terrestrial carbon fluxes, it remains among the least-observed carbon fluxes, particularly outside the midlatitudes. In situ measurements collected in the Soil Respiration Database (SRDB) number only a few hundred worldwide. Similarly, only a single data-driven wall-to-wall estimate of annual average heterotrophic respiration exists, based on bottom-up upscaling of SRDB measurements using an assumed functional form to account for climate variability. In this study, we exploit recent advances in remote sensing of terrestrial carbon fluxes to estimate global variations in heterotrophic respiration in a top-down fashion at monthly temporal resolution and 4∘×5∘ spatial resolution. We combine net ecosystem productivity estimates from atmospheric inversions of the NASA Carbon Monitoring System-Flux (CMS-Flux) with an optimally scaled gross primary productivity dataset based on satellite-observed solar-induced fluorescence variations to estimate total ecosystem respiration as a residual of the terrestrial carbon balance. The ecosystem respiration is then separated into autotrophic and heterotrophic components based on a spatially varying carbon use efficiency retrieved in a model–data fusion framework (the CARbon DAta MOdel fraMework, CARDAMOM). The resulting dataset is independent of any assumptions about how heterotrophic respiration responds to climate or substrate variations. It estimates an annual average global average heterotrophic respiration flux of 43.6±19.3 Pg C yr−1. Sensitivity and uncertainty analyses showed that the top-down Rh are more sensitive to the choice of input gross primary productivity (GPP) and net ecosystem productivity (NEP) datasets than to the assumption of a static carbon use efficiency (CUE) value, with the possible exception of the wet tropics. These top-down estimates are compared to bottom-up estimates of annual heterotrophic respiration, using new uncertainty estimates that partially account for sampling and model errors. Top-down heterotrophic respiration estimates are higher than those from bottom-up upscaling everywhere except at high latitudes and are 30 % greater overall (43.6 Pg C yr−1 vs. 33.4 Pg C yr−1). The uncertainty ranges of both methods are comparable, except poleward of 45∘ N, where bottom-up uncertainties are greater. The ratio of top-down heterotrophic to total ecosystem respiration varies seasonally by as much as 0.6 depending on season and climate, illustrating the importance of studying the drivers of autotrophic and heterotrophic respiration separately, and thus the importance of data-driven estimates of Rh such as those estimated here.

Tracking the fate of deposited nitrogen and its redistribution in a subtropical watershed in China

AbstractVegetation cover and soil infiltration can result in nitrogen (N) redistribution and associative isotopic fractionation. This study investigated N and its isotopic characteristics from deposition to baseflow under different vegetation canopy cover types in a subtropical watershed in China. Results showed that canopy interception via vegetation cover types exhibited notable dilution effects on reactive nitrogen (Nr) deposition during the rainy season, with the highest maximum value of 287.4%. The range of δ15N changed from 3.0‰ to 11.9‰ in throughfall, which may have resulted from an increase in N taken up by vegetation and particles washed away from plant leaves during rainfall events. Throughfall, soil water, precipitation, and rainfall run‐off were the sources of D and 18O‐H2O in baseflow river water, wherein the rainwater and rainfall run‐off contributed 73% ± 8% and 15% ± 12%, respectively. Soil water was the main source of 15N‐NO3 in baseflow river water, which contributes 78% ± 4% of overall source, but precipitation only contributes 10%. The average annual Nr deposition flux was 24.4 ± 4.2 kg·ha−1·yr−1, whereas the annual total nitrogen exportation flux was 1062 ± 269.8 kg·yr−1. The 46% of HDO or D2O for river water came from rainfall run‐off, which would transport a maximum nitrate flux of 354 kg·yr−1. Moreover, vegetation cover and soil infiltration resulted in δ15N enrichment and a decrease in Nr in river flow.

Numerical study on the response of the largest lake in China to climate change

Abstract. Lakes are sensitive indicators of climate change. There are thousands of lakes on the Tibetan Plateau (TP), and more than 1200 of them have an area larger than 1 km2; they respond quickly to climate change, but few observation data of lakes are available. Therefore, the thermal condition of the plateau lakes under the background of climate warming remains poorly understood. In this study, the China regional surface meteorological feature dataset developed by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), MODIS lake surface temperature (LST) data and buoy observation data were used to evaluate the performance of lake model FLake, extended by simple parameterizations of the salinity effect, for brackish lake and to reveal the response of thermal conditions, radiation and heat balance of Qinghai Lake to the recent climate change. The results demonstrated that the FLake has good ability in capturing the seasonal variations in the lake surface temperature and the internal thermal structure of Qinghai Lake. The simulated lake surface temperature showed an increasing trend from 1979 to 2012, positively correlated with the air temperature and the downward longwave radiation while negatively correlated with the wind speed and downward shortwave radiation. The simulated internal thermodynamic structure revealed that Qinghai Lake is a dimictic lake with two overturn periods occurring in late spring and late autumn. The surface and mean water temperatures of the lake significantly increased from 1979 to 2012, while the bottom temperatures showed no significant trend, even decreasing slightly from 1989 to 2012. The warming was the strongest in winter for both the lake surface and air temperature. With the warming of the climate, the later ice-on and earlier ice-off trend was simulated in the lake, significantly influencing the interannual and seasonal variability in radiation and heat flux. The annual average net shortwave radiation and latent heat flux (LH) both increase obviously while the net longwave radiation and sensible heat flux (SH) decrease slightly. Earlier ice-off leads to more energy absorption mainly in the form of shortwave radiation during the thawing period, and later ice-on leads to more energy release in the form of longwave radiation, SH and LH during the ice formation period. Meanwhile, the lake–air temperature difference increased in both periods due to shortening ice duration.

The importance of sewage effluent discharge in the export of dissolved organic carbon from U.K. rivers

AbstractThe flux of fluvial carbon from the terrestrial biosphere to the world's oceans is known to be an important component of the global carbon cycle, but within this pathway, the flux and return of carbon to the river network via sewage effluent has not been quantified. In this study, monitoring data from 2000 to 2016 for the dissolved organic carbon (DOC) concentration, biochemical oxygen demand, and chemical oxygen demand of the final effluent of sewage treatment works from across England were examined to assess the amount of DOC contributing to national‐scale fluvial fluxes of carbon. The study shows that the median concentration of DOC in final effluent was 9.4 compared with 4.8 mg C/L for all surface waters for the United Kingdom over the study period and that the DOC in final effluent significantly declined over the study period from 11.0 to 6.4 mg C/L. Rivers receiving sewage effluent showed a significant, on average 19%, increase in DOC concentration downstream of sewage discharges. At the scale of the United Kingdom, the flux of DOC in final effluent was 31 ktonnes C/year with a per capita export of 0.55 kg C/year and compared with an average annual flux of DOC from the United Kingdom of 859 ktonnes C/year, that is, only 3.6% of national‐scale flux. The lability of this DOC was limited, with only 7.4% loss of final effluent DOC concentration over in‐stream residence times of up to 5 days. The direct decline in DOC concentration from sewage treatment works was not large enough on its own to explain the declines observed in DOC concentration in U.K. rivers at their tidal limit.

Response of the sediment geochemistry of the Changjiang River (Yangtze River) to the impoundment of the Three Gorges Dam

Based on the measurement of major and trace elements in suspended sediments in the low reaches of the Changjiang River during throughout a whole hydrologic year, the origins, seasonal variations, and fluxes of multielements and the human impacts on multielements transport processes have been analyzed along with the influence of weathering in the Changjiang River basin. The results show that most element contents were high in both autumn and winter and low in summer, which was largely caused by the dilution of discharge. Weathering detritus in the Changjiang River basin is the main source of most elements in suspended sediments. However, riverine pollution could bring more loadings of Cd, Pb, As and Zn into river water than it did a few decades ago. The annual average fluxes of Cd, Pb and Zn, which are major contamination elements, to the sea were 179 ± 21 tons/year, 7810 ± 675 tons/year, and 12,000 ± 1320 tons/year, respectively, in which approximately 8.7%, 11.9% and 2.7% of their loadings, respectively, were contributed by pollution inputs. Element exports mainly occurred in the summer (44.4%–57.4%) in the lower part of the Changjiang River. A general relationship between sediment retention and element content suggests a positive feedback mechanism for the decreased number of particles, in which element riverine loadings are reduced due to the enhanced trapping effect by the Three Gorges Dam (TGD). Compared to those in 1980, current element shares of the Changjiang River compared to the global budget have declined due to the construction of the TGD.